Water Quality & Habitat Health of the Three Bays Estuarine System Progress Report from the 1999 Field Season
Background: One of the most significant long-term threats to coastal ecological health, both locally and globally, is the over-fertilization or eutrophication of our near shore coastal salt ponds and embayments. Functioning as the "sentinels of the coast", these circulation restricted embayments are among the most productive of our coastal waters. Healthy systems support extensive and diverse plant and animal communities and have important aesthetic, commercial and recreational value. At the same time, however, these systems are also the first to be impacted by human activities within their watersheds. The steady and progressive increase in nutrient inputs to these fragile environments, resulting primarily from on-site septic systems, fertilizers and runoff associated with increased coastal development, can have long ranging impacts on both ecosystem health and the economic health of coastal communities which depend on the commercial, recreational and aesthetic resources provided by these systems. As the problem of nutrient overload to coastal water bodies has only been seriously recognized in recent decades. Our limited understanding of the long-term impacts of chronic nutrient loading, the natural processes controlling nutrient cycling and the ability of these systems to assimilate additional nutrients frequently hinders the development of management strategies. In addition, as new approaches for restoring embayments are implemented evaluation of "success" is needed, but requires long-term water quality data.
The potential detrimental impacts of chronic nutrient overloading to coastal systems are clear, with the primary source of this loading being associated with the steady and progressive increase in development along our coastlines. Degradation of coastal waters and development are tied together through inputs of pollutants in runoff and groundwater flows, and to some extent through direct disturbance, i.e. boating, oil and chemical spills, and direct discharges from land and boats. Excess nutrients, especially nitrogen, promote phytoplankton blooms and the growth of epiphytes on eelgrass and attached algae, often with adverse consequences. Where waters are made turbid by excessive plankton blooms, light penetration to benthic plants (like eelgrass) is reduced, thereby reducing their rate of photosynthesis and growth. Algal slime and other epiphytic growth on these plants may shade them further, so that they sicken or die. Decaying phytoplankton and macro-organic matter build up on the bottom, increasing the oxygen demand upon bottom waters. At excessive levels of nitrogen inputs, bacterial decay processes combine with animal respiration and night-time plant respiration to draw down the dissolved oxygen supply, especially in the hot, still weather typical of July and August. Fish and shellfish kills and changes in overall benthic animal communities are a common result. This process of increasing nutrient inputs resulting in algal blooms, low oxygen and stressed animal communities is called eutrophication and if the increased nutrient loads result from human activities, cultural eutrophication.
Three Bays Nutrient Related Health: Overall the Three Bays Estuary is showing relatively good nutrient related health compared to most of the embayments on the south shore of Cape Cod. However, there are some areas within the Estuary which are presently showing nutrient related water quality declines and there is a wide variation in habitat quality within the Three Bays System. In general the quality of habitat in Three Bays shifts from high quality near the inlets to Nantucket Sound to poor quality (eutrophic) in the inland reaches. At present, Princes Cove, and the region of the mouth of the Marstons Mill River through the narrows to North Bay are showing poor nutrient related environmental health. However, the loss of eel grass beds from most of the Three Bays proper indicates that the system has undergone nutrient loading related shifts. The effects of watershed nutrient inputs can be seen in diminished water transparency, increased chlorophyll and nitrogen levels in the upper system and North Bay. Sampling of benthic animals and high frequency sampling of chlorophyll and oxygen levels will help to better define the level of habitat health in these component systems. Specific findings and recommendations for the data collection in the coming field season are given below. It should be noted that the monitoring program and all proposed additional data collection are specifically to support the linked watershed-embayment quantitative assessment the direct target of which is the restoration and protection of the health of the Three Bays Estuary.
(1) The Three Bays Estuary appears to be nitrogen limited, i.e. additions of nitrogen will increase algal production. Therefore, managing the nutrient related health of these marine waters requires management of nitrogen inputs from the watershed and removal by tidal exchanges to adjacent Nantucket Sound.
(2) The upper estuary from the mouth of the Marstons Mills River through the channel to North Bay and Princes Cove is currently showing poor nutrient related health. Additional high frequency sampling performed by Three Bays since summer of 2000 clearly supports that Princes Cove, Warrens Cove and much of North Bay is currently being over-fertilized (i.e. it is eutrophic).
(3) The Marstons Mills River appears to be a major "point source" of nitrogen loading to the Estuary. The River carries nitrogen gathered through surface and groundwater inflows to the Bay.
(4) The organic matter within the Three Bays Estuary appears to be produced by phytoplankton supported by inputs of watershed nitrogen and recycled nitrogen within the Bays, as opposed to entering the system in surface water flows.
(5) Fecal coliform levels are generally low within the lower estuary , but high (relative to shellfish harvest limits and safe swimming levels) within the upper estuary. However, examination of the long-term record of fecal coliform levels collected by DMF (1985-1998) indicates that levels over the shellfish limit occasionally occur in the near shore regions throughout most of the Three Bays System.
(6) Elevated fecal coliform levels are not always associated with rain events, suggesting potential direct discharges to the upper system from waterfowl, boats, direct wastewater discharges (in violation of health code). Additional sampling by Three Bays Preservation since the summer of 2000 are aimed at further elucidating the fecal coliform sources.
(7) Water quality monitoring will support water quality modeling of the effect of widening the Cotuit entrance channel. This effort would be part of the on going quantitative water quality study, but would rely (in part) on the monitoring data obtain over the past 14 years. Computer modeling will aid in the calculation of this water quality improvement.
(8) Shellfish upwellers to take advantage of high phytoplankton biomass, but avoid the periodic low bottom water dissolved oxygen. Upwellers are a practical and efficient method to grow out shellfish to a sustainable size that allows for free planting in the estuary.
(9) Deployment of moored measuring instruments as was done in Princes Cove in 2000 and other areas of the embayment that are of special concern to better elucidate the frequency and duration of low oxygen conditions and phytoplankton blooms.
(10) The establishment of a freshwater inflow and nutrient load gauging station on the Marstons Mills River and the Little River was completed in 2000. The data provided essential data input to the linked watershed-embayment water quality models and are central to validating the freshwater balance of the system. A flow station was established during the summer 2000 as part of the Water Quality Monitoring Program, using a grant from the Cape Cod Commission for hardware.
(11) Nutrient related environmental stress is clearly evident within the upper portions of the Three Bays System and other areas have undergone declines from historic levels. Nitrogen loading to the Three Bays System is still continuing both through increasing watershed loading and the time lag between the initiation of a new nitrogen source and the arrival of the associated nitrogen in the Bay. In addition, bacterial contamination within the upper estuary is occurring. While it is clear that additional water quality monitoring is needed, the implementation of the state-of-the-art linked watershed-embayment management study is warranted. This quantitative assessment and modeling of the Three Bays System is needed to develop realistic and efficient watershed management options focused on improving habitat quality. This estuarine restoration and management effort requires a solid foundation of nutrient-related water quality monitoring information (multi-year) from which to base the higher level modeling and management effort. However, some aspects of estuarine restoration and management planning have already begun, based upon the initial monitoring results (i.e. the implementation has started and should continue to be fostered).
