Wadden Sea Eutrophication: Long-Term Trends and Regional Differences

Abstract

The Wadden Sea is a shallow intertidal coastal sea, largely protected by barrier islands and fringing the North Sea coasts of Netherlands, Germany, and Denmark. It is subject to influences from both the North Sea and major European rivers. Nutrient enrichment from these rivers since the 1950s has impacted the Wadden Sea ecology including loss of seagrass, increased phytoplankton blooms, and increased green macroalgae blooms. Rivers are the major source of nutrients causing Wadden Sea eutrophication. The nutrient input of the major rivers impacting the Wadden Sea reached a maximum during the 1980s and decreased at an average pace of about 2.5% per year for total Nitrogen (TN) and about 5% per year for total Phosphorus (TP), leading to decreasing nutrient levels but also increasing N/P ratios. During the past decade, the lowest nutrient inputs since 1977 were observed but these declining trends are leveling out for TP. Phytoplankton biomass (measured as chlorophyll a) in the Wadden Sea has decreased since the 1980s and presently reached a comparatively low level. In tidal inlet stations with a long-term monitoring, summer phytoplankton levels correlate with riverine TN and TP loads but stations located closer to the coast behave in a more complex manner. Regional differences are observed, with highest chlorophyll a levels in the southern Wadden Sea and lowest levels in the northern Wadden Sea. Model data support the hypothesis that the higher eutrophication levels in the southern Wadden Sea are linked to a more intense coastward accumulation of organic matter produced in the North Sea.

Interactive impacts of meteorological and hydrological conditions on the physical and biogeochemical structure of a coastal system

“…Water column processes dominate the organic matter turnover in coastal systems, amounting to about 80 % in the deeper parts of the German Bight to around 50 % in the Wadden Sea (Heip et al, 1995;van Beusekom et al, 1999). Based on these estimates, within the outer German Bight, we do not expect a large sensitivity of our results to the resolution of benthic processes, but this may be the case for the shallower sites and the Wadden Sea.…”

“…Surface DISi, DIN, DIP and integrated net primary production rate during July for the years 2012 and 2013 and scenarios 2013-R12, 2013-M12 and 2013-W12. 1990; van Leeuwen et al, 2015). Following the flood event, these interactions have moved away from the coast to further offshore regions of the German Bight.…”

“…12), as these areas were limited by light, rather than by nutrients (see also Loebl et al, 2009). In reality, an even stronger light limitation in the vicinity of the mouth of the Elbe estuary is likely, due to the increase in the SPM towards the Elbe estuary (van Beusekom and Brockmann, 1998;Gayer et al, 2006), which is only partially accounted for by the model (see Appendix B2). It should be noted that the riverine influence within the coastal zone may be overestimated by our simulations, given the lower-thanobserved salinities (Fig.…”

“…Mixing of riverine freshwater with the surrounding saline marine waters at the coasts is driven by a set of hydrodynamical processes intriguingly linked together (for a recent review, see Geyer and Maccready, 2014). The freshwater inputs by rivers may lead to haline stratification in the coastal region, in the absence of any thermal stratification (van Aken, 1986). Horizontal density gradients caused by riverine freshwater inputs govern gravitational circulation (i.e., exchange flow), where the seaward flow of the lighter water at the surface is counteracted by a landward flow of the saltier, denser waters near the seafloor (see Burchard et al, 2018).…”

Long‐term changes in the phytobenthos of the southern Åland Islands, northern Baltic Sea

“…Efforts to reduce nutrient output in e.g. Chesapeake Bay and the Wadden Sea, have partly resulted in recovered macrophyte communities (Kemp et al , van Beusekom et al ). Also, the nutrient levels in Danish waters have decreased during the 2000s, with improved water quality and benthic communities as a result (Rienmann et al ).…”