The energy-mass (Em) flux framework proposes that climate, humans and other biota regulate lake ecosystems by altering the influx of energy (E) and mass (m) through diverse direct and indirect pathways.  In this view, lakes are open ecosystems that retain E and m by production of particles that are deposited as sediments, have limited capability of accessing sequestered materials, and are dependant on a continuous influx of E and m from the ectosystem, the region surrounding the lake.  Physical models of the environmental forcing of lakes reveal that neither E nor m pathways are intrinsically paramount, and instead predict that the magnitude of effect of individual inputs depends on the ratio of influx to lake content.  Consequently, because increases in m influx associated with continued economic growth (~3%/yr) far outstrip is projected changes in E influx associated with global warming, we predict that lakes within inhabited basins will mainly respond to future changes in land use rather than thermal forcing.

Results/Conclusions

This hypothesis was tested using variance partitioning analysis of highly-resolved, 100-yr long paleolimnological time series of algal assemblages in lakes within agricultural and reference basins of central North America.  Analyses of six lakes reveal that climate warming accounts for <5% of historical variation in algal abundance and community composition since 1900, some 4-fold less than the combined effects of agriculture and urbanization.  Thus while global warming will undoubtedly affect isolated lakes strongly, this framework forecasts that thermal effects may be obscured in lakes with anthropogenic forcing.