Abstract
International harmonisation of management goals for eutrophication processes in coastal waters, requiring reduction of discharges and depositions of nutrients and organic matter, needs coordinated assessments and measures. This is especially necessary in open areas, connected by currents and mixing processes with trans-boundary exchanges. Management goals, defined nationally as local thresholds for nutrients and chlorophyll-a, had been applied recently (2006–2014) within international eutrophication assessments in the North Sea (OSPAR) and Baltic Sea (HELCOM). Consistency of thresholds for nitrogen nutrients and chlorophyll-a concentrations is tested by mixing diagrams and correlations between nitrogen nutrients (total and inorganic nitrogen) and chlorophyll-a. Results indicate mean consistent relations, but single deviations as in the continental coastal water of the North Sea surpassed means by a factor up to 5 for chlorophyll-a in relation to inorganic nitrogen. Thresholds differed across national borders significantly. Correlations of thresholds and assed data reflect the degree of regional deviations by comparison. Consistency of regionally applied thresholds can be achieved stepwise, by application of regionally correlated means, by adaptation to mixing and parameter relations, and finally by relations of thresholds to natural background concentrations. By this, consistency of international assessments can be improved generally, allowing coordinated management of open coastal waters.
Highlights
Eutrophication is still one of the most harmful threats for coastal waters (Cloern, 2001)
Thresholds for nitrogen nutrients and chlorophyll-a have been transferred as examples from recent assessments in the North Sea and Baltic Sea (HELCOM, 2017; OSPAR, 2017), together with the assessed recent surface concentrations (2006–2014) and combined with salinities from the ICES database which was the main data source for the assessments as well
Thresholds and recent data were transformed for correlations to square means of 145.23 km2 as grids, approaching ICES boxes in North Sea, or were compiled as basin means in the Baltic Sea from a HELCOM compilation (HELCOM, 2017)
Summary
Eutrophication is still one of the most harmful threats for coastal waters (Cloern, 2001). The coastal ecosystems in the North Sea and Baltic Sea are still affected significantly, despite considerable regional reductions of nutrient concentrations in main rivers discharging to the North Sea (Nienhuis, 2002a; Nienhuis, 2002b a, b, Duarte, 2009, Brockmann et al, 2018, Greenwood, 2019) and to the Baltic Sea (HELCOM, 2018a, 2018b, 2018c). Atmospheric nitrogen inputs to the North Sea were reduced by 30% since 1990 (OSPAR, 2017). Decreasing nutrient discharges to the Baltic Sea were reported for 2011–2016 (HELCOM, 2018a, 2018b, 2018c) but recent stagnations keeping eutrophication at elevated levels. Ninety-seven percent of the open Baltic Sea were still assessed to be below good eutrophication status and 86% of the coastal waters. Levels of nutrient indicators were indicated generally as “furthest away from good status”
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