THE EFFECTS OF LONG-TERM MANIPULATION OF NUTRIENT SUPPLY ON COMPETITION BETWEEN THE SEAGRASSES THALASSIA-TESTUDINUM AND HALODULE-WRIGHTII IN FLORIDA BAY

Long term (8 yr) continuous fertilization (via application of bird fec es) of established seagrass beds in Florida Bay, FL, USA caused a chan ge in the dominant seagrass species. Before fertilization, the seagras s beds were a Thalassia testudinum monoculture; after 8 yr of fertiliz ation the seagrass Halodule wrightii made up 97% of the aboveground bi omass. Fertilization had a positive effect on the standing crop of T. testudinum for the first two years of the experiment. The transition f rom T. testudinum-dominated to H. wrightii-dominated was dependent on the timing of colonization of the sites by H. wrightii; the decrease i n T. testudinum standing crop and density at the fertilized sites occu rred only after the colonization of the sites by H. wrightii. There we re no trends in the standing crop or density of T. testudinum at contr ol sites,and none of the control sites were colonized by H. wrightii. The effects of fertilization on these seagrass beds persisted at least 8 yr after the cessation of nutrient addition, suggesting that these systems retain and recycle acquired nutrients efficiently. Results of these experiments suggest that Halodule wrightii, the normal early-suc cessional seagrass during secondary succession in Caribbean seagrass c ommunities, has a higher nutrient demand than Thalassia testudinum, th e normal late successional species, and that the replacement of H. wri ghtii by T. testudinum during secondary succession is due to the abili ty of T. testudinum to draw nutrient availability below the requiremen ts of H. wrightii.