Nutrient dynamics in a back barrier tidal basin of the Southern North Sea: Time-series, model simulations, and budget estimates

Abstract

In the tidal inlet of the back barrier area of Spiekeroog Island (Southern North Sea), nutrient concentrations (silica, phosphate, ∑ nitrite + nitrate) were determined hourly by an autonomously analysing system on a permanently installed time-series station from April 2006 to December 2008. Based on the high frequency of analyses we studied nutrient dynamics on annual, seasonal, and tidal time scales. By comparing the nutrient input to the tidal flat area via freshwater through a flood-gate and pore water discharge from tidal flat sediments, we conclude that nutrients are primarily supplied to the water column by pore water advection, while the freshwater contribution is negligible. To assess the annual nutrient contribution of our study area to the German Bight, we used a numerical Euler-Lagrangian model (EcoTiM) to calculate annual budgets of silica and phosphate. The model results indicate that the back barrier area of Spiekeroog Island exports inorganic silica (128*106 mol a-1), phosphate (3*106 mol a-1), and nitrite plus nitrate (29*106 mol a-1) to the North Sea. Extrapolation of these data to the entire Wadden Sea along the southern North Sea reveals that the back barrier areas export silica and phosphate in the same order of magnitude and nitrite plus nitrate one order of magnitude lower than the combined rivers Elbe, Weser, and Ems.