THE SIGNIFICANCE OF AUTOTROPHIC AND HETEROTROPHIC PICOPLANKTON IN HYPERTROPHIC ECOSYSTEMS

Hypertrophic aquatic ecosystems are biologically important because the y represent an environment where homeostatic mechanisms are strongly r educed and extreme oscillations occur in physical and chemical paramet ers as well as in the growth of many planktonic organisms. Hypertrophi c lakes may represent not only the ultimate state of eutrophication, b ut also systems where abundance and production of picoplanktonic organ isms (0.2-2 mu m in diameter) may theoretically be greatest. In this r eview we examined the microbial ecology literature for studies on hype rtrophic lakes and assessed the significance of heterotrophic bacteria (BACT) and autotrophic picoplankton (APP). Our main Conclusions are a s follows: (a) The relative contribution of APP to total phytoplankton biomass is strongly reduced in hypertrophic systems. (b) The absolute abundance and biomass of APP are highly variable among these systems; depending mainly on differences in light quality and quantity, the ex istence of nitrogen limitation and presence of pollutants. (c) The abu ndance of BACT does not increase as rapidly as algal biomass with incr easing trophy. (d) A clear dichotomy exists between lakes dominated by Microcystis where bacterial biomass and production are relatively low , and Planktothrix-dominated lakes where bacterial biomass and product ion are high. (e) Bacterial production per unit biomass tends to incre ase from oligotrophic to hypertrophic systems supporting the previous hypothesis that bacteria do not play a lesser role in lake metabolism at the upper end of the trophic scale. In general, however, our knowle dge of the microbial ecology of hypertrophic systems has advanced very little since the 1980s, largely because most studies of these systems have focused on their remediation.