A REVIEW OF REGULATION MECHANISMS OF METAZOAN PLANKTON IN RIVERINE ECOSYSTEMS: AQUATIC HABITAT VERSUS BIOTA

摘要

This review summarizes the key factors of a series of articles about animal plankton in rivers, a topic that remains poorly investigated. Running water, in which sampling processes are variable and often difficult, acts as a constraint as well as a resource for plankton. Assessment of the potamoplankton reveals that good swimmers of the true plankton are mixed with numerous poor swimmers among littoral and epibenthic forms. Small taxa dominate communities in most of rivers all over the world and, in comparison with crustaceans; the successful development of rotifers rests on their shorter development times. According to hydraulic and geomorphological features, the diversity of the habitats and the ability of individuals to avoid washout was examined, decrease in flow creating standing zones favourable to zooplankton development. Many species can find suitable conditions for the development of dense populations in stream habitats and examples of taxa-specific response, regardless of the habitat structure, are observed. While the challenge for riverine plankton remains 'eat to be fruitful and multiply', their activities have been poorly investigated in turbulent conditions, those able to continue growing in a current of 20 cm s~(-1), such as brachionid species, being usually dominant in rivers. Examination of the distribution of plankton populations, in the Middle Loire and in other rivers, showed that the flow regime, which plays a central role in organizing river habitats, explains the presence/absence of these fast-growing organisms. While reservoirs are major sources of crustaceans, lateral exchanges as well as downstream transport are important in determining the plankton. These ecosystems are primarily governed by abiotic factors, biological control taking place after physical control has happened. Because animals arrive by chance in any habitat during dispersion, zooplankton in rivers remains primarily governed by unpredictable physical processes and depends on the age of the water and the availability of habitats. In this way, the hydro-dynamics of river habitats need to be more documented with regard to their key role in the planktonic life. Preservation of the lentic areas remaining at the heart of the matter, the contrasting ecology of the rotifers should be a reliable tool with which to estimate the changes in habitats diversity of main channels, particularly during droughts, while crustacean's diversity would be more representative of flood plain annexes, particularly during flood events.