BIOMASS DISTRIBUTION IN FRESH-WATER PLANKTON COMMUNITIES

Both resource control of heterotrophic biomass and heterotrophic regulation of plant populations imply that heterotrophic biomass should constitute an increasingly smaller proportion of total system biomass as the turnover time of autotrophs increases. Although this trend is widely accepted, it has seldom been tested, perhaps because comparable data for many ecosystems are hard to collect. The plankton are an exception to this difficulty. Because the biomasses of both autotrophs and heterotrophs can be estimated with relative ease and because workers use similar techniques, data are available from a wide range of lakes. Both literature data, representing a wide geographical range, and data from a localized set of lakes show that the ratio of heterotrophic to autotrophic biomass (H/A ratio) is well above unity where autotrophic biomass is low and declines where autotrophic biomass is high. A similar pattern is found in marine and terrestrial systems. In terrestrial systems, this pattern has been explained by changes in the turnover time of the autotrophic biomass, but in lakes energetic subsidies from the littoral and the watershed are likely needed to support the relatively high heterotrophic biomass of many oligotrophic systems.