Implications of changing terrestrial organic carbon export on lake productivity: Merging process and habitat specific responses to an integrated ecosystem level understanding

Background/Question/Methods

Export of terrestrial organic carbon (OC) affect a multitude of physicochemical properties of recipient lake ecosystems, which, in turn, result in both positive and negative effects on the biomass production of different food webs compartments. Still, the knowledge of the net effects of terrestrial OC on lake food webs remains poor, largely due to a paucity of studies integrating all relevant ecosystem processes as well as the difficulties to carry out controlled experimental studies at the scale of whole ecosystems. Also, recent findings indicate that effects of terrestrial OC on lake food webs are more complex than previously believed. In this talk we present results from comparative studies of small sized boreal lake ecosystems with contrasting input of terrestrial OC. We quantified the effects of terrestrial OC by analyzing the metabolism and consumer stable isotopic composition in benthic and pelagic habitats with the aim to assess the net effect of terrestrial OC on the biomass production of whole lake ecosystem.

Results/Conclusions

The results show that terrestrial OC input had a pronounced impact on the structure and function of recipient lake ecosystems by affecting the magnitude and pathways of biomass production and resource use at all levels of the food web. Terrestrial OC input resulted in a dominance of heterotrophic over autotrophic production and a dominance of pelagic over benthic production. The contribution of terrestrial OC to the growth of intermediate and top consumers was similar across habitats and generally increased with the relative supply of terrestrial OC. Discrepancies between OC supply and consumer resource use show that autochthonous OC was selectively transferred to higher trophic levels, suggesting that autochthonous resources cannot be completely replaced by terrestrial resources and that there is an upper limit of terrestrial support of lake food webs. Further, high terrestrial support of lake food webs seems to result in low biomass production, implying that the net effect of terrestrial OC input is a repression rather than a stimulation of biomass production. Our results emphasize the need to integrate different processes (autotrophic-heterotrophic) and habitats (benthic-pelagic), in order to understand net effects of terrestrial OC on food webs and lake productivity. Studies based on single processes or habitats may have limited predictability on whole ecosystem responses to changing terrestrial OC export for a large majority of lakes in the boreal region.