(12) Water quality monitoring is a long-term commitment required for proper management of coastal embayments. The data is initially used to guide higher level studies for restoration or protection efforts. Over time monitoring is used to gauge the effectiveness of implemented management practices and to support adaptive management of these systems.
Our near shore coastal salt ponds and embayments are among the most productive components of our coastal oceans. Coastal salt ponds and embayments represent a fundamental commercial, recreational and aesthetic resource for coastal communities like Barnstable. These circulation- restricted embayments support extensive and diverse plant and animal communities providing the foundation for many important commercial and recreational fisheries. The aesthetic value of these systems is also significant to both year-round and seasonal residents and the tourist industry alike. Maintaining high levels of water quality and ecological health in these systems, therefore, is fundamental to the enjoyment and utilization of these valuable resources for virtually all coastal communities.
Within Massachusetts alone the quality of our marine environment results in nearly $2 billion in annual earnings, over $600 million of which is related to recreation and tourism and $660 million to fishing and seafood sales. While much of the economics relies on a healthy environment, the Massachusetts population in coastal communities grew at more than two times the rate as inland communities state-wide. Globally, almost 40% of the worlds population lives within 60 miles of the coast.
As a result of land-use changes associated with an increasing coastal population, our coastal waters, specifically the near shore systems of coastal salt ponds, embayments and harbors, have seen an apparent and steady decline in water quality in recent decades. This is not only a local issue, but one of global concern as development increases along the world's shorelines. Because of their obvious proximity to the coast and their frequently heavy utilization, many coastal communities like those on Cape Cod have increasingly turned their attention toward protecting these systems from further decline or attempting their restoration. Quantifying long-term trends in water quality and addressing the actual causes of decline are fundamental to developing management strategies for these systems, both for protection from further degradation, and restoration and remediation where necessary.
The progressive and steady decline in nutrient related water quality represents one of the biggest problems facing coastal communities today. The Three Bays System is no exception. Declining nutrient related ecological health of coastal environments manifests itself as large-scale shifts in animal and plant habitat quality with a resulting shift away from commercial and recreational fish and shellfish species. Additional economic impacts may also result at higher levels of nutrient related water quality degradation (eutrophication) where aesthetic and recreational values are impacted by algal blooms, odors and/or increasing wrack deposition.
Fortunately new approaches have been developed to support the conservation or restoration of coastal embayments. These approaches are based upon the linkages between embayments and their watersheds and are based upon state-of-the-art ecological analysis and modeling coupled to engineering and policy initiatives. Managing environmental health requires a quantitative understanding of the biological and physical processes which control nutrient related water quality within a specific embayment and the role of watershed inputs in the materials balance of the receiving waters.
The first step in developing this understanding for any coastal embayment system is the establishment of an embayment-wide water sampling program. The sampling program must be conducted throughout most of the year, with the emphasis on summer-time (worst case) conditions. The goal of the monitoring program is to (1) determine the relative ecological health of each of the system's sub-embayments, (2) gauge the decline or recovery of various sub-embayments over the long-term, and (3) provide the foundation (and context) for the detailed quantitative measures required for proper nutrient and resource management.
To this end, the Three Bays Water Quality Monitoring Program was initiated in 1999 as a collaborative effort between the School of Marine Science and Technology (SMAST) at UMass Dartmouth and Three Bays Preservation. The role of the monitoring program is initially focusing on the level and spatial distribution nutrient related environmental health within Three Bays System and how patterns in environmental health varied through time. In its assessment phase, the Program has sought to focus on measuring those parameters indicative of habitat quality/environmental health. All of the present effort, however, will feed directly into future restoration efforts.
The SMAST provides technical support for a variety of monitoring programs throughout S.E. Massachusetts and has a national and international reputation in coastal research. The Coastal Systems Program has been acknowledged by the EPA, USGS, EOEA, Mass DEP, MCZM, and CCC as highly qualified to fulfill this role as well as serving as the provider of the quantitative ecological measurement and modeling required for site-specific restorations The Coastal Systems Program is currently involved in restoration efforts or protection efforts for over 89 coastal embayments.
Nutrient overload is the major ecological threat to water quality within the Three Bays System, primarily via the ecological degradation which results when loading exceeds the assimilative capacity (also called critical nutrient threshold) of the system for new nutrient inputs. Of the various forms of pollution that threaten coastal waters (nutrients, pathogens and toxics), nutrient inputs are the most insidious and difficult to control. This is especially true for nutrients originating from non-point sources, such as nitrogen transported in the groundwater from on-site septic treatment systems (the primary mechanism for waste disposal within the Three Bays watershed). Identifying, understanding and predicting the potential impact of these nutrients is required for management of the Three Bays Estuary, but important to this is state-of-the-art scientific analysis and modeling. While these higher level studies are developing, ongoing watershed management decisions can be made based on the incoming monitoring data and existing watershed loading analysis and we can begin to develop suitable management strategies for protection of these valuable coastal environments. However, comprehensive restoration efforts must await the results of the quantitative system-wide program.
THREE BAYS WATER QUALITY MONITORING PROGRAM
It has been clear to citizens that portions of the Three Bays System are declining in quality. In general, areas with potentially lower circulation (such as Princes Cove) tend to have lower water quality and are more sensitive to impacts from nutrient loading. Not surprisingly, these also tend to be most likely the areas with the most significant nutrient inputs. As a result of public concern the Three Bays Water Quality Monitoring Program was initiated in 1999. In this collaborative effort, Three Bays Preservation Inc. provides the support, coordination and oversight of the program through its director, Lindsey Counsell and SMAST provides technical guidance, analytical support and data interpretation.
The monitoring program uses both volunteers and staff for the field data collection efforts. The sampling teams are equipped and trained prior to sampling and given refreshers as needed during the field season. Analysis of samples collected from the effort and compilation of field and laboratory data provide an ecological overview of water quality conditions within the system. The methods employed are directly comparable to other data collection efforts associated with SMAST which involve over half of the embayments in Massachusetts.
