Hydrobiologia 289: 1-7, 1994. J.-P. Desty, C. S. Reynolds & J. Padisdk (eds), Phytoplankton in Turbid Environments: Rivers and Shallow Lakes. D 1994. Kluwer Academic Publishers. Printed in Belgium 1 Are phytoplankton dynamics in rivers so different from those in shallow lakes? C. S. Reynolds l , J.-P. Descy 2 & J. Padisdk 3 1 NERC Institute of Freshwater Ecology, Windermere Laboratory, GB-LA22 OLP Ambleside, U. K; 2 Unite d'Ecologie des Eaux Douces, Facultds Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium; 3 Balatoni Limnol6giai Kutat6intdzete, H-8237 Tihany, Hungary Key words: phytoplankton, rivers, shallow lakes, turbulence, turbidity Abstract This paper introduces a series of contributions to the ninth meeting of the International Association of Phytoplankton Taxonomy and Ecology, held in Belgium during July, 1993. It draws from the original papers a synthesis which supports the view that the successful species in rivers and turbid shallow lakes are selected primarily on their ability to survive high-frequency irradiance fluctuations as they are circulated through steep light gradients. The selective distinction is less than that which discriminates between plankton of deep lakes and shallow lakes or even between clear and turbid shallow ones. River plankton is, however, dependent on fast growth rates but its survival in rivers is aided by a suite of water-retentive mechanisms. The ecology of turbid systems is dominated by physical interactions, those biotic interactions traditionally believed to regulate limnetic communities being suppressed and rarely well-expressed. Introduction The ninth meeting of the International Association of Phytoplankton Taxonomy and Ecology was convened at the Station Scientifique des Hautes Fagnes (SSHF), the field station of the Universit6 de Liege, Mont Rigi, Ardennes, Belgium, between 10 and 18 July, 1993. The original purpose of the meeting was to consider the taxonomy, physiological adaptations and popula- tion dynamics of planktonic algae that inhabit rivers and shallow lakes, on the premise that both kinds of habitat are to be readily perceived as being characteris- tically 'turbulent' and often also 'turbid'. The suppos- edly strong environmental constraints of these habitats might guide wholly analogous selective interactions towards the assembly of analogous community struc- tures. The pre-meeting circular invited contributors to address their papers to the validity, or otherwise, of this contention. The task of this introductory article might then have been simply to provide an editorial context for the collected presentations and to relate the individual contributions to the overall thesis. In fact, few of the contributed papers attempted to quantify the physical character of the location concerned (just how turbulent was it?) or, for that matter, the level of tur- bidity (just how murky was it?), which would permit useful, reasonable comparison to be made among the selection of habitats considered by the authors collec- tively. That this is so does not disappoint the editors; it is simply a recognition that these are concepts which are not well-formed beyond intuitive acceptance and that biologists are only ready or willing to treat them in qualitative terms. Our task is therefore altered to one of seeking patterns, of identifying unresolved problems and of setting future objectives for ecological research into these interesting and important habitats. The volume includes seventeen original papers, nine of which are devoted to rivers, four to standing waters of differing relative transparency and anoth- er two examine the impact of impoundments in river catchments. The remaining two attempt to generalise over both turbid lakes and rivers. The contributions are variously taxonomic (what lives there?), functional (how does it do so?) or synthetic (what generalisations can be made?). Little is made of the environmental differences between rivers and shallow lakes, perhaps