From historical backgrounds towards the functional classification of river phytoplankton sensu Colin S. Reynolds: what future merits the approach may hold?

River phytoplankton has been studied to understand its occurrence and composition since the
end of the nineteenth century. Later, pioneers addressed mechanisms that affected river phytoplankton by ‘‘origin of plankton’’, ‘‘turbulent mixing’’, ‘‘flow heterogeneity’’, ‘‘paradox of potamoplankton maintenance’’ and ‘‘dead zones’’ as keywords along the twentieth century. A major shift came with the recognition that characteristic units in phytoplankton compositions could be linked to specific set of environmental conditions, known as the ‘‘Phytoplankton
Functional Group concept’’ sensu Reynolds. The FG concept could successfully be applied to river phytoplankton due to its close resemblance to shallow lakes phytoplankton. The FG approach enables one to separate the effects of ‘‘natural constraints’’ and ‘‘human impacts’’ on river phytoplankton and to evaluate the ecological status of rivers. The FG classification has mainly been advocated in the context of how the environment shaped the functional composition
of phytoplankton. It may be further developed in the future by a trait-based mechanistic classification of taxa into FGs, and by the exact quantification of FGs on ecosystem functioning. These improvements will help quantify how global warming and human impacts affect river phytoplankton and corresponding alterations in ecosystem functioning.