Effects of nitrogen and phosphorus load reduction on benthic phosphorus release in a riverine lake

Abstract

The Lower Havel in Berlin is a polymictic riverine lake, thermally stratified for some days or weeks in summer. It is characterized by a relatively high mean total phosphorus concentration (145 µg l−1) and frequent cyanobacteria mass development in summer. We quantified the potentially mobile P pool in sediments, determined P fluxes based on conventional dialysis sampler-, gel probe- and benthic chamber measurements, and combined this with column experiments and P budget calculations to evaluate whether a) a reduction of P loading would be counterbalanced by P release from sediment, and b) release of redox sensitive P would even increase with reduction of nitrogen loading. The potentially mobile P pool was relatively small (228 t) compared to mean annual external load (190 t a−1) and gross P release (1.2–36 mg m−2 day−1), and was thus of little relevance for long-term P availability. Despite 38 % of P in the sediment being redox sensitive, the seasonal course of P gross release was mainly driven by redox independent organic matter mineralization of diagenetically young surface sediment. Under anoxic conditions, P release was higher than under oxic conditions and to some extent controlled by nitrate under laboratory conditions. However, ambient nitrate availability was too low to affect P release which was more dependent on mineralization and Fe availability than on redox. Therefore, the Lower Havel would benefit from P load reduction but internal P loading would be largely unaffected from further N load reduction.