mollusks (Jones, 1950; Pratt, 1953; Thorson, 

 1956; Sanders, 1958; McNulty, 1961; Brett, 1963). 

 Another feature of most healthy stations was the 

 occurrence of a red alga, Gracilaria sp., which is a 

 source of organic detritus and provides a base foj- 

 attachment of epiphytic mollusks. 



Salinity at healthy stations was between 18 jxp.t. 

 (upper bay stations C, C-2, and C-3) and 23 p.p.t. 

 (transect 10). The combination of relatively high 

 mollusk diversity and reduced salinity at the 

 upper bay stations indicated that a factor other 

 than salinity prevented the establislunent of an 

 equivalent variety of mollusks at most marginal 

 and unhealthy stations. 



MARGINAL STATIONS 



At the 18 stations classified as marginal at least 

 50 percent of the live mollusks were indicator 

 species, and the average incidence of indicators 

 at these stations was 80 percent. The average niun- 

 ber of species per station represented by live ani- 

 mals was only 3 ; the mean number of individuals 

 was 102 (table 3) . In comparison with the healthy 

 stations, marginal stations had about one-fourth 

 as many species of mollusks and about one-half as 

 many individuals. 



Sediments at marginal stations ranged from fine 

 sand to medium silt. The average sediment tyi^e 

 was very fine sand (3.090) — a somewhat finer par- 

 ticle size than the average size at healthy stations. 

 Sediment sorting was poor (1.40) and vei"y close 

 to the figure for liealthy stations. 



Bottom vegetation (Gracilar'ia sp.) was found 

 at only one marginal station. That station liad a 

 substrate of fine sand and more species of mollusks 

 than any other station. 



Low salinity (less than 4 p.p.t.) near the mouth 

 of the Alafia Eiver was probably responsible for 

 fewer species of mollusks at stations C-8, C-8-1, 

 and C-8-2. The only species present in this area 

 was Tagelus plebeiu-s. 



Data for DO (dissolved oxygen) indicated that 

 from June through August bottom water in the 

 bay between transect 10 and McKay Bay becomes 

 anaerobic (EWPCA, personal commimication ; see 

 footnote 3). At other times, however, DO values 

 are generally above 3 ml. /liter and would not prove 

 limiting. Changes in the DO at stations regarded 



as marginal may create a more favorable environ- 

 ment for mollusks during other seasons. 



UNHEALTHY STATIONS 



No live mollusks were collected at 19 stations 

 classified as unhealthy (fig. 1). Two of these sta- 

 tions were on the eastern shore of the bay (8-10 

 and 9-10) and adjacent to an extensive area of 

 gypsum spoil — a byproduct of the phosphate in- 

 dustry. The gypsum forms a crust on the bottom 

 that virtually eliminates macrobenthic organisms. 



Sediments at other unhealthy stations had a 

 mean grain size that varied from —1.90 

 (granule) to 7.84 (very fine silt) — see table 4. 

 Sediments were coarse at stations near spoil 

 islands left from channel construction and on a 

 natural, shelly shoal (C-7). Absence of mollusks 

 in coarse sediments probably resulted from the 

 grinding action of large particles powered by wave 

 action. Stations with fine sediments were in 

 comparatively deep water. There the sediments 

 had a high concentration of the toxic compound, 

 hydrogen sulfide, and are probably anaerobic, or 

 nearly so, at all times (Florida State Board of 

 Health, 1965). 



ECOLOGICAL ZONES 



Isopleths were drawn between similar stations 

 to represent approximate boundaries of healthy, 

 marginal, and unhealthy zones in Hillsborough 

 Bay (fig. 2). Calculation of the area within each 

 zone showed that only 22 percent of the bay falls 

 in the healthy categoi-y, 36 percent is marginal, 

 and 42 percent is unhealthy. Most healthy zones 

 were near the mouth of the bay where the solid 

 and soluble products of pollution were least con- 

 centrated. Marginal zones were on the bottom 

 slopes between the three unhealthy zones that were 

 along the eastern and western shores and in mid- 

 bay ship channels. Observations in Kai'itan Bay 

 (Dean and Haskin, 1964) and Biscayne Bay (Mc- 

 Nulty, 1966) suggest that pollution abatement in 

 Hillsborough Bay would favor progi-essive re- 

 population of marginal zones by a more normal as- 

 semblage of benthic plants and animals. In heav- 

 ily silted areas of the unhealthy zones, however, 

 biological restoration would probably requii-e a 

 long period of time. 



198 



U.S. FISH AND WILDLIFE SERVICE 



