MOLLUSKS AND BENTHIC ENVIRONMENTS IN HILLSBOROUGH BAY, 



FLORIDA' 



BY JOHN L. TAYLOR, JOHN R. HALL, AND CARL H. SALOMAN, FISHERY BIOLOGISTS 



BUREAU OF COMMERCIAL FISHERIES BIOLOGICAL LABORATORY 

 ST. PETERSBURG BEACH, FLA. 33706 



ABSTRACT 



Analysis of benthic mollusks and sediments at 45 

 stations showed that the diversity and abundance of 

 mollusks was affected by bottom conditions which were 

 influenced in varying degrees by domestic and industrial 

 pollution and dredging. Nineteen stations had no living 

 mollusks, 18 stations had one or more of the four mol- 

 lusk species that were predominant, and 8 stations had 

 mollusks well represented by numerous species and 

 large numbers of individuals. Stations with no living 

 mollusks were termed unhealthy, and others were 



This rei^ort treats the relation of diversity and 

 abundance of molhisks to bottom conditions in 

 Hillsborough Bay, Fla., where dredging and pollu- 

 tion from domestic and industrial sources now con- 

 trol the ecology. The data are from benthic and 

 hydrological surveys by the Bureau of Commercial 

 Fisheries Biological Laboratory, St. Petersburg 

 Beach, Fla., during August and September 1963. 



The problem of pollution in coastal waters has 

 stimulated research to establish environmental 

 quality criteria based on physical, chemical, and 

 biological components of marine and brackish 

 water connnunities. Mollusks are useful in such 

 studies because the group is well described taxo- 

 nomically and contains species that vary gre4^tly 

 in habitat selection, mode of feeding, and tolerance 

 to environmental change. Furthermore, most mol- 

 lusks are sedentary as adults and the remains of 

 their shells provide a semipermanent record of 

 their occupancy. 



The ecology of mollusks in natural waters has 

 been studied by a number of authors. Previous 

 studies on the ecology of mollusks in natural and 

 l^olluted waters of the southeastern United States 

 provided a basis for the interpretation of collec- 

 tions from Hillsborough Bay. Reports on mollusk 

 assemblages in unpolluted estuaries included work 

 by Ladd (1953), Parker (1960), and Brett 

 (196.']). Within the same geographic area, studies 

 of mollusks in polluted estuaries include work 



' Contribution No. 56, Bureau of Commercial Fisheries Biological Lab- 

 oratory, St. Petersburg Beach, Fla. 33706. 



Published March 1970. 



FISHERY BULLETIN: VOL. 68, NO. 2 



designated marginal or healthy on the basis of the 

 mollusks present. From station data, isopleths connect- 

 ing similar areas indicated that 42 percent of the bay 

 bottom was unhealthy, 36 percent marginal, and 22 

 percent healthy. Infrequent occurrence of the American 

 oyster (Crassostrea virginica) further suggests that the 

 major portion of Hillsborough Bay was seriously con- 

 taminated. An appendix has a checklist of the 64 species 

 of mollusks collected in the bay. 



on the ecological effects of petroleum wastes 

 (Mackin and Hopkins, 1961), pesticides (Butler, 

 1966), siltation and dredging (Mackin, 1961), 

 channelization (Chambei-s and Sparks, 1959), and 

 domestic sewage (McNulty, 1966). The work by 

 ]\IcNulty, and an earlier series of studies with 

 collaborators, represent a comprehensive study 

 over a period of 11 years in Biscayne Bay, Fla., 

 before and after pollution abatement. 



ECOLOGICAL FEATURES OF 

 HILLSBOROUGH BAY 



Hillsborough Bay lies in the upper part of 

 Tampa Bay. east of Interbay Peninsula and north 

 of a line between Gadsden Point and Newman 

 Branch (fig. 1). The 56-km. shoreline encom- 

 passes a water area of about 10,360 ha. Forty per- 

 cent of this area is 1.8 m. or less, and except for 

 dredged ship channels up to 10.5 m. deep, the great- 

 est depth in the bay is about 5.4 m. Tidal range is 

 normally 0.9 in. or less, and maximum tidal cur- 

 rent is under 51 cm./second (1 knot) — see Olson 

 and iVIorrill (1955) and Taylor and Saloman 

 (1969). Portions of the bay around Davis Island, 

 Seddon Island, McKay Bay, and Port Sutton have 

 l>een dredged for fill material or deepened for 

 shipping (fig. 1). Other dredging in the bay cen- 

 ters around oy.ster shell deposits which are used 

 for the construction industry (Dawson, 1953). 

 These deposits are extensive and show that the 

 American oyster, Grassostrea virginica^ once 



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