Phosphorus speciation, transformation and retention in the Scheldt estuary (Belgium/The Netherlands) from the freshwater tidal limits to the North Sea

Abstract

Nutrient delivery to the Belgian coastal zone is to a great extent through the Scheldt estuary. We have measured the phosphate (PO 4), poly-phosphate (poly-P), dissolved organic phosphorus (DOP), particulate inorganic phosphorus (PIP) and particulate organic phosphorus (POP) concentrations at the freshwater tidal limits of the Scheldt estuary, along the salinity gradient and in the coastal zone during one year. In addition, nitrate (NO 3), nitrite (NO 2), ammonium (NH 4), dissolved organic nitrogen (DON) and particulate nitrogen (PN) were determined in the freshwater Scheldt sub-basin and NO 3, NO 2 and NH 4 in the Rupel sub-basin, the Scheldt estuary and the coastal zone. The behaviour of each P species along the continuum from the freshwater tidal limits to the coastal zone is discussed. Phosphorus (P) and nitrogen (N) budgets were made for the Scheldt sub-basin during the productive period (May–September). The main P and N transformation processes were identified as PO 4 sorption, algal uptake and nitrification. Retention of P and N in the entire freshwater tidal area was estimated. In the brackish part of the estuary, there was no net source or sink of dissolved inorganic nitrogen (DIN). Desorption of PO 4 was the most important transformation process for P along the salinity gradient. The PO4 pool increased and the PIP pool decreased, while total P behaved conservatively. Mass balance calculations show that the transformation of particulate P to PO 4 enhances the PO 4 flux from the Scheldt to the coastal zone.