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Massachusetts Estuary Project(MEP)
<br /> Linked Watershed Emba ment Model Peer Review
<br /> important to understand that the MEP models do not directly predict eelgrass recovery or any measure of
<br /> uncertainty in eelgrass recovery. The models only predict the concentrations of nitrogen in the water
<br /> column. Application of the models to eelgrass recovery assumes that the thresholds determined by the
<br /> MEP process Iare supportive for eelgrass. Below,the Panel discusses both the strengths and weaknesses
<br /> associated with this process of model application and nitrogen thresholds.
<br /> Key Issue 2-Sentinel Station Protocol by which the Absolute Nitrogen Threshold is Determined
<br /> The protocol MEP uses to establish the nitrogen threshold for eelgrass in an embayment is based on
<br /> developing a direct correspondence between the nitrogen concentrations(either total or bioactive,
<br /> depending on the embayment)and eelgrass condition at one or more sentinel stations. Using this
<br /> approach it is assumed that: 1)the status of eelgrass at the sentinel station represents a"healthy
<br /> functioning eelgrass condition,2)eelgrass is in an equilibrium state at the sen i e, station;and 3)the
<br /> nitrogen concentration'observed at that station is the most significant factor,affecting eelgrass status. The
<br /> underlying strength to this approach is that it is based on data;there are direct measurements of nitrogen
<br /> concentrations and observations of the correspondence of nitrogen with-eelgrass condition. Additional
<br /> confidence in this approach- gained by studies in eelgrass systems#hat corroborate a'similar relationship
<br /> in the correspondence between nitrogen and eelgrass condition:- Further support for this approach is
<br /> attained by using more than one data set to evaluate the status of eelgrass condition,including MEP's
<br /> evaluation and interpretation of eelgrass mapping data provided by the MDEP and field surveys
<br /> conducted by the MEP team.
<br /> Uncertainties arise from potential weaknesses in the basic assumptions of the correspondence approach.
<br /> Scientific consensus indicates that a healthy eelgrass ecosystem has a very'significant influence on the
<br /> local environment with many positive feedbacks that affect;biophysical processes and support eelgrass
<br /> growth and survival,including•parameters such as nutrient cycling and storage,sediment stabilization,
<br /> optical water quality and biological oxygen demand(Moore,2004,Moore et al. 1996,Havens et al.2001,
<br /> Homer and Bondgaard,2001,Carr et al.2010,van der Heide 2011). If it is assumed that the sentinel
<br /> station is a healthy functioning eelgrass system,the environmental conditions at that station would be
<br /> affected by the presence of eelgrass and the nitrogen concentrations at the sentinel site will reflect this. In
<br /> the transition from impaired to unimpaired state where eelgrass is sparse or absent,the positive feedbacks
<br /> will not be functioning and nitrogen delivered to the system will be available for other primary'producers
<br /> .to utilize. Empirical and modeling-studies demonstrate'that eutrophication in southeastern Massachusetts
<br /> is derived mostly from a combination of nitrogen enhanced chlorophyll abundance in the water column
<br /> and excessive macroalgal growth(Valiela e.t al. 1997b,Hauxwell et al.2001). The MEP program has not
<br /> specifically identified nitrogen threshold values for either of these two components,so it is not possible to
<br /> predictif the threshold concentrations would limit these primary producers from outcompeting eelgrass
<br /> (Pedersen and Borum 1996,Mcdlathry et al.2007).
<br /> The threshold approach assumes that the transition from impaired to unimpaired conditions will'operate '.
<br /> as a simple switch when nitrogen reaches a specified threshold concentration. This approach
<br /> Oversimplifies the complex process,of eutrophication that results in the impairment of several ecosystem
<br /> functions and changes in the bio-physical properties of the sediment. The positive feedbacks associated
<br /> with sustaining a healthy eelgrass state;presumably present where the threshold value is determined(e.g,
<br /> _:La•
<br /> it
<br /> riai.e.i ciar.ity,•nutlient storage,oxygen production),'.are not'.present'.is,-the.impair eo.conalLion.'.�i'�i 09.—n �
<br /> now interacting with other potential stress factors.'Many of the most impaired enib ynien-s"have changed
<br /> state,ll#avens et al.20,01,van der Heide et al.201 1,Carr et al.''20"W,Fonseca,2u 1 1 j.and these other
<br /> .lift vxitiai-.,ttcSafat,taars. t'ri7Lit,a:,water<utaS;atd.:,susca..ftuo€i'..-,t, iiiicncs,.Gi„i
<br /> av ei. rS blb ;7 r feugicssc v a D fn
<br /> recovery (Uoodiiiaii et ai. 1993,Moore et al. l99%/S',emp et al.LUfJ4t,Wazniak et a1.2-00 f,tialfegos eta].
<br /> o ae111V l..a ixud ceig ass resTvi[x,wn_goals tho pi—Zits aiii aiss h.wie to e:zp d adiuss
<br /> epth grarli nf`-into deeper waterwhere liglit availability W-111'become a ctiticAly signiiis.__it factor.
<br /> Deceniber 30,2011 —�
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