FACTORS REGULATING THE FLUX OF PHOSPHATE AT THE SEDIMENT - WATER INTERFACE OF A SUBTROPICAL CALCAREOUS LAKE: A SIMULATION STUDY WITH INTACT SEDIMENT CORES W. ECKERT, A. NISI-LRI and R. PARPAROVA Israel Oceanograph;c and Limnological Research, The Ytgal A lion Kmneret Lamnologlcal Laboratory, P.O.Box 345, Tiberlas 14-102, Israel Abstract. Different factorswhichinteractively controlthe flux of soluble reactive phosphorus (SRP) at the sediment- water int~ace (SWI) of Lake Kinneret were studied seasonally. The influence ofpH, Eh and microbial activity on SRP flux at the SWI was inv~tigatedby manipulating the conditions in the overlying water of intact sediment cores. The calculated diffusive SRP flux out of the sediment was lower in cores sampled during winter and spring than during the period of amixis. Potential SRP release, as measured in the absence of microbial activity, was strongly cnhanced upon the transition ti'omoxic to anoxic conditions indicating P release from iron(lll)-bound phosphorus. In springand sunmaer cor~, an enhanc~ SRP flux fi'omsediments at pH 7 m comparison to plt 8 indicated P release from carbonate-bound P which sedimented previously as result of high pH values during the algal spring bloom. Microbial uptake at the SWI was the most important sink for SRP and no net-flux occured under oxic conditions. The higher net-flux of P under anoxic conditions was linked to carbon limitation of the bacteria at the SWI. Keywords: phosphorus, P flux, microbial activity, redox, simulation, Lake Kirmerel,sediment, accumulative P release 1. Introduction The regulatory' effect of sediments for the phosphorus (P) cycle of freshwater lakes is well established (e.g. Li et al., 1972; Baccini, 1985; BostrOm et al., 1988). On one hand, sediments may act as a P sink by accumulating high concentrations of allogenic apatite minerals and orgamc and inorganic P complexes. On the other hand, they may act as a P source releasing phosphorus back into the water column. Sedimentary P release is triggered by the concentration gradient between dissolved phosphorus in the porewater and the ovcrlying water (OW). Elevated P concentrations in porewater are the result of dissolution and desorption processes and mineralization of organic-bound P (BostrOm et al, 1982: Encll and I.ofgren, 1988). Following the classical works of Einsele (1936), Ohlc (1938) and Mortimer (1941) sedimentary P cycling is considered to be linked mainly to the iron cycle. As the redox conditions drop below ca. 200 mV, iron(Ill) is reduced chemically causing the mobilization of the sorped phosphate (Bostrom et al., 1982). In sediments with an oxidized surfacc laycr, upward dill'using Fc > is reoxidized Ibrrning a micro-layer with a high sorption capacity for phosphate at the sediment water interface (SWI) (Lofgren & Bostrom, 1989; Sinke, 1992). The stability of the iron(III)-P complex is strongly pH-dependent. An increase in pH favors P desorption due to substitution of phosphate by OH. In calcareous lakes this P may be re- precipitated as hydroxy apatite or adsorbed to CaCO 3 (Bostr~3m ct al. 1988). In contrast to Water, Air and Soil Pollution 99: 401-409, 1997. 9 1997 Kluwer Academic Publishers. Printed in the Netherlands.