Eutrophication

Rivers are the major source of Wadden Sea eutrophication. The nutrient inputs of the major rivers impacting the Wadden Sea continue to decrease at an average pace of about 2.5 % per year for TN and about 5.4 % per year for TP. During the past decade, the lowest inputs since 1977 were observed but these declining trends are levelling out, i.e., the TP trend has stagnated. Phytoplankton biomass (measured as Chla) remains at a low level. In tidal inlet stations with a long-term monitoring, summer phytoplankton levels correlate with riverine TN loads but stations located within the Wadden Sea behave more complex. Regional differences are observed with highest levels in the southern Wadden Sea and in the mouth of the Elbe estuary and lowest levels in the northern Wadden Sea. No standardized macroalgal monitoring is carried out. In Lower Saxony and Schleswig Holstein, macroalgal aerial monitoring is carried out. In Schleswig Holstein, the areal extent (>20 %) is decreasing and the observations from the last decade are clearly below the earliest observations. In Lower Saxony, present extent is below the earliest observations in the 1990s but interannual variation is large and not predictable from nutrient inputs. The trilateral goal is to reach a Wadden Sea that can be regarded as a non-problem area with regards to eutrophication. The aim is to have good ecological status in the sense of the EU Water Framework Directive. Measures to reduce nutrient inputs in the past have led to a significant improvement of the eutrophication status. However, given that nutrient levels, N/P ratios, chlorophyll levels and macroalgal coverage still are clearly above background levels, we conclude that the goal of an eutrophication non-problem area has not been met yet. The aim of the WFD is to reach a good ecological status, not background levels. However, these targets are bilateral and not trilateral precluding a general WFD related assessment for the entire international Wadden Sea