Patterns and trends in nutrients and phytoplankton in Dutch coastal waters: comparison of time-series analysis, ecological model simulation, and mesocosm experiments

Abstract

In the Dutch coastal zone, nutrient and chlorophyll concentrations show gradients of up to one order of magnitude perpendicular to the coast within the first 30-50 km offshore. Time-series analysis reveals significant decreasing trends for dissolved inorganic phosphorus (40%) and total phosphorus (35%) and an increase in the dissolved inorganic N:P ratio from 25-30 to 40-55 over the period 1988-1995. Trends in nitrogen ( 15%), silicate (stable), and chlorophyll are smaller and generally not statistically significant. The trends in phosphorus reflect a proportional and immediate response to decreasing riverine inputs. The observed trends, spatial gradients, and long-term seasonal patterns are simulated quite well with a coupled physicalecological model with high spatial resolution for the coastal zone. The model results indicate no effect of decreasing phosphorus, but an important role for both nitrogen and light climate in primary production and algal biomass. These results have been reproduced in mesocosm experiments. Moreover, these experiments indicate a strong response of primary production and chlorophyll to nitrogen load, whereas secondary production (macrobenthos) remains relatively stable. Ecological efficiency of secondary production increases from 7% to >10%, with a decrease in nitrogen loading by 50% from the present level. In the absence of a significant nitrogen reduction in coastal waters, the mesocosm results cannot be related to field data as yet. However, the expectation is that reducing nitrogen inputs will not affect productivity at higher trophic levels to any great extent.1998 International Council for the Exploration of the Sea

Our evolving conceptual model of the coastal eutrophication problem

“…Nixon et al 1996, Eyre & Balls 1999. For example, nutrient budgets show that: the coastal ocean is a source of N that overwhelms all human sources to the Straits of Georgia/Juan de Fuca (Mackas & Harrison 1997); internal recycling is the dominant source of N in Aarhus Bay, Denmark (Jør-gensen 1996); local sources from fish farming dominate in the Archipelago Sea (Bonsdorff et al 1997b); municipal wastewater contributes 60% of the N input to Long Island Sound (National Research Council 2000); riverine loading is the dominant source of nutrients to the Dutch coastal zone (De Vries et al 1998). Among these different coastal ecosystems, there is great divergence in the scale of human disturbance of the pristine nutrient budgets.…”

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

“…Indeed, good spatial correlations are found in the North Sea between annual primary production and winter nitrate concentrations (e.g. van Beusekom and Diel-Christiansen 1994; de Vries et al 1998;Hydes et al 1999). However, due to the potentially large influence of denitrification, nitrate is a less useful indicator of the eutrophication status in shallow coastal areas.…”

Global Warming Can Lead to Depletion of Oxygen by Disrupting Phytoplankton Photosynthesis: A Mathematical Modelling Approach

“…Phytoplankton (u) produce oxygen (c) with the per capita rate A f (c) during the daytime photosynthesis. Function M(c, u) describes the rate of oxygen decrease due to different processes such as its consumption by phytoplankton during the night-time, breathing of marine fauna, decay in the oxygen concentration due to (bio)chemical reactions in the water (e.g., decay and remineralization of detritus) [22,[30][31][32][33][34], etc. The phytoplankton growth rate g(c, u) is known to be correlated with the rate of photosynthesis [35].…”