BEON-Garden: giftige algen en de reductie van de nutriëntenbelasting, eindrapport
Riegman, R.; Los, H.; Peeters, C. (1998). BEON-Garden: giftige algen en de reductie van de nutriëntenbelasting, eindrapport. BEON Rapport = BEON-report, 98(4). RIKZ: Den Haag. 53 pp.
Part of: BEON Rapport = BEON-report. Programma Bureau BEON: Den Haag. ISSN 0924-6576
The major goal of this project was to establish the ability to compete for nutrients and/or light of different algal species. The effects of a reduction in nutrient discharges on the species composition in the Dutch coastal waters would be more predictable, when knowledge is available on natural selection of algae. Depending on the availability of nutrients the algal species composition in the coastal waters might change. This possibility was simulated in the BEON-GARDEN project in experimentally in the laboratory. Additionally, the achieved knowledge about algal species selection was implemented in the coastal algal growth model MANS-BLOOM {Delft Hydraulics) in order to achieve more reliable predictions on the impact of nutrient discharge reduction scenarios. Amongst others, a dynamic grazing module was implemented in the model. For the laboratory study, we used Alexandrium tamarense, Fibrocapsa japonica, Heterocapsa triquertra, Prorocentrum micans, Gymnodinium simplex, Chrysochromulina polylepis, Emiliania huxleyi type B, Rhodomonas sp., Synechococcus, Chaetoceros socialis, Cymatosira belgica, Ditylum brightwellii, Lauderia borealis, Odontella aurita, Pseudonitschia pungens and Streptotheca tamesis ,Gyrodinium aureolum, Heterosigma akashiwo, and Ceratium tripos. Occasionally, samples from the Marsdiep area (The Netherlands) were used as an inocculum for the cultures to explore the possibility that other algae, not being included in this study, might be good competitors as well. There have been carried out three series of competition experiments. In the first series a mixture of diatoms and non-silicate using algae was exposed in discontinuously diluted batch cultures to various regimes of ammonium-, nitrate, phosphate-, and light-limitation to establish the competitive ability of diatoms. After it had been demonstrated that in principle the diatoms always win the competition, in contrast to non-diatoms, a second series was carried out with mixtures of dinoflagellates and Prymnesiophyceae. Finally, in a third series, it was investigated what the impact of nitrogen speciation (ammonium, nitrate, urea, or humic acids) would be on the algal species composition. In this experiment, diatoms, dinoflagellates, Prymnesiophyceae and a cyanobacterium were included. In all series the performance of species was studied under an ammonium-, nitrate-, phosphorus-, and irradiance-limitation. Additionally, nutrients and irradiance were supplemented in various amounts and frequencies. It could be concluded that: diatoms are better competitors than Prymnesiophyceae or dinoflagellates, but their natural distribution will depend on the availability of silicate. .Prymnesiophyceae (e.g. Phaeocystis, Emiliania, Chrysochromulina) can grow very fast, and are better competitors than dinoflagellates. Every algal species has its own characteristic response to the type of growth rate limiting factor. On basis of these conclusions, combined with available knowledge in the literature, the following statements could be made: the dominance of larger algal species (> 10µm) in algal blooms is most likely due to their reduced losses by microzooplankton and/or their species specific life cycle and the needs for growth in each stage of the life cycle. Eutrophication enhances the change that algal blooms will be toxic and/or harmful, since top-down control increases with enhanced primary production creating a habitat for harmful species. It is realistic to expect shifts in algal species composition in natural nutrient controlled populations as a consequence of changes in nutrient supply ratios. There is no difference between N- and P- controlled systems with respect to the possibility that a phytoplankton bloom will be dominated by harmful species. A shift in the controlling factor will unavoidable lead to a change in algal species composition. This means that species will disappear or appear in the ecosystem. This includes harmful species. Therefore, shifts in NIP discharge ratios facilitate the risk of disturbance which will affect the entire pelagic food web. At higher nutrient discharges these disturbances will have a more catastrophic profile than at low nutrient discharges. In a coastal area where eutrophication problems are not related to the algal species composition, but only to the algal biomass, the N- and P-load should be reduced in such a way that the NIP ratio in the system remains the same. When toxic algae dominate, an additional effort in changing the NIP ratio in the coastal area by manipulation of the nutrient discharges could be considered. A powerful tool for coastal water quality management would be an empirically established relationship between the occurrence of harmful algal blooms in relation to residence time and nutrient load of a coastal area.
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