PLANKTONIC AND MICROBIAL CONTRIBUTIONS TO PHOSPHORUS RELEASE FROM FRESH AND AIR-DRIED SEDIMENTS

There was a fivefold increase in the concentration of P (dissolved rea ctive phosphorus) after intact sediment cores from North Lake, Western Australia, were air-dried and reflooded. Effects of plankton and micr obial biomass on phosphorus release were investigated. Freshly killed plankton released P before drying, but less P was released when plankt on biomass was air-dried. Bacteria were important contributors of P du ring drying and rewetting, and their contributions occurred through tw o processes: (1) when sufficient moisture and oxygen were available, b acteria rapidly removed soluble P from the water and incorporated it i nto the particulate phase, the amount of P taken up being positively c orrelated with bacterial respiration; and (2) upon drying, bacterially stored P was partly returned to the water, the release increasing wit h increasing bacterial uptake. The bacterial contribution of P upon dr ying was further investigated by sterilizing air-dried and wet sedimen ts with a low dose of gamma irradiation (10 kGy). It was thus possible to discriminate among P contributed from the native (initial) microbi al biomass (P,) before drying, P released from the increased (develope d) microbial biomass (P-ii) during drying, and P stored in bacteria th at had survived air-drying (P-s). It was estimated that air-drying kil led about 76% of the microbial biomass. At relatively low external P c oncentrations (<1000 mu g L(-1)), the increased P brought about by air -drying was mainly derived from killed microbial biomass.