flourished in Hillsborough Bay as it does today 

 along most of the Gulf Coast between Cape Sable 

 and the Rio Grande (Butlei-, 1954). The annual 

 mean and range of salinity (22.20 and 12.65-27.84 

 p.p.t.), water temperature (24.96° and 11.65°- 

 34.00° C), and other hydrological features of the 

 bay have been reported by Saloman and Taylor 

 (1968). 



In addition to considerable physical alteration 

 of the bay, water chemistry and resident biota have 

 changed decidedly as a result of domestic and 

 industrial sewage. The principal identified pollu- 

 tants are compounds of phosphorus and nitrogen, 

 and highly organic suspended .solids. Regional 

 sanitation plants provide only primary sewage 

 treatment for 120,000 m.Vday (30 m.g.d.— million 

 gallons per day) and serve a population of about 

 300,000. The treated effluent carries more than 50 

 percent of the suspended solids present before 

 treatment and adds an enormous load of phos- 

 phorus and nitrogen.^ The solids are deposited as 

 sludge, and phosphorus and nitrogen ai-e available 

 as nutrients for plants and animals. The phosphate 

 industry provides additional sediment and 

 phosphorus, and natural land drainage provides 

 substantial amounts of phosphorus, nitrogen, iron, 

 copper, and organic compounds (Odum, 1953; 

 Dragovich and May, 1962; Dragovich, Kelly, and 

 Goodell, 1968). Dragovich et al. (1968) estimated 

 that the Hillsborough and Alafia Rivers together 

 add 557 metric tons of phosphorus to the bay each 

 year. In the bay the annual mean concentration of 

 total phosphorus is 19.38 ^g.at./liter, and the total 

 nitrogen (Kjeldahl) is 80.17 ^g.at./liter. Com- 

 parative figures for Tampa Bay entrance 

 (P = 14.39; N = 45.08 /ug.at./liter) and the near- 

 shore Gulf of Mexico (P = 3.6; 5^^=23.4 ixg.nt./ 

 liter) give some idea of the extraordinary mineral 

 enrichment that exists in Hillsborough Bay (Sal- 

 oman and Taylor, 1968). In Biscayne Bay, Fla., 

 McNulty, Reynolds, and Miller (1959) and Mc- 

 Nulty (1966) found that domestic sewage advei-sely 

 affected the biotic environment. There, daily dis- 

 charge of 120,000 to 200,000 m.yday (30-50 m.g.d.) 

 of raw sewage raised the average concentration of 

 total phosphorus to 3 ^g.at./liter or about one- 

 sixth of the concentration now in Hillsborough 

 Bay. 



Enrichment of Hillsborough Bay by phospho- 

 rus and nitrogen causes excessive gro^rth of phy- 

 toplankton and filamentous algae (Dragovich, 

 Kelly, and Kelly, 1965). The heavy growth of 

 algae and the phytoplankton blooms cause marked 

 fluctuations in dissolved oxygen. In periods of 

 photosynthetic activity, oxygen concentrations 

 have exceeded 8 ml./liter but at other times, BOD 

 (biochemical oxygen demand) may reduce dis- 

 solved oxygen to 1 ml./lit«r or less at the bottom 

 (Saloman, Finucane, and Kelly, 1964; Saloman 

 and Taylor, 1968; FWPCA, personal communi- 

 cation^). 



Other consequences of pollution in Hillsborough 

 Bay include high water turbidity (annual average, 

 19.19 Jackson Turbidity Units), low light trans- 

 mission (annual average, 30.3 percent of incident 

 radiation at 60 cm. Wow the water surface), and 

 \-ery little growth of marine grasses (Taylor and 

 Saloman, 1966; Saloman and Taylor, 1968; and 

 Taylor and Saloman, 1969). In their comparative 

 study of macrofauna in major geographic areas 

 of Tampa Bay, Sykes and Finucane (1966) pro- 

 vided further biological evidence of pollution in 

 the bay. From quantitative sampling, their work 

 showed that catches of fish and crustaceans were 

 lower in Hillslwrough Bay than in any other re- 

 gion of the estuary. The gi-eatest catches came 

 from Old Tampa Bay where environmental con- 

 ditions differ from those in Hillsborough Bay 

 mainly in terms of fewer and smaller sources of 

 pollution, lower turbidity, lower nitrogen concen- 

 tration, higher dissolved oxygen at the bottom, 

 more sandy sediments, a more natural shoreline, 

 and extensive beds of sea gi-asses. 



PROCEDURES 



We sampled mollusks together with bottom 

 vegetation and sediments with a bucket dredge 

 and rigid-frame net at 45 stations between Au- 

 gu.st 13 and September 5, 1963 (fig. 1). The dredge 

 dug 5 cm. into the bottom and had a capacity of 

 15 liters. It filled with sediment after covering 

 an area of about 30 by 100 cm. The net skinuned 

 the bottom and had an opening of 30.5 by 91.4 cm. 

 It was hung with square-mesh netting with open- 

 ings of 3.2 mm. (Taylor, 1965). At intertidal sta- 

 tions, the lx)ttom was sampled by sho\'el and the 



= Hillsborough County Hcilth Department, Tumpa, Fla. 33601, 

 personal coninuinication, 1969. 



3 Federal Water Pollution Control Administration, Tampa- 

 Hillsborough Bay Project, Tampa, Fla. 33605, 1968. 



MOLLUSKS AND BENTHIC ENVIRONMENTS IN HILLSBOROUGH BAT, FLA. 



193 



