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

Florida Bay is a shallow, seagrassdominated 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 (1 O100 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 0.05to0. 20%;C(29.4-39.5%)andN(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. The dominant primary producer in Florida Bay is the seagrass Thalassia testudiAcknowledgments We thank K. Halama, M. McManus, S. Pultz, and A. H. Powell for assistance with field collections and D. Yozzo, R. Cutter, and D. Osgood for help with laboratory analysis. S. L. Williams, R. M. Chambers, D. L. Childers, and two anonymous reviewers made comments on earlier drafts of this paper that strengthened the finished work. Salinity data in Fig. 2 were collected in conjunction with J. Bugden and R. Jones of Florida International University. The National Audubon Society research office in Tavemier, Florida, provided laboratory space and logistical support. M. Robblee of Everglades National Park research center supplied boats, laboratory space, and discussion. Financial support was provided by a John D. and Catherine T. MacArthur Foundation grant to the National Audubon Society and a National Park Service Cooperative Agreement (contract CA-5280-0-9009) with J.C. Z. at the University of Virginia. num. Over a range of two orders of magnitude in standing crop, areal leaf productivity of T. testudinum shows a consistent and linear relationship to leaf standing crop (r2 = 0.92, Zieman et al. 1989). This relationship holds over a wide range of sediment properties and water depths. Because of this pattern, any factor controlling the standing crop of T. testudinum in the bay will also control the primary production of this dominant autotroph. Florida Bay is a roughly triangular embayment on the southern tip of the Florida peninsula (-25”N, 8 low; Fig. 1). It is bounded on the north by the mainland of Florida and on the south and east by the emergent Pleistocene reef ridge of the main Florida Keys. Its western margin is open to the Gulf of Mexico, and it receives freshwater runoff from the Everglades that lie directly to the north. In all but the wettest years, it is a negative estuary, with hypersalinity caused by evaporation exceeding freshwater inputs (Fig. 2). The entire bottom of this shallow (< 3 m) bay is dominated by seagrasses, but there is a strong gradient in the development of the seagrass community from the enclosed northeastern sections of the bay to the open western sections (Zieman et al. 1989). The communities of northeast Florida Bay are generally dominated by sparse T. testudinum, with somewhat denser cover on localized areas of increased sediment accumulation. Seagrass cover increases dramatically toward the west, where dense stands of T. testudinum are often intermixed with the seagrasses Halodule wrightii and Syringodium filiforme. The sparse T. testudinum communities of northeast Florida Bay are nutrient limited (Powell et al. 1989, 199 1). Upon fertilization with bird excrement, seagrass density increases and H. wrightii replaces T. testudinum as the dominant species. Seagrass beds from many