VARIABLE RATES OF PHOSPHATE-UPTAKE BY SHALLOW MARINE CARBONATE SEDIMENTS - MECHANISMS AND ECOLOGICAL SIGNIFICANCE

We determined phosphate uptake by calcareous sediments at two location s within a shallow lagoon in Bermuda that varied in trophic status, wi th one site being mesotrophic and the other being more eutrophic. Phos phate adsorption over a six hour period was significantly faster in se diments from the mesotrophic site. Uptake at both sites was significan tly less than that reported for a similar experiment on calcareous sed iments in an oligotrophic lagoon in the Bahamas. The difference in pho sphorus adsorption between our sites did not appear to be related to s ediment characteristics often cited as important, such as differences in surface area (as inferred from grain size distributions), total org anic matter content, or iron content. However, the sediment total phos phorus contents were inversely related to phosphorus uptake at our sit es in Bermuda, and at the previously studied Bahamas site. We hypothes ize that phosphate uptake in these calcareous sediments is a multi-ste p process, as previously described for fluvial sediments or pure calci um carbonate solids, with rapid initial surface chemisorption followed by a slower incorporation into the carbonate solid-phase matrix. Acco rdingly, sediments already richer in solid phase phosphorus take up ad ditional phosphate more slowly since the slower incorporation of surfa ce-adsorbed phosphate into the carbonate matrix limits the rate of ren ewal of surface-reactive adsorption sites. Although carbonate sediment s are a sink for phosphate, and thereby reduce the availability of pho sphorus for benthic macrophytes and phytoplankton in the shallow overl ying water, phosphate uptake by these sediments appears to decrease al ong a gradient from oligotrophic to eutrophic sites. If our result is general, it implies a positive feedback in phosphorus availability, wi th a proportionately greater percentage of phosphorus loading being bi ologically available longer as phosphorus loading increases. This patt ern is supported by the significantly higher tissue phosphorus content of the seagrass, Thalassia testudinum, collected from the eutrophic i nner bay site. Over time, this effect may tend to cause a shift from p hosphorus to nitrogen limitation in some calcareous marine environment s.