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 sho w gradients of up to one order of magnitude perpendicular to the coast within the first 30-50 km offshore. Time-series analysis reveals sign ificant 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 nitrog en (-15%), silicate (stable), and chlorophyll are smaller and generall y not statistically significant. The trends in phosphorus reflect a pr oportional and immediate response to decreasing riverine inputs. The o bserved trends. spatial gradients, and long-term seasonal patterns are simulated guile well with a coupled physical-ecological model with hi gh spatial resolution for the coastal zone. The model results indicate no effect of decreasing phosphorus, but an important role for both ni trogen and light climate in primary production and algal biomass. Thes e results have been reproduced in mesocosm experiments. Moreover, thes e experiments indicate a strong response of primary production and chl orophyll to nitrogen load, whereas secondary production (macrobenthos) remains relatively stable. Ecological efficiency of secondary product ion increases from 7% to >10%, with a decrease in nitrogen loading by 50% from the present level. In the absence of a significant nitrogen r eduction 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 gr eat extent. (C) 1998 International Council for the Exploration of the Sea.