The use of enclosed plankton communities in aquatic ecotoxicology : fate effects of mercury, cadmium and selected aromatic organics in a marine model ecosystem

J. Kuiper

Research output: Thesisexternal PhD, WU


<p/>Most investigations in ecotoxicology are carried out in the laboratory. Although laboratory experiments are indispensable and yield useful information, it is difficult if not impossible to extrapolate results of short-term laboratory tests currently in use to real field situations. The need in aquatic ecotoxicology for experiments with complex systems, more closely approximating natural conditions, led to the use of large, flexible plastic bags, isolating part of the ecosystem, and suspended in natural waters. This approach has been used here to study marine plankton communities. The general aim of the study was to develop a method which could act as an intermediate between laboratory and field, for determining fate and effects of pollutants in low concentrations.<p/>This thesis summarizes results of several experiments, reported in detail in different papers, which are partly reproduced in the Appendix.<p/>Chapter 2 summarizes the materials and methods used. During the experiments, which usually lasted 4 - 6 weeks the development of the phytoplankton, the zooplankton and the bacteria was followed (biomass and species composition), as well as several physico-chemical factors, affecting the organisms on the various trophic levels.<p/>In the first experiments (Chapter 3) it was shown that Dutch coastal plankton communities, exposed to identical experimental conditions inside simultaneously filled plastic bags (contents 1.5 m <sup><font size="-1">3</font></SUP>), developed in very similar ways. Therefore this method could be applied for toxicological research.<p/>From 1975 onwards several experiments were carried out in which mercury, cadmium and selected organic compounds (phenol, 4-chlorophenol (4CP), 2,4-dichlorophenol (DCP) an 3,4-dichloroaniline (DCA) were added as pollutants. Most experiments were carried out with Dutch coastal water. In 1979 we participated in POSER (Plankton Observations in Simultaneous Enclosures in Rosfjorden), a project carried out in a south Norwegian fjord. There the fate and effects of mercury on plankton communities in enclosures of different dimensions were studied. Chapter 4 is devoted to the fate and effects of the model pollutants in the different experiments.<p/>The fate of the selected compounds was generally in accordance with the expectations. Cadmium remained in the system and accumulated very slowly into the sediment, which collected at the bottom of the bags. DCA was not degraded, the other organic compounds were degraded in the plankton system as well as in laboratory die-away tests. Some differences were, however, sometimes found between laboratory and semi-field experiments. Sometimes phenol and 4CP were degraded more slowly in the plankton community than in the simultaileous die-away tests, carried out using water from the enclosures. This difference was probably caused by factors working on the ecosystem level, such as competition between different bacteria for substrate and inorganic nutrients and competition between bacteria and phytoplankton for inorganic nutrients.<p/>This finding is of practical importance for the extrapolation of results of laboratory degradation tests to the field. Laboratory tests are often performed in the dark so that competition between bacteria and algae for nutrients is generally absent. Moreover nutrients are often added in large amounts to laboratory cultures. For this reason laboratory tests could easily overestimate the degradation rate in oligotrophic environments, such as most seas and oceans during a large part of the year.<p/>The fate of mercury in the modelecosystems was also unexpected. After addition of mercury(II) chloride to the systems concentrations decreased in the water and increased in 'the sediment and methylation of mercury was also found. A large part of the added mercury however, was lost to the atmosphere, probably as volatile metallic mercury.<p/>Another important result was the indication in one experiment that toxic and stable intermediates were formed during the degradation of 4CP and DCP. In most cases biodegradation tests and toxicity tests are carried out separately. In the model ecosystem fate and effects were studied simultaneously. this also appeared to be important in the case of 5-nitrofuroicacid-2, which lost both the nitrogroup and its toxicity within one day after addition to the enclosures, probably as a result of exposure to light.<p/>In most experiments effects on the enclosed plankton community could be shown after the addition of the model pollutants. Concentrations causing these effects were relatively low as compared with results of laboratory toxicity tests. Apart from this sensitivity of the test system, it is important that it appeared to be possible to experiment with various organisms on a single trophic level, as well as with different trophic levels in one system. Addition of contaminants led to effects on these interactions on, or between trophic levels.<p/>In many cases differences were found in the species composition of the plankton community between controls and contaminated systems. Changes in the relative abundance of organisms on different trophic levels were also found.. These changes may be important in field situations, because small changes, such as the disappearance or the inhibition of one species, can cause large changes in the ecosystem by complex interactions.<p/>Because mercury, cadmium, 4CP and DCP were used in the same concentrations in different experiments, information on the reproducibility of the experimental results was generated. This information is given in Chapter 5. Notwithstanding the fact that the starting conditions differed very much between experiments, it can be concluded that the toxicological results, i.e. fate of the added compounds and the concentrations having effects, are quite reproducible. The intensity of the effects depended very much on the situation in the plankton community.<p/>Chapter 6 is devoted to the possibilities for extrapolation of the results to field situations. Information from the experiments described here and from the literature suggest that the development of a plankton community inside a plastic bag, is qualitatively similar to that of the "free" community at least for periods of up to 4 weeks. With respect to the effects found, it was shown that mercury had similar effects on plankton communities enclosed at different locations (Canada, Scotland, Norway, The Netherlands). These results indicate that results obtained with the model plankton systems can be extrapolated to other sea areas. On the other hand this simple method provides just a first step, and the model systems must be seen as an intermediate between laboratory and field situations. The model system is less complex than the field situation. Even if this less complex system is investigated in a relatively simple way, as was done in this study, concentrations showing effects are among the lowest reported in the literature.<p/>In Chapter 7 some remarks are made on the optimal set-up of ecotoxicological experiments using enclosures. For ecotoxicological experiments, excluding fish or other larger carnivores, bags containing 1 - 2 m <sup><font size="-1">3</font></SUP>appear to be sufficiently large in relatively eutrophic waters, such as Dutch coastal waters. In more oligotrophic environments enclosures with a contents of approximately 10 m <sup><font size="-1">3</font></SUP>are preferred to enable larger samples to be taken. The optimal duration of an experiment is probably 4 - 6 weeks, because the enclosed community diverges more and more from the natural situation with time, making extrapolation of results to field situations more questionable.<p/>The enclosure method can be applied to at least two field of ecotoxicological problems : to assess the impact of specific dumping events and other environmental problems, and to validate laboratory toxicity and biodegradation tests.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Koeman, J.H., Promotor
  • den Hartog, C., Co-promotor, External person
Award date23 Nov 1982
Place of PublicationWageningen
Publication statusPublished - 1982


  • toxicology
  • chemicals
  • pollution
  • aquatic organisms
  • plankton
  • marine areas
  • saline water
  • methodology
  • biological techniques
  • experiments
  • equipment
  • ecotoxicology
  • bioaccumulation

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