Open-File Report 99-219 contents page

CHESAPEAKE BAY HABITAT CRITERIA AND SCORES

The CBP identified salinity-regime based criteria for certain critical water-quality parameters in order to assure the restoration of SAV. These parameters were identified in Table IV-1 (p.27) of the Chesapeake Bay Technical Synthesis Report (Batiuk and others, 1992). A DIP criterion specifically for the tidal Potomac River and Potomac Estuary was given in Table V-10 (page 74) of that same report. These criteria are presented in Table 18, expressed as bounds on the median values of samples collected during the SAV growing season (April-October). Note that the criterion for the light attenuation coefficient is expressed in terms of the more commonly made measurement of Secchi depth (SECCHI) by using the conversion factor assumed for the Chesapeake Bay by the CBP, namely SECCHI=1.45/(light attenuation coefficient); to be conservative, terms are rounded up. Note also that the criterion is equivalent to one expressed as per cent (%) light saturation in reference to a specified depth in the water column, for example, the one-meter restoration goal. The growing season (April-October) medians were calculated for the period 1983 through 1997. These are shown by salinity regime segment in Tables 5-11, and by monitoring station in Tables 12-20.

In order to summarize for management purposes whether or not the criteria have been historically satisfied, the median seasonal sample values for the water quality parameters for the several tidal Potomac River and Estuary segments can be converted into "achievement scores", which are here called "Chesapeake Bay habitat criteria scores". A score is assigned for each water-quality parameter by river segment for each year studied by the steps outlined in 1-3 below.

1. Consider a river segment r which is in salinity regime s (fresh, oligohaline or mesohaline). For each water-quality parameter i, the median value of measurements made for samples taken during the SAV growing season ( April-October) in year t is represented as median (i,r,t). Table 18 presents the criterion (i,s) to which the median in salinity regime s for water quality parameter i is compared.
   
   
2. For i = TSS, CHLA, DIP and DIN, the criteria define an upper bound which the seasonal median should not exceed. The Chesapeake Bay habitat score is computed to be:
   
   Score (i,r,t) = 1- [median (i,r,t ) / criterion (i,s)] ;
   
   but
   
   if Score (i,r,t) < -1, then set Score (i,r,t) = -1.
   
   
3. For i = SECCHI, the criterion defines a lower bound for the seasonal median. The Chesapeake Bay habitat score is computed as follows.
   
   Score (i,r,t) = [median (i,r,t ) / criterion (i,s)] - 1 ;
   
   but
   
   if Score (i,r,t) > +1, then set Score (i,r,t) = +1.
   
   Thus, for each water quality characteristic, the score is bounded; that is,

When a criterion is satisfied for a particular year, Score (i,r,t) = 0, but when a criterion is not satisfied during a particular season, Score (i,r,t) < 0. If Score (i,r,t) = 0, then the criterion has just been met.

Figures 2 through 15 include bar charts to graphically depict the scores for each water quality parameter and river segment derived from the seasonal median values presented in Tables 4 through 17, respectively , and the criteria in Table 18.

Chesapeake Bay habitat criteria scores and coverage by submersed aquatic vegetation
Water-quality data
Open-File Report 99-219 contents page