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Nutrients and light as factors controlling phytoplankton biomass on the Dutch Continental Shelf (North Sea) in 1988-1990
Peeters, J.C.H.; Haas, H.A.; Peperzak, L.; de Vries, I. (1993). Nutrients and light as factors controlling phytoplankton biomass on the Dutch Continental Shelf (North Sea) in 1988-1990. Report DGW (Directorate-General for Public Works and Water Management), 93(4). Dienst Getijdewateren: 's-Gravenhage. ISBN 90-369-0272-X. 64 pp.
Part of: Report DGW (Directorate-General for Public Works and Water Management), more

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  • Peeters, J.C.H.
  • Haas, H.A., more
  • Peperzak, L., more
  • de Vries, I., more

    In the discussion in the Netherlands on the sanitation of the nutrient loads discharged into the North Sea the question has been raised of whether only the load of phosphorus or the loads of both nitrogen and phosphorus should be reduced. To answer this question it should be known which factor(s) limit(s) phytoplankton where and when, and how fast the marine ecosystems react to load reductions. The maximum phytoplankton biomass depends on several limiting factors. In this study the degree of light limitation is determined by P/Pmax' a ratio of photosynthesis at a light intensity averaged over day and depth, and the maximal photosynthesis. First potentially limiting nutrients (P, N, Si) are identified from the ratios of dissolved inorganic nutrients and with bioassays. The degree of limitation is calculated by V/Vmax', the ratio of the rate of nutrient uptake and the maximum rate of nutrient uptake. The calculations are based on dissolved inorganic nutrient concentrations and published values of uptake constants. Other methods to identify a degree of limitation are the ratio of Particulate Organic Nitrogen/Particulate Phosphate (PON/PP), the ratio of the intracellular glutamine/glutamate (GLN/GLU) and actually measured rates of nutrient uptake. It is shown that in winter, light strongly limits phytoplankton at all locations. In spring, light increases and simultaneously phytoplankton increase to a level where nutrients can become limiting and light also. In general, the average daily irradiance in the water column increases from coast to offshore areas. There is a seasonal trend in the ratios of inorganic nutrients and bioassays, indicating that P is the first potentially limiting nutrient in spring at the locations investigated. The decrease in N concentrations, relative to P concentrations, in the course of the year and allover the North Sea often leads to potential N limitation in summer. There is a spatial tendency for P to be potentially the main limiting factor in the coastal zone and for N to be in the offshore areas. There also is a trend that N-Iimitation increase from spring to summer. Nutrient uptake rates measured at the end of the spring bloom of 1990 indicate that N is actually limiting also at coastal stations. At that time Phaeocystis pouchetii was the predominant species. Silicon is limiting for diatoms at all stations during late spring and summer. The nutrient uptake rates measured sometimes suggest simultaneous limitation by several nutrients. An explanation is that individual species of the phytoplankton have widely differing nutrient requirements. Two bioassay species used in this study did differ considerably in their phosphorus and nitrogen requirements. As a consequence in the same environment, P limited one species and N the other. The pattern of nutrient limitation, data on nutrient discharges and internal processes of the ecosystem indicate that both P and N loads need to be reduced to ensure that algal blooms in spring and summer will decline. N reduction will probably lead to N limitation predominating in the coastal zone of the North Sea. A strategy based solely on P reduction has the drawback that in summer benthic phosphorus reserves are released which may retard the reduction of the P concentrations. The resulting N surplus in the coastal zone might be transported to areas which are predominantly N-Iimited, where it could cause an increase of phytoplankton blooms. Theoretically, there is a risk that an extreme reduction of the P-Ioad will lead to the unwanted dominance of a few phytoplankton species.

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