Long-term variability of winter nitrate concentrations in the Northern Wadden Sea driven by freshwater discharge, decreasing riverine loads and denitrification

Abstract

The hypothesis of a recent reversal in the eutrophication of the Wadden Sea and the potential of inshore waters in denitrification is explored. Salinity, temperature and nitrate concentrations in the List Tidal Basin (Northern Wadden Sea) have been measured about twice weekly since 1984. Salinity has a clear seasonal cycle with lowest salinities of about 27 in late winter and highest salinities of about 31 in summer. Mean annual deviations from the long-term mean salinity correlate significantly with riverine freshwater discharge. Winter nitrate concentrations are generally high (about 50 lM on average). The major part of the variability is related to salinity (*35%). Temperature had a minor impact (*1%). Superimposed on this, a long-term decrease of about 1 lM per year was found. Together, these processes account for about 45% of the nitrate variability. The long-term decrease of about 2% per year is similar to continental riverine trend in total nitrogen loads. In contrast to the List Tidal Basin, salinity explained more than 90% of nitrate variability in the off-shore German Bight. Salinity (30) normalised winter nitrate data of the German Bight also show a long-term decreasing trend. Most of the List Tidal Basin data are either on or below the nitrate-salinity relation found in the German Bight. This observation suggests that denitrification has a major impact on the winter nitrate concentrations in the Northern Wadden Sea compared to the German Bight. It is hypothesised that a large part of the unexplained variability is related to weather-dependent changes in residence time of tidal water masses in the Wadden Sea and circulation patterns within the German Bight.

228Ra, 226Ra, 224Ra and 223Ra in potential sources and sinks of land-derived material in the German Bight of the North Sea: implications for the use of radium as a tracer

“…Moreover, weather-dependent fluctuations in the residence time of Wadden Sea tidal water masses have recently been hypothesised as pivotal in controlling regional eutrophication (van Beusekom et al 2008), and is has been shown that pore waters from tidal flats represent an important part of the nutrient and carbon cycle in the North Sea (Billerbeck et al 2006a, b). Therefore, additional tracers that could improve evaluations of groundwater and pore water discharge, water mass residence times or exchange rates would be a useful tool in this environment.…”

Nitrous oxide production associated with coastal marine invertebrates

“…The N 2 O emission potential of marine invertebrates has so far been neglected, although coastal marine sediments are presumably hot spots of N 2 O emission, since they are densely inhabited by filter-and depositfeeding invertebrates (Williams et al 2004, Philippart et al 2007) and exposed to high nitrate concentrations (Kieskamp et al 1991, Van Beusekom et al 2008. High N 2 O emission can also be expected from aquaculture facilities in which animals are typically reared at high densities and high nitrate concentrations.…”

Helgoland Roads, North Sea: 45 Years of Change

“…This is likely due to a reduction in nutrient input (Hickel et al 1993;van Beusekom 2004;van Beusekom et al 2008) and also due to the fact that the current direction is less influenced by coastal waters, and more open North Sea water reaches the site than 45 years ago (Fig. 6).…”