The sampling teams conduct field measurements of physical parameters as well as collect water samples for subsequent nutrient analysis by the Coastal Systems Laboratory at SMAST. The physical parameters include: total depth, Secchi depth (light penetration), temperature, pond state, weather, wind speed and direction. Samples are also collected and "fixed" in the field for Winkler Titration to determine dissolved oxygen content. Laboratory analyses include: plant available forms of nitrogen and phosphorus (nitrate + nitrite, ammonium, ortho-phosphate), organic forms of nitrogen and carbon (dissolved organic nitrogen, particulate organic nitrogen, dissolved organic nitrogen and particulate organic carbon), the primary phytoplankton pigment, chlorophyll a, as a gauge of algal biomass and salinity. Samples for physical parameters are measured at surface and bottom or in depth profile if low tide water depths reach 3 meters. Nutrient samples are generally collected at mid-water except in deep basins where profiles are collected. These parameters form the basis for evaluating the nutrient related health of a coastal embayment. In addition, samples for the indicator of potential pathogen contamination, fecal coliform numbers, are collected from 6" (15 cm) depth at each station and assayed by a membrane filtration technique by the Barnstable County Department of Health and Environment laboratory.
The following measurements were conducted on each sampling (O = On Site; L = Lab):
(O) Total Depth
(O) Light Penetration (Secchi depth)
(L) Nitrate + Nitrite
(L) Dissolved Organic Nitrogen
(L) Particulate Organic Nitrogen
(L) Total Dissolved Nitrogen
12. Chlorophyll & Pheophytin a
(O) Oxygen Content
(O/L) Periodic Sulfide
Sampling is monthly and concentrated from May - November, when nutrient related water quality declines are most prevalent. Fourteen stations within the estuary (Figure 1A) and five within the Marstons Mills River System (Figure 1B) are occupied during each event. Stations were selected to provide adequate spatial coverage with replication within the larger sub-embayments.
Data is compiled and reviewed by the laboratory for accuracy and evaluated to discern any possible artifacts caused by improper sampling technique. Modifications to the Program are made based upon annual review. Modifications are only made if data quality is compromised, useful long-term data is created by consistent sampling of a fixed network of stations through time. SMAST has completed a Quality Assurance Project Plan (QAPP) for the program in which details of sampling and analytical procedures are provided. All analytical methodologies have been previously approved for use by EPA, MCZM, DEP, NOAA and NSF.
As the program progresses, additional "special projects" are being undertaken to identify problem areas or to support interpretation of the monitoring data. Based upon 1999 results (see below) the Program has begun monitoring river discharge volume from the Marstons Mills River (with partial support from a Cape Cod Commission Grant). In addition, moored instruments to monitor dissolved oxygen and chlorophyll have been deployed within Princes Cove to better elucidate the level of nutrient related decline in this over fertilized system.
THREE BAYS NUTRIENT RELATED WATER QUALITY
The Three Bays Estuarine Complex consists of the estuary, the freshwater lakes, ponds and rivers, and the encompassing watershed. The Three Bays Water Quality Monitoring Program focuses on gathering data on the nutrient related health and bacterial contamination of the fresh and salt water aquatic systems. Ultimately, a linked watershed-embayment modeling effort will be used to seamlessly integrate all three components. However, the monitoring program data alone is a significant increase in our understanding of the health of the Three Bays System and potential causes of degradation if encountered. Before the initiation of the Monitoring Program there was no system-wide data available for gauging the environmental health of this marine system. Therefore, although only a single season of sampling has been completed, we have attempted to bring forward the initial results. It should be cautioned that a proper assessment generally requires multiple years of data collection to allow for wet versus dry years or windy versus calm etc. However, there are some points which can be made unequivocally and assessments which additional data would be unlikely to alter. These include findings of significant depletions of oxygen or sustained algal blooms or macroalgal mat formation or loss of eelgrass beds. What follows is our initial assessment of the Three Bays System which will be updated and refined as additional data is collected.
The aquatic resources within the Three Bays Monitoring Program are dominated by the marine bays. The bays encompass almost 1200 acres compared to the 3 major fresh ponds, 375 acres (Table 1). It is also clear from the relative upland contributing area for each sub-embayment that the upper estuary is most likely to be nutrient impacted. This is further supported by the greater distance to the high quality source waters of Nantucket Sound. The upper estuary (Marstons Mills River Mouth through the narrows to North Bay and Prince Cove) is the receiving area for watershed inputs from almost half of the total watershed of Three Bays, but has little dilution as these upper basins are relatively shallow (Table 1). The nutrient load to this region projected by the Cape Cod Commission indicates that nutrient overloading has already occurred.
North Bay which is still in the upper region of the estuary receives nutrients from almost one third of the entire watershed to the Bays, as well as nutrients passing through the upper estuary. While North Bay receives tidal flows though two channels and is a large system, it is the sub-system which needs to be most closely monitored for shifts in health. By contrast, Cotuit Bay and West Bay receive nutrients from proportionally less watershed area than North Bay. Of course the actual watershed loading is based upon both area and land-use.
Water column sampling during the 1999 field season was randomly distributed relative to rain events (Figure 2). The high frequency of rainfall in this region makes this a common feature of embayment monitoring programs. It should be noted that 1999 did have a exceptionally dry June.
One of the key features of a healthy shallow embayment is the presence of eelgrass beds. These communities require a high level of water clarity so that light can penetrate to the bay bottom to support plant grow. One of the key responses to nutrient overloading is a decline in water clarity due to shading created by phytoplankton blooms. A simple direct measure of water clarity is made using a Secchi disk, which is a round white/black disk lowered into the water. The depth at which the disk can no longer be seen can be related to the depth to which light penetrates. This technique is standard in monitoring of bays and lakes.
Within the marine regions of Three Bays there was a gradient in light penetration, with greater water clarity in the Seapuit River and Lower West Bay (which approach Nantucket Sound levels) and higher turbidity in North, Upper West and Cotuit Bays and still higher turbidity in Prince Cove (boxes in Figure 3). The low light penetration within Prince Cove is consistent with the projected watershed nutrient loads. The percent of the water column above the Secchi depth (diamonds in Figure 3) is a gauge of light levels at the bay floor. A value of 100% indicates clear water from surface to bottom and a high probability that eelgrass would be able to grow (if the condition persists year-round). Only portions of West and Cotuit Bays (including Seapuit River) appear to have high light at the sea floor. This is consistent with the current distribution of eelgrass within the Bay, although a detailed survey is not yet complete. The water clarity was likely much higher in the early 1900's given the scallop fishery within the Bays, even before the second inlet to the Sound was opened.
