Phosphorus release with carbonate dissolution coupled to sulfide oxidation in Florida Bay seagrass sediments

Jensen, Henning S., Ole I. Nielsen, Marguerite S. Koch, Inmaculada de Vicente

Limnol. Oceanogr., 54(5), 2009, 1753-1764 | DOI: 10.4319/lo.2009.54.5.1753

ABSTRACT: We hypothesized that CaCO3 dissolution, coupled to sulfide oxidation, is an important mechanism by which solid-phase inorganic P (iP) becomes available to seagrass in tropical carbonate sediments. To examine this supposition, we measured field sulfate reduction rates and simulated the acidity (10-50 µmol H₂SO₄ cm^-3 sediment) generated by subsequent sulfide oxidation from high (western) and low (eastern) total P (TP) sediments in Florida Bay. Dissolution experiments were conducted using sediment slurries at field pH (pore-water pH ~6.5-7.5). While CaCO₃ dissolution (maximum 1.3% of sediment dry weight) was evidenced at all sites by leaching of Ca²⁺ into slurries, at three of the four sites PO₄^3- was primarily recovered in the sediment exchangeable pools and at a lower P : Ca ratio than observed in source sediments. In contrast, no PO₄^3- was recovered from an eastern bay site with the lowest TP and finest-grained sediments, suggesting that PO₄^3- was either tightly adsorbed or incorporated into the carbonate matrix post-acidification. The potential for tight PO₄^3- resorption by sediment from the low-TP site was also supported by the rapid rate of isotopic exchange of ³³PO₄^3- into the sediment matrix. These adsorptive and incorporation processes may explain the low PO₄^3- in pore waters in the eastern vs. western regions of Florida Bay, even upon dissolution of carbonates. Carbonate dissolution coupled to sulfide oxidation could potentially provide 1.8 and 23.5 µmol iP m^-2 d^-1 at eastern and western bay sites, respectively, meeting 5% of eastern and 29% of western bay seagrass (Thalassia testudinum) P requirements.