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.