Experimental Evidence for Nutrient Limitation of Seagrass Growth in a Tropical Estuary with Restricted Circulation

We studied the impacts of additions of nutrients to a seagrass community on a carbonate mud bank in Florida Bay. Shallow mud banks dampen lunar tide in Florida Bay, and empoundment and channelization of the upland watershed (the Everglades) have reduced freshwater imput, resulting in restricted circulation and reduced nutrient availability. Nutrients were supplied by seabirds defecating from experimental roosts. Seabirds used the roosts 87% of the time so the input of nutrients was constant and quantifiable. The birds delivered approximately 2–4 g of excrement per day, resulting in an average loading rate of 0.052 gN and 0.009 gP⋅m^–2⋅d^–1. Only a portion of the excrement is immediately released as inorganic NH₃ and PO₄; about 80% reaches the sediment surface in a relatively insoluble form. There was a significant buildup of phosphate and ammonium in the pore water at the enriched, sites. The ammonium profile of low concentrations at the surface and then increasing with a steep slope through 20 em suggests a rapid uptake and demand for mineralized nitrogen. Phosphorus in contrast had relatively high levels at the surface. Nutrient addition significantly increased areal leaf production and standing crop of Thafassia testudinum and Halodufe wrightii. Above ground biomass at enriched sites averaged twice controls while below grounc biomass was not significantly different between fertilized and control plots. Increased standing crop was produced primarily through longer, wider blades by Thalassia and longer blades and increased short shoot density by Halodufe. Thalassia areal leaf production was 60% greater at enriched sites than at controls. Halodule areal leaf production increased by three orders of magnitude at enriched sites. Tissue nutrient content and nitrogen fixation assays suggest that phosphorus availability limits seagrass growth in unenriched conditions, but that nitrogen becomes limiting with the addition of bird excrement.