Table l.-The relative abundance (%) and the optimal habitats of the common diatoms occurring both in the 

 stomachs of Atlantic menhaden and on a rock from the Slocum River estuary, Mass. Only those populations 

 having greater than 1% mean relative abundance in the stomachs are listed. 



Diatoms 



Nitzschia Irustulum var. perminuta Grun. 



N. subtilis var. paleacea Grun. 



Navicula cincta (Ehr.) Ralfs 



Melosira nummuloides (Dillw.) Ag, 



Skeletonema costatum (Grev.) CI. 



Cyclotella sp. cf. glomerata Bachm. 



Eunotia pectinalis (Dillw.) Rabh. var. pectinalis 



Achnanthes minutissima Kutz. 



Bacillaria paradoxa Gmel. 



Cyclotella sp. cf. atomus Hust. 



Melosira varians Ag. 



Navicula diserta Hust. 



Navicula capitata var. hungarica (Grun.) Ross 



Eunotia pectinalis var. minor (KiJtz.) Rabh. 



Fragilaria construens var. venter (Ehr.) Grun. 



Navicula gregaria Donk. 



Rhoicosphenia curvata (KiJtz.) Grun. 



Nitzschia Sigma W. Sm. 



Thalassiosira sp. cf. nana Hust. 



Achnanthes wellsiae Reim. 



Nitzschia parvula Lewfis 



Cyclotella striata (KiJtz.) Grun. 



Fragilaria construens var. intercedens (Grun.) Hust. 



Total 



73.8 



71.0 



'F = freshwater, B = bracl<ish, M = marine. 



2). They accounted also for more than 50% of all 

 individuals. Brackish and marine populations were 

 present in about equal numbers, but more common 

 populations were brackish. Nearly 35% of all 

 individuals belonged to brackish populations. All 

 common populations in the stomachs were at least 

 present on the rock, and 17 of the 23 also accounted 

 for greater than 1% relative abundance in the 

 epilithic assemblage (Table 1). Additionally, 24 

 rare populations were found in both the stomach 

 and the epilithic samples. The greater number of 

 rare populations found in the stomachs as com- 

 pared to the rock is attributable to the greater 

 sample size associated with the stomachs. These 

 rare populations were primarily species of Ach- 

 nanthes, Amphora, Cocconeis, CymbeUa, Eunotia, 



Table 2.-The distribution of numbers of common and rare 

 diatom populations from the stomach and epilithic assemblages 

 among their optimal habitats (F = freshwater, B = brackish, M 

 = marine). Populations interpreted as growing equally well in 

 two habitats are divided equally between them. 



'Total sample size ~ 1,800 individuals. 

 ^Total sample size = 200 individuals. 



Fragilaria, Gomphonema, Navicula, Nitzschia, 

 and Synedra. 



Based on the examination of about 200 in- 

 dividuals from the epilithic assemblage, 43 

 populations were identified to species or variety. 

 Twenty identified and three unidentified popula- 

 tions were common in the fish stomachs (Table 1). 

 Only four other populations were unidentified, and 

 they represented less than 3% of all individuals in 

 the sample. All populations are benthic. Given the 

 means of collection of the epilithic assemblage, 

 those populations usually considered planktonic 

 were clearly benthic. They accounted for about 7% 

 of the total number of individuals in the assemb- 

 lage, as they did in the stomach samples. Nearly 

 90% of all populations found on the rock were 

 recorded in the stomachs. Freshwater populations 

 accounted for about 50% of both the common and 

 all populations (Table 2). Brackish and marine 

 populations occurred equally among all popula- 

 tions, but among the common ones, brackish 

 populations were more frequent. A population of 

 the freshwater diatom, Nitzschia parvula, consti- 

 tuted 25% of the whole assemblage. 



Discussion 



The benthic microbial communities of estuaries 

 and the adjacent freshwater reaches of rivers, as 



691 



