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

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 guile well with a coupled physical-ecological 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. (C) 1998 International Council for the Exploration of the Sea.