A high resolution estimate of the inorganic nitrogen flux from the Scheldtestuary to the coastal North Sea during a nitrogen-limited algal bloom, spring 1995

Massive short-term (4-8 wk) blooms of Phaeocystis have been observed in coa stal North Sea waters in the spring for a number of years now. Researchers have shown that these algal blooms, which lead to eutrophication of the loc al water mass, are limited by the supply of inorganic nitrogen from the var ious bordering estuaries. We demonstrate using the case of a typical heavil y polluted macrotidal estuary, the Scheldt in Belgium and the Netherlands, that the short duration of the algal blooms requires estuarine flux estimat ion methods with a high temporal resolution. We use the fully transient, mu lticomponent reactive transport model CONTRASTE to compute inorganic nitrog en fluxes through the mouth of the Scheldt estuary into the North Sea. The model simulations use a detailed dataset of upstream river discharges and s olute concentrations along with tidal forcings for a 210 day period between December 1, 1994 and June 30, 1995. Our temporally resolved estimate shows that widely used estuarine flux estimation methods which rely on a steady- state approximation underestimate the inorganic nitrogen loading available to sustain primary production in the North Sea during the period of the alg al bloom by 100%. Copyright (C) 1999 Elsevier Science Ltd.