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Bay Health Index The Bay Health Index measures the nutrient-related health of each of the Bay's major harbors and coves. It does not include bacteria monitoring and is not an index of swimmability or shellfish bed status. The index is calulated from the scores of mean summertime water clarity, phytoplankton pigments, organic nitrogen, inorganic nitrogen, and the lowest 20% of dissolved oxygen concentrations. Central Buzzards Bay—which exhibits excellent water quality—would score close to 100 percent on the Health Index. The index provides a simple mechanism for the comparison of sites within and between embayments and allows for a "bay at a glance" picture of conditions throughout Buzzards Bay. The Program's primary parameters, nitrogen (inorganic and organic), chlorophyll a pigments, oxygen, and light penetration (Secchi Depth) allow for the production of a single "health index". While the index is only an approximate gauge of the health of an embayment, in application to specific embayments it does appear to agree with other health indicators, such as eelgrass distribution or organic rich sediments.
The health index is based upon independent water quality parameters which are directly related to the level of nutrient related health or level of nitrogen fertilization (eutrophication) of an embayment. The index includes the plant nutrient, nitrogen, as its availability generally limits plant production within the Bay. Total nitrogen is divided into inorganic and organic forms for the calculations. Inorganic nitrogen is the predominant form of nitrogen (nitrate, nitrite and ammonium) which enters the Bay from the watershed and stimulates the growth of phytoplankton and other plants. In the upper regions of embayments or in highly eutrophic embayments, inorganic nitrogen levels can be high. When the inorganic nitrogen is incorporated by plants it is transformed into organic nitrogen which is found within Bay waters in particulate (within cells or plant and animal matter) and dissolved (lost from plants and animals by excretion, leaching, or during decay) forms. As nitrogen loading to an embayment increases, so does the quantity of organic nitrogen found within the embayment's waters. Under eutrophic conditions high levels of inorganic nitrogen can typically be found within the headwaters of inlets with high levels of organic nitrogen throughout most of the system. Since organic nitrogen can also enter embayment waters from runoff or resuspension of bottom sediments or in dissolved forms from the watershed, we use the plant pigments chlorophyll a and pheophytin a as a gauge of the organic matter produced within the embayment waters. These pigments are found within phytoplankton and other plants within the Bay and are not typically entering the Bay from the surrounding watershed. This allows an estimate of the response of the algae within the Bay to the inorganic nitrogen entering from the watershed. Dissolved oxygen is an important indicator of the tolerance level of an embayment to the level of nitrogen it is receiving and phytoplankton (chlorophyll) it is growing. Dissolved oxygen concentration represents the balance between inputs from photosynthesis and from the overlying atmosphere and outputs due to respiration of animal and plant communities and decaying organic matter. When Bay waters show low oxygen levels, it clearly indicates a disruption of the balance due to an overabundance of respiration and decay relative to the amount of oxygen input that the system receives. In addition, low oxygen levels are themselves directly stressful to animal and plant communities. In order to account for changes in measured oxygen levels resulting from changes in water temperature and salinity (which control the level of oxygen in water in balance with the atmosphere, with no other inputs or outputs), oxygen concentrations (mg/L) are converted to percent of air saturation. Percent saturation (%Sat) values less than 100% indicate that the waters have more oxygen consumption than supply and greater than 100% indicate excess supply, usually due to high levels of phytoplankton photosynthesis. However, since it is the degree of low oxygen conditions that control ecosystem health and since these may occur only periodically, use of the average of all oxygen measurements would tend to obscure the level of stress. For this reason, only the lowest 20% of the summer oxygen measurements are used within the index. This is a shift from 33% used previously (Baywatchers Report 1996). However, this appears to increase the sensitivity and accuracy of the index. In some embayments dissolved oxygen was measured in the surface water, rather than just above the sediment surface. In these systems the oxygen scores would represent "best case" conditions. However in the shallow estuaries monitored, we have found that surface and bottom waters generally do not show significant differences in dissolved oxygen level; when oxygen levels decline, they decline throughout the watercolumn.
The final index parameter, light penetration, relates to the ability of waters to support bottom plants (eelgrass and macroalgae) but is primarily another measure of embayment response. Light penetration within the embayments to Buzzards Bay is primarily controlled by the amount of phytoplankton within the water, although localized input of other types of particles or high levels of humic acids can also affect this parameter. Typically, the higher the level of eutrophication, the less light penetrates into the watercolumn. The measure of light penetration is based upon Secchi depth. To generate the Health Index Score from the five base parameters, the summer averages were calculated. The average value for each parameter was then given a "health score" ranging from 0-100, where 100 is excellent and 0 is eutrophic (unhealthy). The excellent conditions parallel environmental conditions of healthy eelgrass beds, diverse and productive animal and plant communities, clear waters, high oxygen levels; in contrast the eutrophic conditions are where eelgrass beds have been lost, bottom animal and plant communities are depauperate, there is periodic low dissolved oxygen and occasionally even fish kills. Scores between 100-0 indicate conditions of intermediate environmental health. These values are based upon observations in Buzzards Bay and other regional embayments. Other regions may have to adjust the index to meet site specific conditions. The upper and lower levels of each parameter and the resulting index score are shown in the previous table. The value for each parameter contributing to an embayment's health score, is calculated using a non-linear (natural logarithim) relationship of the measured parameter to the "working" range for that parameter.
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