EFFECT OF BIOMANIPULATION ON THE STRUCTURING OF THE PLANKTONIC FOOD-WEB AND WATER TREATABILITY BY COAGULATION

The impact of planktivorous fish on the plankton community structure a nd character of aquatic organic matter was studied by a meso-scale enc losure experiment in a stratified reservoir. Aims of the study were (i ) to examine the response of the communities with and without herbivor ous zooplankton to an increased input of phosphorus and (ii) to determ ine the composition and coagulation properties of the organic matter p roduced by these communities. The concentration and composition of org anic matter were affected both in the enclosure with planktivorous fis h (F), where high algal biomass developed, and in the enclosure withou t fish (Z), where algae were maintained at a low concentration by zoop lankton grazing. Although differences between the enclosures in concen trations of total dissolved organic matter and its hydrophobic, hydrop hilic and neutral/basic fractions were relatively small, coagulation p roperties of the organics differed substantially. Particulate and diss olved organic matter produced in enclosure F had a positive effect on the efficiency of alum coagulation in contrast to the less readily sep arable organics produced in enclosure Z. The results indicate that pel agic food web manipulations to maintainin low phytoplankton biomass by zooplankton grazing may not always have positive effects on the treat ability of water by coagulation. (C) 1998 IAWQ. Published by Elsevier Science Ltd.