Stable isotope analysis of zooplankton carbon nutrition in humic lakes

Carbon stable isotopic composition was determined for zooplankton, POM, DOM and, in two cases, phytoplankton from 12 small forest lakes in southern Finland during summer 1997. The lakes were selected to provide a wide range of water colour (12-365 mg Pt l(-1)) but minimum variability in other limnological characteristics. POM and DOM showed similar stable isotope ratios (delta(13)C around - 28 parts per thousand, comparable with values commonly reported for terrestrial C-3 vegetation) and these showed no relationship to water colour. Zooplankton were consistently depleted in C-13 relative to the bulk POM on which they might feed. This relative C-13 depletion increased significantly with water colour. Therefore the zooplankton must have been feeding selectively on isotopically light food sources and doing so to a greater extent in the more humic lakes. Although phytoplankton were C-13-depleted relative to POM, their abundance and their variability between lakes did not appear to explain adequately the observed trend in zooplankton stable isotope ratios. We propose that some zooplankton were grazing on methanotrophic bacteria which utilise isotopically light methane derived from allochthonous organic matter decomposed under anaerobic conditions. This process would be more pronounced in coloured lakes with a higher loading of allochthonous organic matter and greater development of hypolimnetic anoxia.