Microcystins, toxins produced by cyanobacteria, may play a role in fish kills, although their specific contribution remains unclear. A better understanding of the eco-toxicological effects of microcystins is hampered by a lack of analyses at different trophic levels in lake foodwebs. We present 3 years of monitoring data, and directly compare the transfer of microcystin in the foodweb starting with the uptake of (toxic) cyanobacteria by two different filter feeders: the cladoceran Daphnia galeata and the zebra mussel Dreissena polymorpha. Furthermore foodwebs are compared in years in which the colonial cyanobacterium Microcystis aeruginosa or the filamentous cyanobacterium Planktothrix agardhii dominated; there are implications in terms of the types and amount of microcystins produced and in the ingestion of cyanobacteria. Microcystin concentrations in the seston commonly reached levels where harmful effects on zooplankton are to be expected. Likewise, concentrations in zooplankton reached levels where intoxication of fish is likely. The food chain starting with Dreissena (consumed by roach and diving ducks) remained relatively free from microcystins. Liver damage, typical for exposure to microcystins, was observed in a large fraction of the populations of different fish species, although no relation with the amount of microcystin could be established. Microcystin levels were especially high in the livers of planktivorous fish, mainly smelt. This puts piscivorous birds at risk. We found no evidence for biomagnification of microcystins. Concentrations in filter feeders were always much below those in the seston, and yet vectorial transport to higher trophic levels took place. Concentrations of microcystin in smelt liver exceeded those in the diet of these fish, but it is incorrect to compare levels in a selected organ to those in a whole organism (zooplankton). The discussion focuses on the implications of detoxication and covalent binding of microcystin for the transfer of the toxin in the foodweb. It seems likely that microcystins are one, but not the sole, factor involved in fish kills during blooms of cyanobacteria.
Floodplains of the rivers Rhine and Meuse in the Netherlands are moderately polluted with heavy metals and toxic compounds. In these floodplain areas a number of nature rehabilitation programmes are being planned and executed. The question arises whether or not the contaminants of concern pose a risk to organisms exposed, and, if so, whether this will hinder the expected ecosystem recovery. This study focuses on the effects of contaminants on aquatic and terrestrial macro-invertebrates in the field situation. In three shallow lakes in the floodplains of the river Rhine, showing different degrees of contamination, chemical analyses and laboratory bioassays have been carried out on sediment samples together with intensive biological field surveys (a so called TRIAD-approach). A nearly identical set-up was chosen to assess 18 terrestrial floodplain sites. The concentrations of several contaminants exceed generic critical risk levels. Risk assessment models show that actual effects on top predators cannot be excluded. Species of lower trophic status might be at risk as well. In the aquatic compartments chemical, ecotoxicological and ecological results point in the same direction. They can be summarised as moderate biological effects, which can mainly be attributed to the elevated levels of a mixture of priority pollutants (metals and polyaromatic hydrocarbons). No large differences were found among the three lakes. At terrestrial sites, however, fewer effects were found in field surveys and bioassays, while the chemical analyses revealed the same levels of pollution as in the lakes study. This suggests a lower bioavailability of the contaminants than expected. In conclusion, no extreme effects have been found. This conclusion only partly supports the predictions made by present day-risk assessment models. The advantages of nature rehabilitation, that is, improvement of ecological quality through habitat diversity, seem to be larger than the disadvantages of increasing toxic stress by exposing a larger diversity of species to floodplain contaminants. To confirm this statement rehabilitation programmes should include overall as well as in-depth monitoring studies.
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