Anoxic degradation of organic matter in lakes : implications for carbon cycling and aquatic food webs

Sammanfattning: Considerable evidence from laboratory studies and marine environments suggests thatdegradation of organic matter (OM) is restricted under anoxic conditions compared towhen molecular oxygen (O2) is present. However, other studies contradict this viewsince they found similar OM degradation rates and bacterial growth rates under bothoxic and anoxic conditions in aquatic environments. Studies from freshwaterenvironments are rare, and have been primarily based on bacterial production estimates.Anoxic degradation of OM in lakes is commonly considered to be slow and of littleimportance for overall lake food webs compared to oxic degradation. The present thesisand the work it is based on challenge this view. First, the performance of a commonlyused method to measure bacterial production was tested in both oxic and anoxic lakewater. Then, the oxic and anoxic potentials of bacterial growth and OM mineralizationwere compared in lake water and sediment. In addition, I assessed the potential ofcarbon transfer from methane (CH4; i.e. an end-product of anoxic degradation) topelagic food webs. Three methods for measuring water column methane oxidation wereevaluated. Then, the potential transport of methane carbon into the microbialcommunity via methane oxidation, and further -up the food web- into the zooplanktoncommunity was estimated. Results indicate 1) that OM degradation and bacterial growthmay be similar in oxic and anoxic lake environments, 2) that OM characteristics may bemore important for the mineralization than the O2 regime per se in the short term (daysweeks),and 3) that methane can be a significant source of carbon and energy for pelagicfood webs. This suggests that the anoxic carbon metabolism may be extensive andpotentially important for pelagic organisms in many lakes.