Plankton in large rivers : ecological and ecotoxicological importance

Celia Joaquim-Justo
University of Liege, Belgium
Celia.Joaquim-Justo@ulg.ac.be

Planktonic organisms are key components of large river ecosystems as they are at the basis of the food chain. Classically, it is considered that the food chain of these ecosystems is based on phytoplankton which is grazed by metazooplankton (mainly rotifers, cladocerans and copepods), the latter being in turn preyed upon by planktivores (fish and macroinvertebrates). Carbon (and thus energy) fluxes along this “traditional” food web has been quantified in some large rivers and it appears that it can be important especially when flow constrains are low (low flow) and high rotifer densities are present.
During the last decade however, studies have emphasized the potential importance of microbial food webs in carbon (energy) transfer from bacterial production to higher trophic levels in such ecosystems.
Indeed, it has been shown that protists (ciliates and flagellates mainly) ingest a significant part of the bacterial production that takes place in the potamon. Since many metazooplanktonic organisms can prey on protists and some can even feed directly on bacteria, energy transfers from the bacterial compartment to the traditional food web via these trophic interactions are potentially high especially considering the high abundance and high turn over rates of the organisms constituting the microbial food web.

Besides their ecological importance, planktonic organisms are also precious indicators for the assessment of water quality in large rivers. Rotifers and ciliates, for example, have been extensively used to characterise the saprobity of large rivers.
Planktonic organisms are also used in toxicity tests; a decisive advantage of using these organisms in bioassays is that their life cycle is short (a few days to a few months); so the tests developed can easily be performed on the whole life cycle of the test organisms and are thus susceptible of being more representative and more sensitive to any perturbation (pollution) of the medium. Successful efforts have been made to develop easy, rapid, cheap and sensitive tests with these organisms.

Such bioassays are more particularly essential for the risk assessment of existing an newly produced chemicals. Indeed, the risk assessment procedure as performed in the EU (for example) is based on the comparison of predicted environmental concentrations (PECs) of the substance under evaluation to predicted no effect concentration (PNECs). PECS are calculated through models based on the physico-chemical properties of the chemical and the processes involved in its production, processing and uses. PNECs are based on bioassays using micro-organisms, planktonic organisms and fish together with safety factors.

Planktonic organisms are also used for the development of early warning tests, the advantage of such tests being to provide a highly sensitive system that will allow a precocious detection of a pollution and thus limit the consequences of the pollution problem before damages caused to the environment are important.