Nitrogen levels within the Three Bays System are consistent with watershed areas, circulation and the different requirement of fresh versus salt water ecosystems for nitrogen. The higher nitrogen levels within the River (MR-C, MR-D, MR-E) versus Middle Pond (MR-B) result from both the high nitrogen load directly to the river coupled with nitrogen removal mechanisms operating within the Pond. However, it is the River which transports nitrogen directly to the upper estuary and both the amount and the prevalence of plant available N forms (DIN=ammonium and nitrate) suggest a potentially high level of loading from the watershed. The nitrogen entering the estuary is rapidly processed and diluted. However, the response of the estuary can be clearly seen by the shifting of the plant available nitrogen (DIN) to particulate nitrogen forms (Figure 4). These particulate forms are phytoplankton which are growing at high rates on the nitrogen entering from the River. The nutrient enriching effect of watershed inputs and summertime nitrogen release from the embayment sediment persists throughout the estuary although the level of enrichment diminishes due to dilution approaching Nantucket Sound. Only the Seapuit River shows nitrogen levels at the same level as the Sound station. Using a general total nitrogen level of 0.35 mg N/L as an upper threshold for eelgrass survival, it appears that much of Three Bays eelgrass bed loss is likely due to nitrogen overloading. Additional study is ongoing to confirm this hypothesis.
To confirm that nitrogen is the critical nutrient to manage within the Three Bays Estuary, we can examine the ratio of plant available nitrogen versus phosphorus within Bay waters. These ratios can be compared to an empirical average ratio (Redfield Ratio=16) and values falling substantially below 16 indicate nitrogen, whereas above 16 would suggest phosphorus is critical (Figure 5). As with most other embayments in New England, nitrogen appears to be the nutrient stimulating phytoplankton production (limiting growth) within Three Bays during the critical summer period. The indication of "abundant" nitrogen during May has been observed in other local systems, however, the nitrogen is rapidly depleted by the summer phytoplankton.
The increase in particulate organic nitrogen in the estuary versus River (Figure 3) and the lower water clarity within the Bays versus Sound are most likely the result of phytoplankton growth. However, in systems where rivers enter, these observations can be caused by organic matter transported by the fresh inflowing waters. This difference would have significant consequences to any management strategy. To evaluate these different sources of organic matter, we examined the relationship between phytoplankton chlorophyll levels and particulate organic carbon concentrations. The result is that within the River (MR-D and MR-E) there was no clear relationship indicating that organic detritus and erosional material may be playing a significant role. In strong contrast, the Bay stations showed a clear direct relationship (Figure 6). This is strong support for the contention that it is the nutrients entering the bay that result in the particulate production (phytoplankton growth) and increased turbidity of the System.
A fundamental feature of a high degree of nitrogen over-enrichment of estuarine waters is the depletion of bottom water oxygen. Oxygen is needed to support both plant and animal life within the marine waters and even a partial depletion (to level of less than 4 mg/L) can cause significant stress and shifts in communities. During the monitoring in 1999, oxygen levels below 4 mg/L were not observed (Figure 7). However, large oxygen depletions compared to Nantucket Sound waters were seen primarily within the upper estuary. These depletions occur when the a system becomes "unbalanced" and oxygen consumption exceeds oxygen re-supply. While monitoring data is a good indicator of potential low oxygen areas, the high temporal variation in oxygen levels within shallow bays generally ensures that the oxygen minimum or the duration of low oxygen events is not detected. As a result of the 1999 data, we deployed a continuous oxygen recorder within Prince Cove during 2000. The data from the late summer 2000 clearly show significant oxygen depletion (to about 2 mg/L) on a frequent basis (data not shown here). The oxygen data are consistent with the other key parameters indicating a gradient from low environmental health in the upper estuary to a high level of nutrient related health in the lower estuary.
Based upon work in Buzzards Bay and by Falmouth Pond Watch a health index has been developed that integrates all of the above water quality parameters. The index is empirically based using habitat quality and monitoring data from about 30 embayments to generate the 3 levels of health, good-excellent (>65), fair (35-65), partially impaired and poor (<35), nutrient over-enriched or eutrophic. The health or Eutrophication Index for the systems within Three Bays shows significant nutrient over-enrichment resulting in poor environmental health within the upper estuary (Figure 8). However, North Bay, Eel River and Cotuit narrows also show the symptoms of nutrient enrichment, although their values still indicate good water quality. West Bay, lower Cotuit Bay and the Seapuit River currently show excellent nutrient related water quality. Note that the Nantucket Sound station also shows an excellent score, which is consistent with other habitat quality observations.
Although fecal coliforms are not indicators of habitat quality or environmental health, they do directly impact human use of the Bays resources. In addition, if the fecal coliforms originate from human sources they should be controlled as they present public health issues. The Monitoring Program collects fecal coliform samples from the surface waters of each station during each event. In addition, Massachusetts Division of Marine Fisheries has been collecting fecal coliform samples periodically as part of shellfish harvest regulations. Three Bays has obtained the DMF data (peers. comm. Neal Churchill) to provide a longer term view of the issue of bacterial contamination within Three Bays.
It is clear from the Monitoring Program data that the region of the Marstons Mills River mouth is a source of fecal coliforms to the estuary. While river samples did periodically show high bacterial levels (>200 colonies per 100 mL), the River mouth region typically yielded the highest counts (Figure 9). High values were occasionally observed within Prince Cove which appear to be supplemented by a source within the Cove, since the inner station typically showed higher levels than the mouth.
Within the Three Bays System, fecal coliform levels appear to be related to freshwater inputs, higher values at lower salinities (Figure 10). This is consistent with other embayment survey studies which indicated that inputs through surface water flows (particularly storm water) are enriched in fecal coliform bacteria, typically with a wildlife source. Tidal wetlands are also important sources of coliform bacteria. However, these sources do not appear to completely explain the monitoring results, since high coliforms were not always related to rain events or proximity to surface water inflows. This is supported by the DMF data which showed moderate frequencies of counts greater than 14 colonies per 100 mL (shellfish threshold) in areas without surface flows or obvious wildlife sources (Figure 11, Table 2). However, the DMF data does clearly confirm the importance of wildlife in that the one of the most frequent "exceedance" stations is adjacent Sampson's Island at the mouth of Cotuit Bay, but directly adjacent a wildlife area.
In general, the data support likely surface freshwater discharges and wildlife as fecal coliform sources. However, the data also suggest that some human related point sources may exist, primarily within Prince Cove and North Bay. These are being examined by a targeted study during the summer of 2000. Overall the level of fecal coliform contamination within the Three Bays Estuary is relatively low and with some source identification it could be improved.
