The production, biomass and structure of phytoplankton in large rivers
Reynolds, C. S.; Descy, J.-P.
Most temperate rivers achieving fourth order or greater carry a distinctive and often well-defined phytoplankton with regular and predictable fluctuations in abundance and composition. This paper attempts to reconcile the maintenance, production ecology and species selection of river plankton with the intuitive difficulties posed by supposedly unidirectional flow, sharply fluctuating discharges and vigourous turbulence. It examines several of the early theories about the maintenance of plankton in river waters, many of which are accepted implicitly even though they have been scarcely challenged or verified. We consider a range of adjacent and interacting habitats, including side-arms and the benthos, as refuges and sources of river plankton. River plankton comprises relatively few of the limnetic genera, even in those instances where it is influenced by intermediate lakes and reservoirs. Physical factors dominate these selective traits. Under appropriate conditions of flow, clarity and temperature, phytoplankton may sustain high levels of photosynthetic production, contributing positively to the carbon metabolism of middle-order rivers. In lower reaches of higher order, however, turbidity may so lower primary productivity that the phytoplankton diminishes and the river ecosystem is driven increasingly by allochthonous carbon. In turn, the capacity for in-channel recruitment of plankton by growth is strongly conditioned by characters of station and hydrological stage. Population increase is often finely balanced against respirational loss, yet, at times, there may be evident a prodigious downstream recruitment to populations. The process is apparently assisted not by enhanced growth but by extended opportunities, which are supposed to be supported by fluvial hydraulic storage and, to an extent, as a function of channel morphology (section, gradient, sinuosity). Nutrients and grazing have rarely been shown to exert critical control over plankton dynamics, except where the continuous and severe physical constraints are alleviated. Perennation of phytoplankton in rivers has to be supported by a facility for periphytic or benthic survival and a testable hypothesis is advanced that a capacity for meroplankty is an essential property of fluvial phytoplankton. The review is illustrated by case studies and, wherever possible, with special emphasis on those carried out on the Danube.