Long-term trends in nutrient budgets of the western Dutch Wadden Sea (1976–2012)

Abstract

Long-term field observations of nitrogen [N] and phosphorus [P] concentrations were used to construct nutrient budgets for the western Dutch Wadden Sea between 1976 and 2012. Nutrients come into the western Dutch Wadden Sea via river runoff, through exchange with the coastal zone of the North Sea, neighbouring tidal basins and through atmospheric deposition (for N). The highest concentrations in phosphorus and nitrogen were observed in the mid-1980s. Improved phosphorus removal at waste water treatment plants, management of fertilization in agriculture and removal of phosphates from detergents led to reduced riverine nutrient inputs and, consequently, reduced nutrient concentrations in the Wadden Sea. The budgets suggest that the period of the initial net import of phosphorus and nitrogen switched to a net export in 1981 for nitrogen and in 1992 for phosphorus. Such different behaviour in nutrient budgets during the rise and fall of external nutrient concentrations may be the result of different sediment-water exchange dynamics for P and N. It is hypothesized that during the period of increasing eutrophication (1976-1981) P, and to a lesser degree N, were stored in sediments as organic and inorganic nutrients. In the following period (1981-1992) external nutrient concentrations (especially in the North Sea) decreased, but P concentrations in the Wadden Sea remained high due to prolonged sediment release, while denitrification removed substantial amounts of N.

Wadden Sea Eutrophication: Long-Term Trends and Regional Differences

“…Also, the present analysis does not suggests a major role of N and P legacies in sediments on the long term eutrophication levels, as the river loads (reflecting interannual dynamics) are a better predictor of the eutrophication level than a simple linear (smoothed) temporal trend. This is in line with the conclusions of Jung et al (2017). Nutrient ratios in rivers impacting the Wadden Sea have increased toward high N/P ratios clearly above the Redfield ratio, with potential consequences for the coastal food webs (see below).…”

“…In case of the rivers Rhine and Maas, we extended the time series to include the December loads of the previous year as suggested by van Beusekom et al (2001), among others based on the fact that the first high winter discharges are in December and on the longer distance between river mouth and Wadden Sea as compared to the Elbe and Weser. Riverine discharges directly into the Wadden Sea like the river Ems and lake IJsselmeer can have strong local effects (e.g., de Jonge and Postma, 1974;de Jonge, 1990;Jung et al, 2017), but less on the adjacent tidal basins. We therefore used only riverine TN and TP loads by the rivers Rhine/Maas as a common driver for the entire southern Wadden Sea.…”

“…They explicitly accounted for sediment import from the North Sea as this part of the Wadden Sea has a sediment deficit due to the construction of a dike (Afsluitdijk) between the Wadden Sea and the former Zuiderzee (now IJsselmeer, see Figure 1). Jung et al (2017) concluded that during the initial eutrophication phase (until 1981) P and N were imported. A net export of N was observed since 1981 and for P since 1992.…”

“…In the present paper, we will describe and compare the eutrophication levels and trends of the international Wadden Sea, a shallow coastal sea along the Dutch, German and Danish North Sea coast, based on long-term observations on nutrients and phytoplankton biomass (chlorophyll a). Since the earliest nutrient measurements in the Wadden Sea during the mid-20th century (Postma, 1954(Postma, , 1966Hickel, 1989) a strong increase in nitrogen and phosphorus concentrations has been documented (e.g., de Jonge and Postma, 1974;Hickel, 1989) reaching maximum values during the 1980s and 1990s (Jung et al, 2017) and decreasing since (e.g., van Beusekom et al, 2001Cadée and Hegeman, 2002). Among the regularly occurring negative effects associated with the increased nutrient inputs are an increased import of organic matter from the North Sea to the Wadden Sea (de Jonge and Postma, 1974), more intense Phaeocystis-blooms (Cadée and Hegeman, 1986), a decline in seagrass distribution (de Jonge and de Jong, 1992), and increased green macroalgal cover (Reise and Siebert, 1994).…”

Temporal change in phytoplankton diversity and functional group composition

“…then, nutrient loads have continuously decreased(Philippart and Cadée, 2000;van Beusekom et al, 2001van Beusekom et al, , 2009Jung et al, 2017), with phosphorus (P) showing a stronger decline than nitrogen (N)(van Beusekom et al, 2019). This suggests that P has become the main limiting nutrient in the region(Philippart et al, 2007;Kuipers and van Noort, 2008;Ly et al, 2014;Leote et al, 2016).…”