Dissolved organic matter dynamics in high-elevation lakes: Effects on bacterial ecology and ecosystem metabolism

Background/Question/Methods

Dissolved organic matter (DOM) is the dominant pool of reduced carbon in aquatic ecosystems. In high-elevation lakes, where rates of primary production are low, DOM fuels bacterioplankton production that equals or exceeds phytoplankton production and is important in structuring ecosystem metabolism. In these systems, microbial metabolism is affected by both the biochemical composition of DOM and the taxonomic composition of the bacterioplankton community. Thus the interaction of community structure and DOM characteristics are important in translating changes in DOM source into broader ecosystem effects, including trophic structure, biogeochemical cycling, and ecosystem metabolic balance. Although high-elevation aquatic ecosystems receive much of their DOM from terrestrial sources, it remains unclear how variability in the concentration and composition of this material affects microbial processes and net metabolic balance. We combine seasonal data from a representative Sierra Nevada lake (Emerald Lake) with synoptic surveys from over 80 additional Sierran lakes to investigate the influence of landscape characteristics on dissolved organic carbon (DOC) and explore the extent to which these dynamics affect bacterioplankton ecology and net metabolic balance. 

Results/Conclusions

There were distinct spatial and seasonal patterns to the characteristics of DOM in high-elevation Sierran lakes. DOC concentration, which ranged from 11–317 µmol L^-1 among lakes, was related to a number of landscape characteristics. Most notably, DOC was inversely related to slope and increased in proportion to the extent of vegetation cover within catchments. Landscape influence on DOM was strongest during spring snowmelt when DOC and total fluorescence were highest and the fluorescence index, a relative measure of allochthonous and autochthonous fulvic acids in the dissolved organic matter pool, indicated organic matter was largely of terrestrial origin. Seasonal sampling from within Emerald Lake suggests terrestrial DOM is replaced by autochthonous DOM in relation to increasing phytoplankton biomass over the course of the growing season. Although seasonal patterns in DOM correspond to changes in bacterial community composition and shifts in ecosystem metabolism, none of the metrics of DOM that we measured significantly explained seasonal variability in net metabolic balance. Instead, NEP was negatively related to bacterial abundance and positively correlated with phytoplankton biomass. Although alpine lakes appear to be slightly autotrophic on average during the growing season, their metabolic balance is sensitive to change: increased terrestrial loading of organic material, which might be expected under climate change scenarios that predict more precipitation will fall as rain, may cause a shift toward net heterotrophy.