PRODUCTIVITY AND BIOMASS OF THALASSIA-TESTUDINUM AS RELATED TO WATER COLUMN NUTRIENT AVAILABILITY AND EPIPHYTE LEVELS - FIELD OBSERVATIONS AND EXPERIMENTAL STUDIES

Thalassia testudinum meadows from 0.5 m and 2.0 m (MLW) depths were studied at 9 sites in the Florida Keys and western Caribbean. Two meadows, one offshore of a populated island with over 2000 septic tanks, and one offshore of a large bird rookery, were similar in having elevated levels of water column nutrients (DIN and SRP), greater epiphyte levels, low shoot densities, low leaf area indices, and low biomass. Increased blade turnover time was partially responsible for increased epiphyte levels offshore of the populated island, but epiphyte communities developed faster on seagrass blades there than at a paired site offshore of an uninhabited island. Results of aquarium experiments approximated the observed phenomena from the field studies: elevated water column nutrients produced increased epiphyte levels and decreased blade turnover rates. Reduced irradiance moderated the effect of nutrient enrichment on epiphyte levels. Elevated levels of water column nutrients, by stimulating epiphyte growth, reduced rhizome growth rates. This could be related to the observed lower shoot density of T. testudinum meadows near sources of water column nutrient enrichment.