Phytoplankton primary production, nutrient concentrations and turbidity were monitored at three stations in the Oosterschelde during 1980-1984 as part of an ecosystem study.From comparisons of dissolved nutrient ratios with the nutrient requirements of phytoplankton, and of ambient nutrient concentrations with half-saturation constants for nutrient uptake by natural phytoplankton populations it was concluded that silicate was a limiting nutrient for diatoms after the spring bloom until the end of the summer. Dissolved inorganic nitrogen and phosphate were not considered to be limiting to phytoplankton growth.In general, the phytoplankton growing season started during the first fortnight of April and ended at the end of September. Column production in the whole Oosterschelde varied between 201 and 540 g C m '2 yr -and was, on average, 25 % higher in the western part than in the eastern part. 'Basin' production in the Oosterschelde varied between 120 and 466 g C m-2 yr -' and was, on average, 55% higher in the western part than in the eastern part; this difference could be explained by differences in the ratio of euphotic depth to mean depth of the compartments.Estimated carbon-specific growth rates in the eastern part varied between < 0.1 and 3 d-' and in the western part between < 0.1 and 1 d -1 . This difference could be explained by the great differences in depth of the compartments. Carbon-specific growth rates are discussed in relation to phytoplankton loss rates. It is suggested that in the eastern part sedimentation must be an important sink for phytoplankton.
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
SUMMARYIn the Dutch sector of the North Sea, sea bird densities are recorded bi-monthly by using airborne striptransect monitoring. From these data we try to estimate: (i) high-resolution spatial patterns of sea bird densities; (ii) low-resolution spatial-average bird densities for large areas; and (iii) temporal changes in (i) and (ii), using data on Fulmaris glacialis as an example. For spatial estimation, we combined Poisson regression for modelling the trend as a function of water depth and distance to coast with kriging interpolation of the residual variability, assuming spatial (co)variances to be proportional to the trend value. Spatial averages were estimated by block kriging. For estimating temporal differences we used residual cokriging for two consecutive years, and show how this can be extended to analyse trends over multiple years. Approximate standard errors are obtained for all estimates. A comparison with a residual simple kriging approach reveals that ignoring temporal cross-correlations leads to a severe loss of statistical accuracy when assessing the significance of temporal changes.This article shows results for Fulmaris glacialis monitored during
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