Taylor and Salo"ian (1968) contrasted 

 undisturbed areas of the bay, where luxu- 

 riant grass grew in sediments averaging 

 94™ sand and shell, with the bottom of 

 dredge canals, where unvegetated sedinents 

 averaged 927. silt and clay. While several 

 studies of Boca Ciega Ray collectively 

 described nearly 700 species of plants and 

 animals occurring there, Taylor and Salo- 

 pian (1968) found only 20% of those same 

 species in the canals. Most of those were 

 fish that are highly motile and thus not 

 restricted to the canals during extreme 

 conditions. Interestingly, while species 

 numbers were higher in undisturbed areas, 

 30% more fish were found in the canals, 

 the most abundant of which were the bay 

 anchovy, the Cuban anchovy, and the scaled 

 sardine. The authors noted that in the 

 few years since the initial disturbance, 

 colonization v.'as negligible at the bottom 

 of the canals and concluded that the sedi- 

 ments there were unsuitable for most of 

 the bay's benthic invertebrates. Light 

 transmission values were highest in the 

 open bay away from landfills, lowest near 

 the filled areas, and increased somewhat 

 in the quiescent waters of the canals. 

 Because of the depth of the canals, how- 

 ever, light at the bottom was insufficient 

 for seagrass growth. Taylor and Saloman 

 (19G8), using conservative and incomplete 

 figures, estimated that fill operations in 

 the bay resulted in an annual loss of 1.4 

 million dollars for fisheries and recrea- 

 tion. 



If seagrasses are only lightly 

 covered and the rhizome system is not 

 changed, regrowth through the sediment is 

 sometimes possible. Thorhaug et al . 

 (1973) found that construction of a canal 

 in Card Sound temporarily covered turtle 

 grass in an area of 2 to 3 ha (5 to 7 

 acres) with up to 10 cm (4 inches) of 

 sediment, killing the leaves, hut not the 

 rhizome system. Regrowth occurred when 

 the dredging operations ceased and cur- 

 rents carried the sediment away. 



the roots, a moderate amount of enrichnient 

 may actually enhance productivity, under 

 certain conditions where waters are well- 

 mixed, as observed by this author in the 

 rich growth of turtle grass and associated 

 epiphytes in the vicinity (within 1 km or 

 0.6 mi) of Miami's Virginia Key sewage 

 plant. This discharge is on the side of 

 the key open to the ocean. In the imme- 

 diate area where these wastes are dis- 

 charged, however, water quality is so 

 reduced that seagrasses cannot grow. Stim- 

 ulation of excess epiphytic production may 

 adversely affect the seagrasses by persis- 

 tent light reduction. Often the effects 

 of sewage discharge in such areas are com- 

 pounded by turbidity from dredging. In 

 Christiansted Harbor, St. Croix, where 

 turtle grass beds were subjected to both 

 forms of pollution, the seagrasses declin- 

 ed and were replaced by the green alga, 

 Enteromorpha . In a 17-year period, the 

 arassbeds in the embayment were reduced by 

 662 (Dong et al . 1972). 



Phytoplankton productivity increased 

 in Hillsborough Bay, near Tampa because of 

 nutrient enrichment for domestic sev/age 

 and phosphate mining discharges (Taylor 

 et al . 1973). Phytoplankton blooms con- 

 tributed to the problem of turbidity, 

 which was increased to such a level that 

 seagrasses persisted only in small sparse 

 patches. The only important macrophyte 

 found in the bay was the red alga, Gra cil- 

 laria . Soft sediments in combination with 

 low oxygen levels limited diversity and 

 abundance of benthic invertebrates. 



Few seagrasses grow in waters of 

 Biscayne Bay that v;ere polluted by sewage 

 discharge in 1955 (McNulty 1970). Only 

 shoal grass and Halophila grew sporadi- 

 cally in small patches within 1 km (0.6 

 mi) of the outfall. Post-abatement stud- 

 ies in 1960 showed seagrasses in the area 

 had actually declined, probably because of 

 the persistent resuspension of dredge 

 materials resulting from the construction 

 of a causeway. 



8.2 EUTROPHICATION AND SEWAGE 



Seagrass communities are sensitive to 

 additions of nutrients from sewage out- 

 falls or industrial wastes. Because 

 seagrasses have the ability to take up 

 nutrients through the leaves as well as 



Physiological studies reveal that 

 seagrasses are not only affected by low 

 levels of light, but also suffer when dis- 

 solved oxygen levels are persistently low, 

 a situation encountered where sewage addi- 

 tions cause increased microbial respira- 

 tion. Hammer (1968a) compared the effects 



86 



