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Determining the nitrogen target to restoring these habitats is the focus of the nitrogen <br /> management threshold analysis provided in Section VIII. <br /> The measured levels of oxygen depletion and enhanced chlorophyll-a levels follows the <br /> spatial pattern of total nitrogen levels in this system (Section VI), and the parallel variation in <br /> these water quality parameters is consistent with watershed based nitrogen enrichment. The <br /> spatial pattern indicated that the magnitude of oxygen depletion, enhancement of chlorophyll-a <br /> levels and total nitrogen concentrations increased from the offshore waters to the main basin of <br /> Waquoit Bay and were highest within the inner portions of the subembayments. <br /> The level of oxygen depletion and the magnitude of daily oxygen excursion and <br /> chlorophyll-a levels within the main Basin (north and south) of Waquoit Bay indicate high levels <br /> of nutrient enrichment and impaired habitat quality. The oxygen data are consistent with high <br /> organic matter loads and the moderate levels of phytoplankton biomass (chlorophyll-a levels) <br /> indicative of nitrogen enrichment of this estuarine basin. The large daily excursions in oxygen <br /> concentration in the main basin of Waquoit Bay combined with the clear evidence of oxygen <br /> levels above atmospheric equilibration throughout Waquoit Bay and its sub-embayments is <br /> further evidence of nitrogen enrichment at a level consistent with habitat degradation. Oxygen <br /> conditions within the northern portion of the main basin of Waquoit Bay showed low to moderate <br /> levels of oxygen depletion consistent with the organically enriched sediments, moderate levels <br /> of phytoplankton biomass and generally low macroalgal accumulations associated with its <br /> observed level of nitrogen enrichment. The southern portion of the main basin of Waquoit Bay <br /> is also showing moderate (to high under algae mat) oxygen stress to benthic communities, with <br /> a gradient of less oxygen depletion moving toward the tidal inlet. The bottom waters have large <br /> daily excursions in oxygen levels (5-7 mg L-'), more pronounced than in the northern portion. <br /> Large daily excursions in oxygen levels are a clear indication of organic enrichment resulting <br /> from nitrogen loading and, within the main basin, manifests itself through organic enrichment of <br /> sediments, large macroalgal accumulations and phytoplankton biomass. Chlorophyll-a levels <br /> paralleled the oxygen levels within the southern portion of Waquoit Bay. The mid region <br /> generally shows only moderately enhanced water column chlorophyll, averaging 7 ug L"' and <br /> exceeded 5 and 10 ug L-' for 89% and 6% of the time-series record, respectively. Slightly lower <br /> levels were found near the inlet, with chlorophyll values averaging 5.4 ug L-' and exceeded 5 <br /> and 10 ug L"' 56% and 2% of the time-series record, respectively. It should be noted that <br /> conditions at the"inlet" location are the highest quality within the main basin of Waquoit Bay. <br /> The western sub-embayments to the Waquoit Bay Embayment System, Eel Pond and <br /> Childs River, exhibit significant summer time oxygen depletion. The upper reaches within Eel <br /> Pond and the main channel of the Childs River have significant and frequent oxygen depletion <br /> of bottom waters, while the basin of Eel Pond adjacent the tidal inlet shows only moderate levels <br /> of oxygen depletion, due to the direct influence of the high quality floodwaters from Vineyard <br /> Sound. The lower basin is strongly influenced by the nutrient and organic enriched low oxygen <br /> waters entering from the upper tidal reaches during out-flowing ebb tides. However, the high <br /> turnover of water in lower Eel Pond reduces its ability to build up nutrients, phytoplankton <br /> biomass and organic matter, while the inflow of high quality floodwaters from Vineyard Sound <br /> results in relatively high water quality for a portion of the flood tide period. The upper portions of <br /> the western branch of Eel Pond and the Childs River are clearly presenting significant oxygen <br /> stress to benthic animals, while the lower Eel Pond basin presently has a lower level of oxygen <br /> stress. The spatial pattern of oxygen stress parallels chlorophyll-a, indicative of underlying <br /> nitrogen enrichment as the ultimate cause of the extent of oxygen depletion. Within the upper <br /> portion of Eel Pond and the Childs River, where significant oxygen depletion was observed, <br /> chlorophyll-a levels were very high over the entire study period. In addition, both the upper <br /> Executive Summary 6 <br />