previously. Peak abundance occurred after inflow, 

 lowest abundance during peak salinity concentrations. 

 Blooms could result in very high abundances, greater 

 than 40,000 individuals per m". This ecosystem was 

 characterized by dominance of only a few species. The 

 diversity index N I was used to calculate the average 

 number of dominant species among all stations at each 

 sampling period, and then was compared to salinity 

 (Figure 5-7c). Generally, no more than two dominant 

 species were present at each sampling period. 

 Diversity, measured as the number of dominant 

 species, increased following periods of low salinity and 

 decreased when salinity was high. 



Macrofauna characteristics have a strong non-linear 

 relationship with saUmty, which appeared to be a bell- 

 shaped curve skewed to the left with a long tail to the 

 right (Figure 5-8). 



A three parameter, log normal model: 



Y^a * exp( -0.5 * (ln(X/ c)/by) 



was used to characterize the nonlinear relationship 

 between biological characteristic (1) and salinity (X). 

 The three parameters characterize different attributes 

 of the curves, where a is the maximum value, b is the 

 skewness (or rate of change) of the response as a 

 function of salinity, and c the location of the peak 

 response value on the salinity axis. The models fit the 

 data reasonably well, indicated by the coefficient of 

 variation for each parameter ranging from 7% to 31% 

 (Table 5-3). Using these parameters, abundance 

 appeared to peak at a high salinity around 32.7 ppt. 



biomass at 18.7 ppt and diversity at 9.1 ppt. Lower 

 skewness (b) parameters indicates more narrow ranges 

 of responses values with respect to salinity. For the 

 three characteristics, the salinity range of response (b) 

 increased as the salinity peak value for the response 

 increased (c) (Table 5-3). 



The direction of salinity change during a sampling 

 penod was important for diversity (Figure 5-8c), but 

 not biomass (Figure 5-8a) or abundance (Figure 5-8b). 

 The lowest values of all biological responses occurred 

 at the highest salinity concentrations, and the highest 

 concentrations always occurred during periods of rising 

 salinity (note the circle symbols in Figure 5-8). Low 

 diversity occurred when salinity values were decreasing 

 (obser\^ations on graph with diamond symbols) and 

 high diversity occurred when salinity values were rising 

 within normal salinity ranges (i.e., < 35 ppt). 



In spite of the low average diversity on any given 

 sampling date, a total of 37 species were found over 

 the five year period of the study (Table 5-4). The 

 polychaete, Streblospio benedicti (Figure 5-9) was an 

 overwhelmingly dominant species at all stations and in 

 the marsh overall (Table 5-5). In fact, S. benedicti 

 represented 84% of all individuals found over the 

 entire course of the study. Only four other species 

 contributed as much of 2% of the community: the 

 polychaete Laeonereis culveri, the snail Assiminea sucdnea, 

 and unidentified species of ostracod, and unidentified 

 chironomid larvae. 



Table 5-3: Parameters from nonlinear regressions to predict macrofauna characteristics from salinity 

 (Figure 5-8). Coefficient of variation for parameters in parentheses. 



Characteristic 



Parameter 



Abundance 

 Biomass 

 N1 Diversity 



45,774 (25%) 

 3.426 (15%) 

 2.361 (7%) 



0.6663(31%) 



0.9048 (27%) 



1.699(13%) 



32.7(17%) 

 18.7(24%) 

 9.08(19%) 



Chapter Five ♦ 5-13 