SUMMARY AND RECOMMENDATIONS
Overall the Three Bays Estuary is showing relatively good nutrient related health compared to most of the embayments on the south shore of Cape Cod. However, there are some areas within the Estuary which are presently showing nutrient related water quality declines and there is a wide variation in habitat quality within the Three Bays System. In general the quality of habitat in Three Bays shifts from high quality near the inlets to Nantucket Sound to poor quality (eutrophic) in the inner reaches. At present, Prince Cove, and the region of the mouth of the Marstons Mill River through the narrows to North Bay are showing poor nutrient related environmental health. However, the loss of eel grass beds from most of the Three Bays proper indicates that the system has undergone nutrient loading related shifts. As there is only a single field season of monitoring data, refinement of the precise health of the various sub-systems awaits additional data collection. However, the effects of watershed nutrient inputs can be seen in diminished water transparency, increased chlorophyll and nitrogen levels in the upper system and North Bay. Sampling of benthic animals and high frequency sampling of chlorophyll and oxygen levels will help to better define the level of habitat health in these component systems. Specific findings and recommendations for the data collection in the coming field season are given below. It should be noted that the monitoring program and all proposed additional data collection are specifically to support the linked watershed-embayment quantitative assessment the direct target of which is the restoration and protection of the health of the Three Bays Estuary.
(1) The Three Bays Estuary is nitrogen limited, i.e. additions of nitrogen will increase algal production. Therefore, managing the nutrient related health of these marine waters requires management of nitrogen inputs from the watershed and removal by tidal exchanges to adjacent Nantucket Sound.
(2) The upper estuary from the mouth of the Marstons Mills River through the channel to North Bay and Princes Cove is currently showing poor nutrient related health. Additional high frequency sampling performed by SMAST during summer 2000 clearly supports that Princes Cove is currently being over fertilized (i.e. it is eutrophic).
(3) The Marstons Mills River appears to be a major "point source" of nitrogen loading to the Estuary. The River carries nitrogen gathered through surface and groundwater inflows to the Bay.
(4) The organic matter within the Three Bays Estuary appears to be produced by phytoplankton supported by inputs of watershed nitrogen and recycled nitrogen within the Bays, as opposed to entering the system in surface water flows.
(5) Fecal coliform levels are generally low within the lower estuary , but high (relative to shellfish harvest limits) within the upper estuary. However, examination of the long-term record of fecal coliform levels collected by DMF (1985-1998) indicates that levels over the shellfish limit occasionally occur in the near shore regions throughout most of the Three Bays System.
(6) Elevated fecal coliform levels are not always associated with rain events, suggesting potential direct discharges to the upper system from waterfowl, boats, direct wastewater discharges (in violation of health code). Additional sampling by Three Bays Preservation during the summer of 2000 and continuing until today is aimed at further elucidating the fecal coliform sources.
(7) Water quality monitoring should support water quality and quantity modeling of the effect of a new tidal flow between Grand and Little Islands. This effort would be part of the quantitative water quality study, but would rely (in part) on the monitoring data.
(8) Shellfish upwellers should be established within the Three Bays System to take advantage of high phytoplankton biomass, but avoid the periodic low bottom water dissolved oxygen. Upwellers should include measurements of nitrogen removals by shellfish.
(9) Deployment of moored instruments (like in Princes Cove in 2000) in embayment areas of special concern to better elucidate the frequency and duration of low oxygen conditions and phytoplankton blooms.
(10) Establishment of a freshwater inflow and nutrient load gauging station on the Marstons Mills River and possibly the Little River. The data provide essential data input to the linked watershed-embayment water quality models and are central to validating the freshwater balance of the system. A flow station was established during the summer 2000 as part of the Water Quality Monitoring Program, using a grant from the Cape Cod Commission for hardware.
(11) Nutrient related environmental stress is clearly evident within the upper portions of the Three Bays System and other areas have undergone declines from historic levels. Nitrogen loading to the Three Bays System is still continuing both through increasing watershed loading and the time lag between the initiation of a new nitrogen source and the arrival of the associated nitrogen in the Bay. In addition, bacterial contamination within the upper estuary is occurring. While it is clear that additional water quality monitoring is needed, a gradual implementation of the state-of-the-art linked watershed-embayment management study is warranted. This quantitative assessment and modeling of the Three Bays System is needed to develop realistic and efficient watershed management options focused on improving habitat quality. This estuarine restoration and management effort requires a solid foundation of nutrient-related water quality monitoring information (multi-year) from which to base the higher level modeling and management effort. However, some aspects of estuarine restoration and management planning have already begun, based upon the initial monitoring results (i.e. the gradual implementation has started and should continue to be fostered).
(12) Water quality monitoring is a long-term commitment required for proper management of coastal embayments. The data is initially used to guide higher level studies for restoration or protection efforts. Over time monitoring is used to gauge the effectiveness of implemented management practices and to support adaptive management of these systems' low bottom water dissolved oxygen. Upwellers should include measurements of nitrogen removals by shellfish. the Three Bays Preservation Water Quality Monitoring Program is on going and continues to collect data measuring the health of our watershed.
NITROGEN AND EUTROPHICATION OF COASTAL WATERS
Substantial changes in both marine and land based activities over the past several centuries, notably the increase in residential development within coastal watersheds, have significantly affected the health of our near shore coastal waters. Although some potentially detrimental activities such as over fishing can be identified with sufficient lead time to implement management strategies, the longer-term impacts of other types of activities such as those which result in nutrient overload operate on longer time scales with impacts being felt years to decades after the activity has taken place. These time lags, notably concerning nutrient loading from on-site disposal of septic wastes, makes it more difficult to implement sufficient management and remediation measures before evidence of detrimental impact becomes apparent. Toxic contamination (e.g. PCB's, pesticides, organic compounds, etc.) can present significant problems for coastal systems, however these tend to be localized and often easily identified. For Three Bays, the major threat to aquatic resources is primarily from increased nutrient inputs resulting from waste disposal practices that occur within coastal watersheds, along with the parallel increase in lawn fertilizer use and runoff from impermeable surfaces. The growth in residential development and increased tourism is generally identified as the cause for nutrient related water quality decline. Although some embayments are potentially under the threat of nutrient related impacts from historic and recent nutrient plumes (e.g. those emanating from the Falmouth Wastewater Treatment Facility and Massachusetts Military Reservation old WWTF), generally it appears to be the rapid growth in residential development during recent decades which is pushing the limits of nutrient assimilative capacities for most of these aquatic resources. In some cases, the problem is simply moved and concentrated from one place to another as is sometimes the case with centralized waste treatment facilities where waste from several watersheds is being relocated, treated and disposed of within a separate embayment watershed. Because of the long time scales often required for groundwater transported nutrients to travel from the point of origin to coastal embayments, it is particularly important to understand both the nutrient transport and transformation processes which occur and to have sufficient understanding of the receiving water body to enable quantitative prediction of the potential impacts, specifically the potential increase in eutrophication which may result from these additional inputs.
