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.