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Massachusetts Estuary Project(MEP) <br /> Linked Watershed EnrbaymentModel Peer Review <br /> these sensitivity analyses were not extended to include either the Pleasant Bay or Bourns Pond <br /> applications. _ <br /> There are important differences among these three systems and the Panel recornmends that the SMAST <br /> Team conduct sensitivity analyses for each specific estuary. For example,the benthic flux loads for Great <br /> Pond,Pleasant Bay and Bourns Pond are responsible for 1,46 and 72 percent,respectively,of the total <br /> nitrogen loads to the water column. The benthic flux load for Great Pond is actually slightly negative, <br /> indicating that the sediments are a net sink and not a net source for nitrogen. The atmospheric deposition <br /> loads for Great Pond,Pleasant Bay and Bourns Pond are responsible for 12,22 and 4 percent,' <br /> respectively, of the total nitrogen loads to the water column. The sensitivity analyses for benthic fluxes <br /> should be designed to encompass not only calibration uncertainties,but also uncertainties due to the <br /> simplifying assumption of a linear relationship between reductions in nitrogen loads from the watershed <br /> and net benthic nitrogen loads from the sediments. <br /> Key Issue 5=Mass Balance Analyses <br /> In addition to the mass conservation checks discussed above in Section 5.2.2.1,the Panel recormnends <br /> that the SMAST;Team consider using output from the calibrated water quality models to conduct mass <br /> balance analyses for the whole Pleasant Bay and Bournes Pond systems,and the principal embayments in <br /> each system. These analyses should include all of the individual nitrogen mass flux components into and <br /> out of the water column. Results from such analyses would provide useful diagnostic information on the <br /> relative contributions of individual nitrogen sources to the most impacted embayments in each system, <br /> and would better inforin management decisions on how to phase wastewater and nutrient management <br /> plans. <br /> 5.3. Data-Based Issues <br /> Key Issue 1-Use of Eelgrass as a Bio-Indicator of Embayment Health <br /> Eelgrass(Zostera marina)is a critical and vital bio-physical resource in the coastal embayments and near <br /> shore waters of Massachusetts (Costello and Kemvorthy,2011). The high rates of primary productivity <br /> and widespread distribution of eelgrass provide essential habitat and food for many species of water fowl <br /> and commercially and recreationally important fish and shellfish resources, as well as many valuable bio- <br /> physical ecosystem services including sediment stabilization,shoreline erosion,nutrient cycling and <br /> nutrient storage(Moore and Short,2006). There are limits to the degree to which eelgrass can act as an <br /> effective nutrient reservoir. These limits have been reached in many coastal ecosystems worldwide where <br /> nutrient enrichment and eutrophication have led to significant impairment and eelgrass declines(National <br /> Research Council,2000, Orth et at. 2006a,Waycott et al.2009,van der Heide et al. 2011).These and <br /> other studies have led to a general scientific consensus that eelgrass is a sensitive bio-indicator of <br /> ecosystem impairment and can be used to identify thresholds of nutrient concentrations for establishing <br /> water quality criteria that will support eelgrass (Dennison et al. 1993,Lee et al. 2004,Kemp et al. 2004, <br /> Biber et al. 2008). In Massachusetts estuaries nitrogen is documented to be the primary driving factor <br /> for eutrophication;this has been confirmed by many peer reviewed scientific studies and in many <br /> different eelgrass systems(e.g., Short et al. 1995,Valiela et al. 1997a, 1997b,Havens et al. 2001, Orth et <br /> al.2006b,Krause-Jensen et al. 2008,Waycott et al.2009). <br /> The strength in the MEP approach is that it uses using eelgrass as one of the key response indicators. <br /> This approach assumes that eelgrass declines result from nitrogen emichinent, an assumption that is well <br /> supported by.empirical studies. MEP further assumes that nitrogen remediation will lead to <br /> enviromnental conditions that support eelgrass recovery in embayments where groundwater nitrogen <br /> sources are reduced so that the concentration in the embayment achieves a threshold value that will <br /> support eelgrass recovery. The expectation that eelgrass will recover following nitrogen remediation is <br /> supported by observations in similar coastal embayment systems(Vaudry et al. 2010). It is very <br /> December 30,2011 20 <br />