A high resolution estimate of the inorganic nitrogen flux from the Scheldt estuary 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 coastal 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 local water mass, are limited by the supply of inorganic nitrogen from the various bordering estuaries. We demonstrate using the case of a typical heavily polluted macrotidal estuary, the Scheldt in Belgium and the Netherlands, that the short duration of the algal blooms requires estuarine flux estimation methods with a high temporal resolution. We use the fully transient, multicomponent reactive transport model CONTRASTE to compute inorganic nitrogen fluxes through the mouth of the Scheldt estuary into the North Sea. The model simulations use a detailed dataset of upstream river discharges and solute 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 algal bloom by 100%.