Planktonic and microbial contributions to phosphorus release from fresh and air-dried sediments

Abstract

There was a fivefold increase in the concentration of P (dissolved reactive phosphorus) after intact sediment cores from North Lake, Western Australia, were air-dried and reflooded. Effects of plankton and microbial biomass on phosphorus release were investigated. Freshly killed plankton released P before drying, but less P was released when plankton biomass was air-dried. Bacteria were important contributors of P during drying and rewetting, and their contributions occurred through two processes: (I) when sufficient moisture and oxygen were available, bacteria rapidly removed soluble P from the water and incorporated it into the particulate phase, the amount of P taken up being positively correlated with bacterial respiration; and (2) upon drying, bacterially stored P was partly returned to the water, the release increasing with increasing bacterial uptake. The bacterial contribution of P upon drying was further investigated by sterilizing air-dried and wet sediments with a low dose of gamma irradiation (10 kGy). It was thus possible to discriminate among P contributed from the native (initial) microbial biomass (P_i) before drying, P released from the increased (developed) microbial biomass (P_ii) during drying, and P stored in bacteria that had survived air-drying (P_s). It was estimated that air-drying killed about 76% of the microbial biomass. At relatively low external P concentrations (<1000 μg L^-1), the increased P brought about by air-drying was mainly derived from killed microbial biomass.