Influence of microphytobenthos on the nutrient flux between sediment and water- a laboratory study

Abstract

The importance of microphytobenthos as a regulator of the flux of inorganic phosphorus and nitrogen between sediment and water was investigated in a laboratory study. Sediment from the SE Kattegat was incubated in nutrient-enriched seawater during exposure to varying light quantities (0, 5, 10, and 30 pE m-' S-'). Microphytobenthos growth was limited by both inorganic nutrients and light quantity. When exposed to no-light conditions, chlorophyll a content of the sediment decreased only slightly and remained at an almost constant level (50 to 55 mg m-2) for several weeks, and increased rapidly when exposed to light. A relationship was observed between light quantity and the rate and direction of the flux of NH: and PO:-, but not NO;, between the sediment and overlying water The indirect influence of the microphytobenthos on the nutrient flux, i.e. by changes in the oxygen concentration, seemed more important than the direct effect by uptake by the algae. The rate of release of phosphate from the sediment exposed to darkness was up to 0.2 mm01 PO4 m-2 d-l The maximum mean rate of ammonia release was 1.6 mm01 m-' d-' in the dark and ca 0.1 mm01 m-2 d-' at 5 yE S-'. When the amount of light was 3 10 pE m-' S-', the microbenthic activity (oxygen production and increased nutrient requirements) prevented the release of PO4 and NH4 from the sediment.