Variability of continental riverine freshwater and nutrient inputs into the North Sea for the years 1977–2000 and its consequences for the assessment of eutrophication

Abstract

We determined the monthly and annual riverine freshwater, nitrogen (N) and phosphorus (P) loading into the North Sea from Belgium, The Netherlands, and Germany for the years 1977-2000. An average of 133 km 3 yr 21 of the 309 km 3 yr 21 precipitation into the watershed is carried by the rivers into the sea. Total freshwater discharge fluctuates with a strong 6-7 yr periodicity, is strongly correlated with precipitation, and exhibits a slight long-term decrease. The temporal changes of regional patterns of precipitation lead to changing ratios of annual discharge of the western rivers compared to the eastern rivers, varying between 2.2 and 3.5. The long-term oscillations in discharge were more pronounced as discharge increased. The annual means of total and dissolved inorganic N and P loads were estimated to be 722 and 582 kt N yr 21 and 48 and 26 kt P yr 21 , respectively. The monthly N loads were much more strongly correlated with discharge, compared to the monthly P loads. Total N and P as well as dissolved inorganic N also demonstrated a 6-7 yr periodicity. The annual N loads decreased by about 17 kt N yr 21 from 1977 to 2000. The total phosphorus and phosphate loads decreased from about 80 and 50 kt P yr 21 in the 1980s to 25 and 12 kt P yr 21 , respectively, in the 1990s. The western rivers contributed the major part of the nutrient loads. The long-term oscillations in their nutrient loads were much more pronounced, compared to the eastern rivers. The area-specific loading rates estimated for all rivers are comparable to earlier estimates using shorter data records, smaller sample sizes, and a less complete watershed monitoring program. The monthly and annual average N:P ratios and their variability increased considerably for individual rivers during the study interval. These results confirm that the water quality of European continental rivers is strongly influenced by intense land use. They demonstrate the necessity for using long time series monitoring results to assess change and evaluate the effects of climate change on the North Sea coastal ecosystems, using ecosystem models on decadal time scales.

Deciphering associations between dissolved organic molecules and bacterial communities in a pelagic marine system

“…DOC concentrations ranged between 68 and 170 μmol C l − 1 and correlated significantly with salinity (Pearson's r = − 0.94, Po0.001; Figure 3a). This finding indicates conservative mixing of high-DOC freshwater, largely consisting of riverine input from Elbe, Weser and Ems at the German coast, as well as Rhine, Meuse and Scheldt in the Netherlands DOM-bacteria associations in the North Sea H Osterholz et al (Radach and Pätsch, 2007), and Atlantic water with lower DOC concentrations (Hansell and Carlson, 1998). According to Thomas et al (2005), the lowsalinity and high-DOC Baltic Sea inflow impacts the North Sea water salinity and DOC concentration in the area along a narrow strip off the Norwegian coast.…”

“…The main water exchange with the Atlantic Ocean (51 000 km 3 year -1 inflow and 56 700 km 3 year -1 outflow) occurs via the northern boundary, while through the Dover Strait between the Netherlands and Great Britain~4900 km 3 year -1 enter the North Sea (Eisma and Kalf, 1987). Compared with these numbers, the flow into the system via precipitation, riverine freshwater input and the Baltic Sea in the East is relatively small and sums up to 895-955 km 3 year -1 (Thomas et al, 2005;Radach and Pätsch, 2007). DOM, the dominant form of organic matter transported by rivers (Wetzel, 1984), is an important source of organic carbon and other nutrients (N, P) for coastal microbial metabolism, respiration and primary production (Azam et al, 1983;Tranvik, 1992).…”

The acclimative biogeochemical model of the southern North Sea

“…1). For eight of these rivers, Radach and Pätsch (2007) and Pätsch and Lenhart (2011) presented a detailed quantitative analysis of nutrient fluxes. Besides the fluxes in inorganic form based on direct measurements, fluxes in organic form have been accounted for, first by calculating the total organic material concentration by subtracting dissolved nutrient concentrations from total nitrogen and total phosphorus, and then by assuming 30 % of the organic material to be in particulate form (i.e., POM; Amann et al, 2012).…”

Nitrogen Source Apportionment for the Catchment, Estuary, and Adjacent Coastal Waters of the River Scheldt

“…For the riverine section of the Scheldt, the corresponding standards are 4 and 3 mg N/L, respectively, for summer mean total nitrogen concentrations (CIW 2008). We compare the estimated loads from the Scheldt into the North Sea that result from a particular measure with those from the English Channel and the Seine, Thames, and Rhine rivers (Lacroix et al 2007, Radach andPätsch 2007).…”