Consequences for pelagic energy mobilisation of a sudden browning episode without a clear increase in DOC concentration: a case of a boreal pristine lake

Abstract

In the boreal zone, episodes of heavy rains are among the most conspicuous events disturbing aquatic ecosystems. Due to climate change they are predicted to become more frequent and intense, leading on browning of lakes. We assessed the possible effects of heavy rains to DOC concentration and water colour and resulting impacts on functioning of a headwater lake with naturally high water colour and concentration of allochthonous DOC. The weekly measurements were carried out in summers 2002–2004; the first 2 years had normal and the last year very high summer precipitation. The rains resulted in rapid increase in water colour, but not in DOC. We hypothesised that due to browning, irrespective of whether it stems from an increase in brownish DOC or other coloured substances, photosynthesis decreases, bacterial activity increases, and the lake becomes more heterotrophic. We also anticipated that due to strengthened heterotrophy, the period with apparent net autotrophy and CO2 flux onto the lake would disappear. To test our hypothesis, we measured autotrophic inorganic carbon uptake of photosynthetic organisms as well as dark fixation of inorganic carbon, community respiration and leucine uptake. Finally, we compared the results to CO2 flux measurements with an eddy covariance technique. Our hypotheses were only partly supported since there were no drastic changes in any of the metabolic processes measured. Although the lake ecosystem became slightly more heterotrophic, primary production was still occasionally higher than community respiration and the lake could act as a sink of CO2. Thus regarding metabolic processes, the ecosystem of this pristine lake was not strongly disturbed by the sudden browning episode.