A historic perspective on Wadden Sea eutrophication

Abstract

In this paper, a reconstruction of the pre-industrial trophic status of the Wadden Sea is presented. A conceptual model is outlined that links the organic matter and nutrient dynamics in the Wadden Sea with riverine nutrient input. Fundamental processes in this model are: a nutrient-limited offshore primary production and the subsequent import of primary produced organic matter from the North Sea into the Wadden Sea. Two approaches have been followed to estimate the production and remineralisation levels under pre-industrial conditions. The first approach is based on present-day relationships between the seasonal cycle of NH4 and NO2 in the western Dutch Wadden Sea and suggests, on average, sixfold lower production and remineralisation rates under pre-industrial conditions (range: four to eight times). The second approach is based on present carbon budgets extrapolated to pre-industrial budgets on the basis of present relationships between winter nutrient concentrations, annual primary production and annual organic matter turnover rates, and suggests a fivefold lower organic matter turnover under pre-industrial conditions (annual primary production: ~55 g C m−2 year−1, annual remineralisation: ~77 g C m−2 year−1). Better pre-industrial light conditions in the Wadden Sea may have allowed a more efficient use of nutrients, a higher annual primary production of about 86 g C m−2 year−1 and annual remineralisation rates of about 108 g C m−2 year−1.