Phosphorus limitation of primary production in Florida Bay: Evidence from C:N:P ratios of the dominant seagrass Thalassia testudinum

Florida Bay is a shallow, seagrass‐dominated embayment on the southern tip of Florida. Variation of C, N, and P content of leaves of Thalassia testudinum was measured on two spatial scales: locally (10–100 m) in relation to a point source of nutrients associated with a bird colony in eastern Florida Bay and regionally (10– 100 km) across all of the bay. Locally, the P content of leaves decreased from a high of 0.16% P (wt/wt) 30 m from the nutrient source to a low of 0.08% 120 m from the source; the C and N content (34.9 and 2.1%) was independent of distance from the nutrient source. Due to variations in P content, C : P and N : P, but not C : N, varied locally. Regionally, P content varied greatly, from 0.05 to 0.20%; C (29.4–39.5%) and N (1.7–2.7%) showed considerably less variation. Variation in C : P and N : P across the bay encompassed a range nearly as great as reported for all seagrasses around the world combined; C : N showed little variation. Local variation around the nutrient point source indicated that C : P and N : P were indicators of P availability, and trend analysis of the regional spatial variation in C : P and N : P showed that P availability was greatest in northwest, and least in eastern, Florida Bay. This pattern mirrored abundance of seagrasses and productivity in the bay. T. testudinum from the bay appears to be P limited and N saturated, even in the sparsest seagrass communities.