Periodic eutrophication (or overproduction) events occur when increased nutrient inputs stimulate the overproduction of algae and phytoplankton which, combined with night-time respiration activities and decomposition, can result in oxygen depletion in these water bodies. Coastal salt ponds, because of their large shoreline area and generally restricted circulation and flushing, are usually the first indicators of nutrient pollution along the coast, where lower rates of dilution and flushing are less effective in ameliorating the effects of additional inputs. In addition, stimulated growth of epiphytes on eelgrass (animals and plants which colonize on the leaf surfaces) as a result of increased nutrient loading can cause the decline of eelgrass beds, important in the production of bay scallops and other commercially valuable species. Because these systems by their nature are highly productive, nutrient rich environments they frequently provide suitable habitat for many species of commercially and recreationally valuable fish and shellfish. Additional nutrient loading beyond the ability of these systems to assimilate new nutrients, however, can upset the delicate balance of these systems resulting in over-fertilization and potentially detrimental eutrophication. Each system's assimilative capacity is specific and the potential for eutrophication determined through a delicate balance of inputs, losses, the ability of the system to mix vertically by wind driven mixing (more difficult in systems with deep basins) and the amount of nutrients and organic matter bound into sediments. Alteration of one or more of these factors, most notably changes to nutrient loading or alteration of system hydrology, can have significant impacts on nutrient related water quality in these systems.
Many of Cape Cod's embayments presently show some signs of nutrient over-enrichment. Some of the more nutrient enriched systems are on found adjacent Vineyard Sound (Little Pond to Popponesset Bay) where advanced eutrophication has occurred as seen by periodic dense algal blooms, malodorous conditions and occasional fish kills from low oxygen conditions resulting from nutrient related oxygen depletion in bottom waters. Although it is often difficult to separate the results of natural processes from those induced by man, increased nutrient conditions resulting from excessive loading due to human activities is certainly an important contributor to declining water quality in these sensitive coastal ecosystems. It is notable that while the population of these watersheds has been increasing steadily since colonial days, only recently have significant signs of eutrophication become apparent in the coastal ponds. One reason for this is that it is both the distribution and the total mass loading of nitrogen which determine the impact, related not to the rate of population increase but to the number of persons. Since there is no evidence that the "natural" sources of nitrogen have changed significantly over the past 350 years and since the assimilative capacity (the ability of the system to receive more nutrients without deleterious effects) has only recently been approached for many of these embayments, evaluation of sources will appropriately focus the "new" sources related to human activities, those most capable of being managed.
Nitrogen is a natural and essential part of all ecosystems, aquatic and terrestrial. For the coastal ponds, as for most temperate coastal systems, nitrogen is limiting to phytoplankton, algal and rooted plant productivity and therefore secondary production, especially shellfish. It would, therefore, seem that increasing nitrogen inputs would be a benefit to the system, increasing fisheries harvests. The apparent paradox stems from the fact that at low levels of nitrogen in coastal waters, increased loading does have a stimulatory effect upon secondary production (e.g.. fish and shellfish); at higher levels increased yields may still be achieved but changes in community structure begin to occur (e.g. phytoplankton species, benthic animal species and impacts to eelgrass habitats). At higher loadings, however, the increased oxygen demand in the water column and sediments from the increased plant production exceeds the rate of oxygen input from photosynthesis and by mixing from the atmosphere, and lowered oxygen concentrations can occur (hypoxia, anoxia). It is the stress associated with low oxygen concentrations which has the most deleterious effects upon plant and animal communities which at higher frequencies and durations of occurrence result in the loss of stable populations and their replacement with opportunistic species. Although eutrophication can occur as the result of natural processes (as is primarily the case with Oyster Pond, Falmouth and its deep basins), cultural eutrophication, or that resulting from human activity, represents the greatest potential long-term threat to coastal water quality.
Sources of nutrient pollution into coastal waters are generally classified into two types: point sources, which tend to be discrete and easily quantifiable; and non-point sources, those which are more widespread, more difficult to measure and generally reach coastal waters through groundwater transport. Point sources have historically been regulated and quantified while non-point sources are a recent area of research and have a larger error associated with their estimates. The difficulty with managing nitrogen loading is its widespread distribution from a wide array of sources. This is especially true for nutrients originating from non-point sources, such as nitrogen transported in the groundwater from on-site septic treatment systems, lawn and agricultural fertilizers, and animal farming. Regardless of the original form of the nitrogen, the form of almost all nitrogen in groundwater is nitrate. For example, while both organic and inorganic nitrogen enter septic systems, as a result of degradation and anaerobic conditions within tanks almost all of the nitrogen released is as ammonium. Even at the very high resulting concentrations, the ammonium is rapidly oxidized by bacteria (nitrification) to nitrate generally after a few meters of infiltration. Once the nitrate reaches the groundwater it is transported nearly conservatively (or unaltered) to coastal waters. At the sediment/water interface at the bottom of a salt pond or harbor, the nitrate either passes up into the harbor (where it is available for plant uptake), or may be "detoxified" by a natural community of denitrifying bacteria which release the nitrogen as harmless nitrogen gas. How nitrate input is partitioned between these processes determines its effect on the biological activity and environmental health of a receiving water body.
Once nitrogen compounds enter the water column of coastal water bodies, the extent of their impact is determined by the rate at which they are lost through tidal exchange or burial in the sediments. Readily available nitrogen (nitrate or ammonia) can be taken up by algae and phytoplankton. These plants may fall to the bottom upon dying, or may be eaten and "processed" digestively by zooplankton (microscopic animals), fish or shellfish. Subsequent microbial activity in the sediments can re-release the nitrogen bound in such decaying organic matter to the overlying water column, where it once again becomes available as a nutrient for plant growth. Thus the harbor sediments act as sort of a "storage battery", continuing to provide a source of nitrogen for biological production even though the original inputs may have diminished or ceased.
The number of times the nitrogen cycles between sediments and the water column, before being flushed out to the ocean or buried permanently in the sediments, is directly related to the potential for eutrophication. Each cycle magnifies the impacts of a one-time input. Since sediments store large amounts of nitrogen, the extent of recycling determines how long nutrient-related problems persist after the original sources from groundwater or surface runoff from land are stopped. Evidence for this magnification of impact and the significance of biological transformations which occur in these systems is represented from observed changes in the dominant form of nitrogen which occurs in different segments of the ponds. In the upper reaches of the bays, readily available dissolved forms of nitrogen such as nitrate and ammonium dominate, however moving down the bays toward Vineyard Sound the dominant nitrogen species shifts toward the particulate form, reflecting transformation and uptake by phytoplankton in the water column as the nitrogen is transported toward open coastal waters. This uptake and transformation is relatively rapid and significant inorganic concentrations are generally only observed in the very headwaters of the ponds. Separate benthic flux measurements show that a portion of the nitrogen, the particulate form, which has fallen to the sediments does indeed become re-released as inorganic nitrogen from the sediments, providing once again a readily available source of nitrogen for plant production in the water column. The significance of this finding revolves around the fact that with each round of particulate-dissolved transformation which occurs in the sediments, oxygen is consumed. In addition, with each new bloom of phytoplankton, night-time respiration by these plants increases the demand for oxygen in the water column when light is not available for photosynthesis. It appears from the data that nitrogen is actively transformed and recycled within the ponds as it moves from headwaters until it is eventually flushed out of the pond and therefore the one time input of nitrogen can impact the system many times until it is eventually lost to open coastal waters.
