Resumen de Nitrogen and Phosphorus Recycling by a Dominant Tropical Seagrass (Thalassia Testudinum) Across a Nutrient Gradient in Florida Bay
Kara L. Rosch, Marguerite S. Koch
Nitrogen (N) and phosphorus (P) recycling by the dominant Atlantic/Caribbean seagrass, Thalassia testudinum Banks and Soland. ex Koenig, was investigated to further understand nutrient dynamics in seagrass-dominated tropical estuaries. Nutrient loss from leaves attached to short-shoots and subsequent decay rates of detached senescent leaves, roots, and rhizomes were determined across a nutrient gradient in Florida Bay. The greatest flux of nutrients occurred from leaves as they senesced on the short-shoot, and loss was higher at nutrient-rich (88% P; 51% N) compared to nutrient-poor (49% P; 25% N) sites. The close relationship between live leaf percent N and P decline (R2 = 0.91), high loss rates of P (max = 95%) and N (max = 86%), and the major loss of nutrients at leaf tips, indicate that seagrass live leaf nutrients, a large nutrient pool in the bay, are being turned over via decomposition and/or leaching, rather than internal plant resorption or decay. Detached senescent leaf decay constants (k = 33-129 × 10-4 d-1) were relatively low, and root and rhizome decay even slower (k = 23-66 × 10-4 d-1), thus detrital decomposition is a slow nutrient recycling process. In contrast, the rapid leaching of nutrients from attached leaves indicates that T. testudinum may serve as a sediment nutrient pump. Scaling to the ecosystem, and applying current nutrient budgets, T. testudinum leaf nutrient flux was estimated to contribute 209 and 4343 mt of P and N yr-1, accounting for -50% and -35% of the annual P and N inputs to Florida Bay, although further studies are required to validate these initial bay-wide estimates.
