Solid-phase Phosphorus Pools in Highly Organic Carbonate Sediments of Northeastern Florida Bay

Abstract

Currently, few studies have investigated sediment phosphorus (P) pools or identified the chemical processes important in the P cycle of fine-grained carbonate sediments, particularly in coastal estuaries with high organic matter. To determine the role of fine-grain calcium carbonate and high organic matter on sedimentary P, we investigated the solid-phase P pools in seagrass sediments of north-eastern (NE) Florida Bay at the Bay–mangrove ecotone. Sediments were fractionated by sequential extractions into seven chemically-defined groups: exchangeable inorganic and organic P, reducible inorganic and organic P (Fe-bound), acid extractable inorganic and organic P (Ca-bound), and residual organic P. Calcium-bound P accounted for approximately 56% of total P and 96% of inorganic P. Our total calcium-bound P was in the range (34–151 μg P g⁻¹) reported for coarse-grained low organic sediments, while the organic P associated with this fraction was slightly (∼10%) higher than those reported for other carbonate systems. The second dominant P fraction was residual organic P (30–71 μg P g⁻¹) accounting for 42% of TP. This high residual pool suggests the importance of fringing mangrove and seagrass detritus in long-term P storage. In contrast to temperate estuaries, the iron-bound P fraction in NE Florida Bay sediments was low (<70 μg g⁻¹) at the surface and undetectable (<1 μg g⁻¹) below 10 cm. Based on our findings, we hypothesize that carbonate chemisorption reactions, along with potential reactive organic surfaces, and the sequestering of P into recalcitrant organic pools, maintain low exchangeable and porewater P concentrations across the NE Florida Bay estuary, and account for the reported autotrophic P-limitation.