WEINSTEIN: SHALLOW MARSH HABITATS AS PRIMARY NURSERIES 



known resistor and periodically checked against 

 salinity standards. Fishes and shellfish were pre- 

 served in 10'7f Formalin and lengths (standard 

 length for fishes, carapace width for crabs, and 

 total length for shrimp) recorded for selected 

 species. Subsampling for lengths was employed 

 when sorted collections contained more than 100 

 individuals of a given species. 



Sediments were collected with a 0.01 m'-^ plug 

 sampler. Three to seven cores, 5 cm deep, were 

 taken at each site, with the actual number at any 

 one location governed by the expected 

 homogeneity of the substratum. Where the bottom 

 was uniform, three evenly spaced cores were taken 

 along a diagonal across the site; where bar forma- 

 tions formed a border of the site, six or seven 

 evenly spaced cores were collected along two 

 transects located parallel to each shore. Cores 

 were processed according to prescribed methods of 

 the American Society for Testing and Materials 

 (American Society for Testing and Materials 

 19631. Hydrometer analysis was used for the fine 

 ft-action, and particle size descriptions were based 

 on the unified scale (U.S. Corps of Engineers 

 1953). 



The degree of similarity among sites was com- 

 pared with the percent similarity index (PSI) 

 (Whittaker and Fairbanks 1958): 



PSI = 100 i(p ,p ) 



wherep^ andp^^ are the proportion of species i in 

 samples a and b, respectively. Since species with 

 single occurrences (singletons) and taxa not iden- 

 tified to species contributed little to similarity 

 among sites or to an understanding of their role in 

 the community, they were omitted from the 

 analyses. Separate matrices were constructed for 

 pooled monthly data at each site on the basis of 

 station and species affinities (recurring species 

 groups) and clustered by the unweighted pair 

 group-average method (Sneath and Sokal 1973). 

 Data were logj^, transformed prior to these analy- 

 ses to give added emphasis to less common species 

 and decrease the effect of extreme values (Clifford 

 and Stephenson 1975). Three separate dendro- 

 grams were constructed — a station association 

 dendrogram in which all nine stations were com- 

 pared, and a pair of dendrograms which reflected 

 station and species associations for seine sites. 

 Results for the latter comparisons were then 

 cross-referenced by construction of a two-way 

 coincidence table (Clifford and Stephenson 1975). 



Partial correlations were calculated with physi- 

 cochemical data and monthly abundances for 

 selected species (Fisher 1973). The latter were 

 based on sufficient sampling densities (taken as at 

 least 10 individuals collected among all sites in 

 any given month) and served to reduce the effect of 

 zero catches in the data. Prior to the analyses, 

 abundance data were \og^^^ transformed in order to 

 stabilize variances (Winsor and Clark 1940). 



RESULTS 



Physicochemical Parameters 



Among the seine sites, two distinct salinity pat- 

 terns were observed (Table 1); high salinity sta- 

 tions (Baldhead and Dutchman Creeks and Shell- 

 bed Island) generally ranged above 15%ii, while 

 Hechtic Creek (well upriver) and Walden Creek 

 (part of a fairly extensive freshwater drainage ba- 

 sin) ranged downwards from this value. Seasonal 

 patterns also were evident, with salinity depres- 

 sions occurring in the spring, especially in March 

 and April, and again during the fall. Although the 

 pattern for rotenone stations was not as distinct as 

 that for seine sites, results were similar, with the 

 exception of the Dutchman Creek station (located 

 at the headwaters of this creek system) which 

 reflected local runoff. 



Sediment characteristics (Table 1) represent 

 means for all cores taken at each site. Fine sands 

 (1.25-3.75 <b) were the major component of the 

 sediment at all stations averaging 809f , with 

 slightly coarser material (-1.0 to 1.25 rf)) con- 

 tributing 20'7f or more of the bottom particle com- 

 position at the Dutchman and Baldhead Creek 

 rotenone sites and at the Hechtic Creek seine site. 

 In general, silts and clays ( >3.75 </>) were minor 

 components of the substratum at stations located 

 in the Baldhead Island complex, although this 

 percentage increased in Dutchman Creek, with a 

 maximum ( lA'i ) at the rotenone site. Both upriver 

 stations. Hechtic and Barnards Creeks, also con- 

 tained more silts and clay. Organic fractions were 

 similarly distributed, with the lowest station 

 mean recorded for the well-scoured Walden Creek 

 system. Dutchman Creek and upriver sites gener- 

 ally had sediments with high organic content, 

 with a maximum at Barnards Creek of SS.l?*^. 



Sorting coefficients (a, ) were calculated (Table 

 1 ) for each site according to the methods of Folk 

 (1974). Sediments were poorly sorted at the 

 Dutchman Creek rotenone station, located at the 



343 



