Does settling seston release soluble reactive phosphorus in the hypolimnion of lakes?

Organic material collected with sediment traps at depths of 20, 40, and 60 m in mesotrophic, phosphorus‐limited Lake Lucerne was decomposed aerobically at a rate of about 0.01 day −1 at 6°C. However, this material is not a source, but rather a sink, for soluble reactive phosphorus (SRP). As a consequence, during stratification, in the upper part of the hypolimnion concentrations of oxygen and SRP decrease simultaneously, and the ratio POC:PP (particulate organic carbon: particulate phosphorus) decreases significantly with increasing sampling depth and hence with increasing age of the seston. Once deposited at the aerobic sediment surface, the particulate matter continues to accumulate SRP. Since settling particles and the aerobic sediment surface both act as sinks for SRP, anaerobic subsurface sediments are the only possible source for the SRP accumulating in the hypolimnion during summer stagnation. The aerobic sediment layer may however become an apparent source for SRP if the supply from deeper anoxic sediments exceeds its biological‐chemical sorption capacity. Based on these observations and on additional information from the literature it is hypothesized that in phosphorus‐limited lakes decomposing organisms do not release, but rather take up, SRP while mineralizing phosphorus‐poor organic material.