CONTRIBUTIONS OF BENTHIC ALGAE TO LAKE FOOD WEBS AS REVEALED BY STABLE-ISOTOPE ANALYSIS

Food webs of tropical, temperate, and arctic lakes can be characterize d by the carbon and nitrogen stable isotope ratios of their constituen t organisms. After assigning trophic levels using delta(15)N, a broad range of delta(13)C is observed at the primary consumer level in nearl y all lakes. The range of delta(13)C is on the order of 20 parts per t housand in tropical lakes Kyoga and Malawi and lakes with low dissolve d inorganic carbon in temperate Canada, but is narrower in shallow lak es of the Canadian arctic. This broad range exists in ecosystems in wh ich terrestrial inputs and/or aquatic macrophytes are often minimal. T he isotopically light end of the range results from phytoplankton phot osynthesis whereas the isotopically heavy end represents benthic algae photosynthesizing within an unstirred boundary layer. This range is s uccessfully predicted by an application of a simple isotopic model for photosynthetic fractionation, originally developed for aquatic macrop hytes, which uses boundary layer thicknesses reported for benthic alga l communities. When benthic photosynthesis becomes light-limited in ve ry turbid lakes of the Mackenzie Delta, then phytoplanktonic carbon do minates the diet of the primary consumers. The organisms on the primar y consumer trophic level appear from their delta(13)C values to harves t preferentially either planktonic or benthic algal carbon but, in tem perate and arctic lakes, higher consumer levels are increasingly omniv orous. Therefore top aquatic predators often have a narrow range of de lta(13)C. In temperate and arctic lakes these top predators have a del ta(13)C near the midpoint of the range at the primary consumer level, which would result from nearly equal dependence on planktonic and bent hic algal carbon This equal dependence would not be predicted from the relative magnitude of planktonic and benthic algal photosynthesis as currently estimated in these systems.