Sediment water exchanges of nutrients and oxygen on tidal flats in the EMS-Dollard estuary☆

Abstract Oxygen and nutrient exchanges have been measured in bell jar experiments in intertidal flats of the Ems-Dollard estuary. At two stations in the outer estuary the exchange of ammonia and silica was correlated with oxygen exchange and light conditions. The Heringsplaat in the Dollard, however, was a sink for nitrate, ammonia and silica throughout the year, in the light and in the dark and even during periods of net oxygen consumption. The difference with the other two stations may be related to the higher nutrient supply and longer emersion period at the Heringsplaat. Phosphate exchange was generally low but in June a release of 1 mmol·m −2 ·d −1 was observed at the Heringsplaat. Estuarine mixing plots show that there was a net release of silica in the Dollard during late summer. The origin of this silica was probably dissolution of suspended material in the water phase. During emersion the inorganic nitrogen nutrients can become nearly depleted at the sediment surface and may limit diatom growth.