Abstract
The Wadden Sea is a shallow intertidal coastal sea, largely protected by barrier islands and fringing the North Sea coasts of The 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 levelling 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.
Highlights
Increased nutrient fluxes to the coastal ocean are one of the main drivers for coastal change, having a pronounced impact on phytoplankton biomass and primary production, harmful algae blooms, seagrass, green macroalgae blooms, and water transparency (Cloern, 2001; Boesch, 2002; Orth et al, 2006; Smetacek and Zingone, 2013)
The present results show that riverine nutrients are a major driver of the long term phytoplankton and nitrogen dynamics in the Wadden Sea
Based on the above discussion we suggest, that due to decreased riverine nutrient loads both summer chlorophyll a – as an indicator of the phytoplankton biomass and phytoplankton growth potential – and the autumn concentrations of NH4 + NO2 as an indicator of organic matter turnover intensity have declined over the entire Wadden Sea
Summary
Increased nutrient fluxes to the coastal ocean are one of the main drivers for coastal change, having a pronounced impact on phytoplankton biomass and primary production, harmful algae blooms, seagrass, green macroalgae blooms, and water transparency (Cloern, 2001; Boesch, 2002; Orth et al, 2006; Smetacek and Zingone, 2013). Frontiers in Marine Science | www.frontiersin.org van Beusekom et al. Wadden Sea Eutrophication in the above mentioned eutrophication symptoms can be expected (e.g., Breitburg et al, 2018). First signs of coastal eutrophication like enhanced algae blooms were already observed in the 1950s and in Europe. Decisions to combat eutrophication were taken since the 1970s and 1980s (de Jong, 2007). Decreasing eutrophication trends have been observed for instance in European waters like the North Sea, Danish coastal waters or in the Baltic (e.g., Carstensen et al, 2006; Emeis et al, 2015; Andersen et al, 2017) during the 2000s or in Chesapeake Bay (Harding et al, 2016; Lefcheck et al, 2018) showing that a reversal of eutrophication is possible
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