ECOLOGY OF BUZZARDS BAY: An Estuanne Profile 43 



diameter greater than 0. 1 8 mm), more comparable 

 to the Ampelisca assemblage (Sanders 1958, 

 1960). 



The similarities in sediment type between the 

 shallow protected and offshore sites are identified 

 as the result of two sets of physical conditions. In 

 the shallow protected areas, eelgrass. which is of- 

 ten prevalent, exerts a dampening effect on cur- 

 rents, resulting in deposits of fine-grained, silt- and 

 clay-rich sand. Near the mouths of harbors sedi- 

 ments are generally fine-grained but poorly sorted, 

 due to stream inputs carrying little or no coarse de- 

 tritus and to deposition in a dynamic flow field with 

 variable wind and wave activity. The sediments in 

 the deeper offshore areas also experience less wave 

 energy and lower current velocities and are afforded 

 some protection by the dendritic troughs of the Pleis- 

 tocene drainage system (Driscoll and Brandon 

 1 973 ), resulting in the accumulation of fine-grained 

 but less organically rich sediments. Offshore areas 

 are generally characterized by water deeper than 9 

 m. The offshore molluscan macrofauna of north- 

 western Buzzards Bay is predominately represented 

 by two species (making up 90% of all collected), 

 Nassahus trivittatus and Yoldia limatula. In con- 

 trast, the shallow, more protected areas are colo- 

 nized by a variety of molluscan fauna, dominated 

 by Crepidula fornicata, Nuncula proximo. 

 Crepidula plana, Bittium alternatum, and 

 Laevicardium mortoni. The most obvious differ- 

 ence in fauna is seen in the abundance of Nuncula 

 proximo in shallow, protected areas and its near 

 absence from other areas. 



Nearshore sediments maintain greater relative 

 abundance of Macoma tenta and Eupleura 

 caudata, with few Nuncula proximo and relatively 

 fewer Nassarius trivittatus than the offshore ar- 

 eas. Open-bay environments, on the other hand, 

 are substantially different from the other three sub- 

 system types. Benthic communities of the open bay 

 are generally characterized by suspension feeders, 

 carnivores, herbivores, or nonselective deposit feed- 

 ers such as Nassarius trivittatus, Chaetopleura 

 apiculata, and Anachis avara. Sanders (1958) 

 suggested that the fauna of stable sand bottoms is 

 probably inherently more diverse than that of mud 



bottoms, most likely because of the more stable 

 (less stressful) environmental conditions at these 

 sites. 



Overall the deeper parts of Buzzards Bay have 

 maintained a stable benthic community for several 

 decades. Nearshore areas that have been organi- 

 cally enriched (possibly by sewage), such as those 

 within New Bedford Harbor, are dominated by 

 Mediomastus ambiseta', this species is an oppor- 

 tunistic colonizer of polluted sediments or those sub- 

 ject to disturbances that limit recruitment of most 

 other benthic organisms. Monitoring of infaunal 

 populations has been conducted at what is known 

 as the 301(h) Site offshore from New Bedford 

 Harbor (Howes and Taylor 1 989), and populations 

 have shown little change from what Sanders found 

 in the late 1 950's and early 1 960's. It appears that 

 benthic populations within the central bay remain 

 relatively "pristine," even in the region of New 

 Bedford, which contributes almost all of the sew- 

 age to Buzzards Bay waters and almost half of the 

 total nitrogen load. Even in this region, the impact 

 on benthic communities appears restricted to 

 nearshore areas (Howes and Taylor 1 989; Costa 

 etal. 1992). 



Although sediment characteristics are important 

 to structuring the infaunal assemblages in Buzzards 

 Bay, the reverse is also true. Bioturbation and sedi- 

 ment reworking by benthic infauna are significant in 

 structuring the biogeochemistry of these sediments. 

 In fact, Pvhoads ( 1 963) estimated that although one 

 species, Yoldia limatula, a deposit-feeding pele- 

 cypod, represented less than 1 0% of the total bot- 

 tom fauna, it was potentially capable of entirely re- 

 working the sediments within its range of distribu- 

 tion in the bay (Fig. 4. 1 ). More than half buried, 

 this clam ingests sediment extracting food and eject- 

 ing waste several centimeters into the water, which 

 eventually settles into small mounds around the si- 

 phon. Typical of deposit feeders, this species acts 

 to mix surface sedimentary layers, alters the char- 

 acteristics of some of the particles through aggre- 

 gation into fecal pellets, and potentially increases 

 the oxidation state of the surface sediments through 

 the presence of its burrow. 