The subsequent deterioration of coastal waters therefore is not directly the result of nutrient loading, but rather a secondary effect of the resulting overproduction of phytoplankton and submerged aquatic plants. Because high nutrient levels are frequently associated with depletion of oxygen, potentially to the point of limiting or prohibiting survival of benthic in fauna, shellfish and fish, it is oxygen depletion that is directly responsible for most of the detrimental effects of excessive nutrient loading in coastal ecosystems. Through these efforts we hope to develop a data base of sufficient quality and duration to enable comparison of nutrient and oxygen conditions throughout the system on time scales relevant to potential changes in development related inputs.
WATERSHED AND ESTUARY CHARACTERISTICS
Management of coastal systems requires not only an understanding of both present conditions, but also of the history of physical and environmental alteration. In addition, within degraded or partially degraded systems, an evaluation of the system's "maximum level of sustainable environmental health" is also needed. Although some of these evaluations are not complete at present, it is clear that there has been significant alteration of the Three Bays hydrologic and biological systems over the past several centuries since the early days of the mills along Goodspeeds River. What follows is a brief description of the Three Bays system focusing on major upland or embayment alterations relating to present system health.
While the nutrient related health of today's Three Bays System is very much linked to changes wrought by human activities, it is the physical structure of the system laid down by the retreat of the Laurentide Ice Sheet about 15,000 years ago that still controls much of the Bays' tolerance to nutrient inputs. The physical structure, shape and depth of a coastal embayment plays a major role in its susceptibility to ecological impacts from nutrient loading. Physical structure (geomorphology), which includes embayment bathymetry, inlet configuration and saltwater reaches, when coupled with the tidal range of the adjacent open waters, determines the system's rate of flushing. System flushing rate is generally the primary factor for removing nutrients from active cycling within coastal bays and harbors like Three Bays. As a result maximizing system flushing is one of the standard approaches for controlling the nutrient related health of coastal embayments.
As the Cape Cod Bay and Buzzards Bay Lobes of the Ice Sheet retreated, the sandy outwash plain that now holds the Three Bays watershed was formed. This sandy outwash produced the highly permeable soils found throughout upper Cape Cod. It is the permeability of the soils which has resulted in the importance of groundwater flow as a major pathway for nutrient transfers from watersheds to adjacent coastal waters in this region. The presence of both groundwater and surface water pathways for input of nutrients into the present estuary has significant impact on its response to changing nutrient loadings with the surrounding watershed from changing land-uses.
As sea level rose and flooded the present basins of the Three Bays, salt marshes began to form and an estuarine ecosystem began to function. At present it is not clear to what extent the embayments basins were formed from flooding kettle ponds versus merely flooding erosional valleys. However, given the shape and depth of the basins compared to other non-kettle systems on the southern shore of the upper Cape, it seems likely that kettle ponds with freshwater stream inflows and outflow were incorporated. With further sea-level rise the present marine beach deposits of Dead Neck, Sampson's Island, Bluff Point, northern tip of Grand Island and near the bridge to Little Island began to develop. The result was a complex estuarine system with a single inlet to Nantucket Sound through Cotuit Bay and major freshwater inputs through the Marstons Mills River and to a lesser extent, Little River.
Based upon studies from other regions of Cape Cod, it is likely that Native Americans utilized the resources of the Three Bays System for several thousand years before ceding the region to Captain Miles Standish in 1648. Native Americans likely used both the upland and estuarine resources. The marine food sources of the system would provide both shellfish (scallops, oysters and quahogs) and fish, particularly herring. According to James Otis, the name Mystic was the Native American term for small streams and ponds, particularly where herring and trout abounded. The largest lake within the watershed is still called Mystic Lake reputedly from this early term.
When Roger Goodspeed, the first European to settle within the Three Bay watershed, settled by the Marstons Mills River (for awhile named Goodspeeds River) in 1653, Three Bays was different from the present system in both its circulation and water quality. The upland was largely forested with some open lands, the Marstons Mills River was free flowing (no dams) and had more extensive freshwater marshes within its lower reaches and the embayment was connected to Nantucket Sound via a single inlet. While this single inlet almost certainly reduced the tidal exchange with high quality Nantucket Sound waters, the very much lower terrestrial input of nutrients suggests a high quality estuarine system. However, it is also likely that, similar to today, within the region of the mouth of the Marstons Mills River and associated salt marshes the sediments and bay waters were among the most nutrient and organic matter rich within the Three Bays System.
However, the aquatic and upland components of this System began to change rapidly. By 1689 a fulling mill was constructed on the Marstons Mills River. In 1704-5 the dam was constructed thus altering the pathway of surface water nutrient transport to the estuary. Town records indicating the leasing of herring rights and the requirement that all mills maintain fish ways is testament to the magnitude of the herring population supported by the system.
During the 1800's utilization of the estuary and its watershed continued to increase. Regions of the watershed were cleared for agricultural land and the Grist Mill at Marstons Mills continued operations past 1842. Land clearing was accelerated by the development of salt works on the shores of the Bay which used fire to fuel evaporation for salt production. This activity peaked in 1812 and then declined. Direct use of marine resources focused on oyster production, where oysters were initially pickled and shipped in barrels to market. In these earlier centuries, as today, oysters were cultured on the Bays' bottom. One of the first growers, Captain George Fisher who was granted a large section of Cotuit Bay, shipped oysters to widespread U.S. markets. With the demand for oysters, the natural beds surrounding Grand Island became depleted and spat were imported from Long Island for grow out. However, at least for awhile, seed could be collected at the mouth of nearby Popponesset Bay on deployed scallop shells to supply the grow out needs. During this period, scallops were harvested within Three Bays in quantity and even at the turn of the century scallops represented a major economic resource. This record of substantial scallop harvest indicates that eelgrass beds were likely prevalent throughout the Bays. This suggests that the water was clearer (greater transparency due to less phytoplankton), hence less nutrient loading from the watershed was occurring. During this period the population was still small, for example there were only 36 homes in Santuit, Little River and Cotuit combined. Throughout the 1800's the residents relied heavily on their coastal resources as salt making, oyster production, fishing, farming, ship-building and coastal trading were the dominant activities.
