Forms and availability of sediment phosphorus in carbonate sand of Bermuda seagrass beds

Primary production by seagrasscs in tropical and subtropical carbonate sediments often appears to be phosphorus (P) limited, and several studies have sought to identify the possible sources of P for long‐term growth. Here, we quantify concentrations of particulate and dissolved P and fluoride (F−) in carbonate sediments, sediment‐water P exchange, and leaf‐tissue P concentrations in three seagrass beds in Bermuda. We also present data on the sequential extraction of P and F‐ from the sediments at each site. Total sediment P (TPsed) in the upper 20 cm ranged from 650 to 1,250 mmol P m−2 and was some 500‐fold larger than the pool of P dissolved in pore waters. Loosely adsorbed inorganic P comprised 2% of TPsed, while a surface‐bound inorganic P pool extracted by dithionite buffer constituted 15‐20%. Most of the remaining P and nearly all of the Ca and F∑ present in the sediment was recovered when the remaing sediment was dissolved in an acetic acid buffer solution. This pool includes calcium‐fluoroapatite (CFA), which is considered the primary diagenetic sink for P in carbonate sands. Dissolved inorganic P and F were both elevated in the rhizosphere pore waters from the three seagrass beds. In combination with our analysis of sediments by sequential extraction, this result indicates that the carbonate matrix of the sediment is being dissolved in the rhizosphere, with a resulting release of P to the pore water and hence to the seagrass roots. We suggest that this is important in the P economy of these subtropical seagrass (Thalassia testudinum) beds and note that the carbonate‐bound P pool is large enough to sustain seagrass P requirements for decades. Nonetheless, primary production in such seagrass systems can remain P limited if the rate of P release from sediment dissolution is too slow to support maximum seagrass growth rates. Analysis of seagrass leaf tissues revealed that P availability was highest at the site with the highest anthropogenic influences: This site was also characterized by the highest sediment P release, indicating that high P availability may also be partly due to higher release of P as sediments are dissolved. Phosphorus was released from the sediment mainly as dissolved organic P causing a net efflux of dissolved P at all sites in April and at two sites in August.