(b) Serve as hosts for large numbers of epiphytes which are 

 grazed extensively, for example, by the mullets. These epiphytes 

 may be comparable in biomass with the seagrasses themselves. 



(c) Provide large quantities of detrital material which serves as 

 food for certain animal species and for microbes which in turn are 

 used as food by larger animals. 



(d) Provide organic matter to initiate reductipn and an active 

 sulfur cycle. 



(e) Bind the sediments and prevent erosion. This also preserves 

 the microbial flora of the sediment and the sediment-water 

 interface. 



(f) Collect organic and inorganic material by slowing down cur- 

 rents and stabilizing the sediments. 



The seagrasses have a rapid rate of growth (up to 9 millimeters per day, 

 average 2 to 4 millimeters per day) and produce between 2.2 and 10.0 grams of 

 dry leaf per square meter per day. 



137. YOUNG, D.K. , and YOUNG, M.W. , "Regulation of Species Densities of Sea- 

 grass-associated Macrobenthos: Evidence from Field Experiments in the 

 Indian River Estuary, Florida," Jourmal of Mavine Reseavah, Vol. 36, 

 No. 4, Nov. 1978, pp. 569-593. 



Field experiments in the Indian River estuary, Florida, were initiated in 

 the seagrass beds, Halodule wrightii, to test effects of (a) excluding preda- 

 tors by caging, (b) enclosing predators inside cages, (c) adding dense popula- 

 tions of suspension feeders, (d) providing organic enrichment, and (e) remov- 

 ing seagrass blades. 



The 11 most abundant species were selected for statistical testing of 

 responses to the experimental treatments. Analyses showed that macrobenthic 

 species differed markedly in their responses to the various treatments over a 

 period of 1 year. Several species increased in densities with organic enrich- 

 ment; whereas, some species increased in densities as a result of protection 

 by the predator exclusion cage. When seagrass blades were clipped, some 

 species increased in densities while others had decreased densities. These 

 variations in response did not consistently correspond to taxonomic groupings 

 or feeding types. 



138. ZIEMAN, J.C., "Origin of Circular Beds of Thalassia (Spermatophyta: 

 Hydrocharitaceae) in South Biscayne Bay, Florida, and their Relationship 

 to Mangrove Hammock," Bulletin of Marine Saienae^ Vol. 22, No. 3, 

 Sept. 1972, pp. 559-574. 



Aerial photos of a mangrove shoreline and an adjacent estuarine area in 

 southwestern Biscayne Bay, Florida, showed the presence of numerous circular 

 to teardrop-shaped areas. The circular areas on shore are hammocks of man- 

 groves and other tropical trees, and they are over depressions in the bedrock 

 which are filled with mangrove peat. The circular areas in the estuary are 

 beds of Thalassia testudinum. Thalassia beds are often surrounded by a white 

 halo of worm and callianassid burrows. The effect of sediment depth on den- 

 sity and length of blades is shown. 



41 



