Abstract: Manuscript on effects of humic substances and nutrients on trophic structure and dynamics (including metabolism) and mitigation possibilities to counteract negative effects of climate change
An increase of dissolved organic carbon (DOC) in streams and lakes has been reported across the northern temperate region but the effects of elevated DOC on whole lake ecosystems when combined with nutrient inputs are as yet unclear. In order to address this uncertainty we performed a mesocosm study. Identical amounts of nitrogen and phosphorus were added at intervals to simulate eutrophic conditions. We found no significant difference between the organic matter treatments in terms of net ecosystem production, respiration or gross primary production. However, dissolved oxygen was lowest in the mesocosms receiving the highest level of added organic matter. In spring, mesocosms receiving high levels of organic matter exhibited the greatest increases of chlorophyll a among the treatment groups, whereas later in the year the high organic matter treatments showed the lowest phytoplankton biomass among the treatments. The highest phytoplankton biomass in summer, autumn and winter was observed in mesocosms receiving low organic matter levels.
A number of the mesocosms became turbid in summer, as a result of cyanobacteria blooms. The concentration of cyanobacteria and duration of the bloom was highest in those mesocosms receiving low levels of organic matter and lowest in those receiving high levels of organic matter. Organic matter concentration also affected higher trophic levels: total macroinvertebrate abundance increased with allochthonous carbon in spring and summer. However, fish abundance did not show consistent trends. The results of the study suggest that as cyanobacteria seemed to benefit from a low increase in humic substances, eutrophic shallow lakes might be faced with increased pressures towards a turbid state with an increase of DOC in the near future.