11/13/2017 BOARD OF SELECTMEN Minutes

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are in effect in these eutrophic waters. Popponesset Bay showed a parallel pattern to Waquoit Bay,with higher chlorophyll-a and lower D,O. in 2016 compared to the 2013-2015 average as well as the long-term average in most of its basins. The causes of reduced DO levels appear to be . the same in both estuaries: the linkage of nitrogen enrichment to organic matter load (chlorophyll-a) to low D.O in bottom waters. Again the observed inter-annual differences can be enhanced by meteorological conditions of low light and wind resulting in less oxygen input through photosynthesis and vertical mixing of oxygen rich surface waters to the bottom, yielding greater depletion between years even at the same level of nitrogen enrichment. This phenomenon has been quantified for.many estuaries throughout the region and is likely tare for estuaries in general. However, if the observed levels in 2016 persist over the next few seasons, it will indicate a further decline in nutrient related health particularly in the Mashpce River and the Quashnet River. While it is not currently known what caused the large prolonged phytoplankton blooms in summer 2016, it may be associated with lower freshwater inflows due to the below average precipitation in 2016. It is likely that results from 2017 will help evaluate this effect. Note that even if conditions improve in 2017,it is the nutrient and organic enrichment of these estuarine basins which keep them poised for low oxygen when other external factors become operative (storms, low light, wind,temperature, etc). NUTRIENT RELATED WATER QUALITY INDEX Integrating the various nutrient related parameters collected as part of the monitoring program in order to provide a simple view of the general nutrient related water quality of the Waquoit Bay and Popponesset Bay Estuarine Systems, the Bay Health Index was constructed based on the monitoring results(long-term, 2010-2012,2013-2014 and 2015-2016). The Bay Health Index was developed for Buzzards Bay embayments based upon levels of nitrogen (inorganic and organic), total chlorophyll-a, bottom water oxygen and the depth of light penetration(Secchi depth). While the index does not provide a quantitative assessment of habitat health and is not suitable for salt marsh dominated estuaries or freshwater, it does give a useful picture of the general level of estuarine water quality and spatial gradients within estuaries. Since clear temporal trends have not been observed in the time-series data (Tables 4 and 5), the average summer conditions throughout the Waquoit Bay and Popponesset Bay Systems were used to parameterize the Index. The Scores for each parameter were calculated and the average score for each station (across the 5 parameters) calculated (Tables 6 and 7). These scores are then compared to a guide of"acceptable"ranges for each parameter to rank the station (Table 8). It should be understood that the resulting Index and the designation of acceptable ranges for each parameter are approximate and provide less certainty than site-specific analysis which include habitat assessments (e.g. eelgrass,benthic infauna). However,the Index does provide a convenient tool for comparing regions within an estuary and between estuaries. It is clear that there are strong gradients in nutrient related water-quality within each estuary which continued through 2016 and that they are similar to the long-term pattern (Figures 14, 16). The overall pattern is similar to that of the separate parameters used in calculating the Index. Both embayinent systems show poor nutrient related water quality within the tidal rivers and tributary basins, with modest improvement within the main basins and with the only regions showing moderate to high quality being adjacent to the tidal.inlets to Nantucket Sound. The region adjacent the tidal inlet is typically the last reach of an estuary to show impairments from watershed nitrogen loading, since this area is being swept with the high quality waters of . 19