By far the greatest changes to the Three Bays watershed and estuary have occurred during the 1900's. The most obvious change has been the dramatic shift in land-use to residential housing during the last half of the 1900's. With this shift and the advent of fertilized lawns, has been a dramatic increase in the amount of plant nutrients (primarily nitrogen) which enter the Bays. It is this recent nutrient load which is responsible for incipient declines in environmental health of portions of the estuary. However, there were likely pulses of nutrients to the system during the 1940's associated with the military training areas within the Bays. The barracks, warehouses and storage tanks would result in a "new" source of nitrogen loading and the paving of the beach from Baxter's Neck to Point Isabella may have also increased bacterial contamination in the adjacent waters.
In the early 1900's there was another major change to the Three Bays Estuarine System. Until this time, tidal exchange with Nantucket Sound was restricted to a single inlet to Cotuit Bay. However, a second inlet was opened which likely increased the flushing of West Bay, which previously had exchanged via the Seapuit River and through North Bay. Regardless of the extent to which this second inlet increased the flushing out of nutrient rich estuarine waters, it will have helped to buffer the Bays against the coming nutrient increases in the latter part of the century. Recent efforts to maintain the Bays for navigation may have also helped to maintain tidal exchanges, but the extent that this may have helped lessen the effects of increased watershed loadings has not been determined. Dredging of the Narrows from Princes Cove to North Bay in 1957 and the inlet to Cotuit Bay (most recently in the late 1990's) are two of the more notable examples of recent efforts. Unfortunately while declines in environmental health of the Three Bays System will be reduced by maximizing tidal exchange with the high quality waters of Nantucket Sound, the growing watershed nutrient loading and the structure of the system will require watershed management to restore the Bays to their former level of environmental quality. Watershed management will almost certainly involve reduction of nitrogen inputs at their various sources and possibly the removal of large loads (e.g. wastewater) from the watershed. Watershed management targeted at embayment restoration will usher in a new phase in the ever changing Three Bays System. While it is unlikely that we can restore the Bays to allow a single scalloper to dredge 80 bushels a week as was the case in 1899, a partial restoration of estuarine habitat should be achievable. Since Three Bays is not highly nutrient overloaded and since the habitat decline is relatively recent, partial restoration should be achievable over decadal time-scales and on a level that should benefit both present and future generations.
FIGURES AND TABLES
Click on an image for a larger view.
Figure 1A. Saltwater stations sampled by the Three Bays Preservation Monitoring Program throughout 1999. Sampling is within the basins from small boats.
Figure 1B. Freshwater stations sampled by the Three Bays Preservation Monitoring Program and the Barnstable Land Trust during 1999. Sampling is from along shore in the River and from small boats in the ponds.
Figure 2. Rainfall measured at the Long Pond Falmouth Gauging Station during 1999. Arrows represent sampling dates in the Three Bays System.
Figure 3. Depth of light penetration (Secchi Depth) and proportion of water column above Secchi Depth. Stations with values >70% may support bottom plants. Diamonds = % above Secchi Depth; Solid boxes = Secchi Disk Depth in meters.
Graph Label Abbreviations
MR-B = Marstons Mills River, Station B
MR-C thru MR-4 = Marstons Mills River, Station C thru Station 4
PC2 and PC3 = Prince Cove, Station 2 and Station 3
NB5 thru NB7 = North Bay, Station 5 thru Station 7
WB8 and WB9 = West Bay, Station 8 and Station 9
ER10 = Eel River, Station 10
CB12 and CB13 = Cotuit Bay, Station 12 and Station 13
SR11 = Seapuit River, Station 11
NS14 = Nantucket Sound, Station 14
Figure 4. Average nitrogen concentration by nitrogen species and as total nitrogen for each monitoring station over the study period, May - November. Note that the estuary is dominated by organic nitrogen forms, with little inorganic nitrogen. In contrast, the freshwaters have high inorganic nitrogen due to watershed inputs. Horizontal line indicates 0.35 mg N/L.
Figure 5. (Top) Inorganic nitrogen versus phosphorus ratio (N/P) as the geometric mean and maximum and minimum values from May - November 1999. Horizontal bar represents the Redfield Ratio, where values above the bar suggest plant growth limited by phosphorus availability and below the bar growth limited by inorganic nitrogen availability. (Bottom) Inorganic nitrogen versus phosphorus ratios for each station on each of the 7 samplings in 1999.
Figure 6. Relationship of phytoplankton chlorophyll a to water column particulate organic carbon concentration throughout the fresh and salt water systems. The direct relationship within the estuary suggests that the predominant source of organic matter is from phytoplankton production as opposed to import via runoff from the watershed. Stations MR-D and MR-E are within the Marstons Mills River System (cf. Figure 1B).
Figure 7. Minimum bottom water dissolved oxygen concentrations sampled during the 1999 field season. Although all values are >4 mg/L, the levels in Prince Cove support the contention that this system periodically has low DO. and is showing nutrient related water quality decline. Continuous DO. measurements are to be undertaken during the 2000 monitoring season.
Figure 8. Eutrophication Index calculated for each monitoring station within the Three Bays System based upon average water quality conditions, May - November 1999. The Index is based upon chlorophyll a, dissolved inorganic nitrogen, total organic nitrogen, Secchi depth, and the 20% lowest dissolved oxygen values for each station. The Index has been calibrated using Buzzards Bay and Falmouth Pond Watch long-term estuarine monitoring data.
Figure 9. Fecal coliform densities within the upper estuary and Middle Pond and the Marstons Mills River in 1999. Numbers above the bars indicate the amount of rainfall (inches) during the 5 days prior to sampling. Note that all high fecal coliform values are not associated with rain events.
Figure 10. (Top) Percent of samples having fecal coliform levels which exceed the limit for shellfish harvest, June - September, versus average station salinity. (Bottom) Geometric mean and maximum fecal coliform densities and average station salinity, June - November 1999.
Figure 11. Summary of Fecal Coliform data collected by Massachusetts Division of Marine Fisheries, 1985 - 1998. Station ID's are as in column 2 of Table 2. The values above the bars indicate the total number of samples collected. Note that significant numbers of samples were found to contain fecal coliform levels >14 FC/100 mL in the Marstons Mills River Mouth (MMR), Prince Cove (PC), North Bay Upper (NB1, NB2) and Mid (NB4, NB5), the Marina & Marsh in West Bay (LIM), the Little River and boat landing in Cotuit Bay (LR, BL), and the bird area associated with Sampson's Island at the western end of the Seapuit River.
Table 1. Physical Characteristics of Component Aquatic Systems of the Three Bays Complex.
Table 2. Fecal Coliform data collected by Massachusetts Division of Marine Fisheries, 1985 - 1998. Sampling stations have been grouped into functional spacial units. Shellfishing limit is geometric mean of 15 sets of 14 FC per 100 mL or 10% samples >29 FC per 100 mL. Beach limit is 200 FC per 100 mL, DMF assay typically cannot determine over 50-64 FC/100 mL or <1.4-2 FC/100 mL.