Murky waters: discerning among sources of natural variation under high uncertainty and at multiple scales

Abstract

Fresh waters account for an inordinately large portion of Earth’s carbon burial and methane production. As such, they are major components of energy flow within and between ecosystems. Yet, the roles of lakes and rivers as sources, conduits, and sinks of energy, and as providers of ecosystem services are incompletely understood. High elevation aquatic systems are of particular concern due their high susceptibility to variation under changing regional climates. I investigated the proportion of consumer biomass derived from terrestrial sources, allochthony, in three metabolic classes of high elevation lakes using a modular, Bayesian, stable isotope mixing model. Additionally, I quantified the influence of local- and regional-scale environmental drivers on flow and thermal regime across three flow-source classes of rivers using dynamic factor analysis. The most probable estimate of allochthony across consumer taxa was 41% in small-montane lakes. For large-montane and alpine lakes, allochthony was just 4 and 3%, respectively. These results corroborate previous findings that lake size, depth, and light penetration are dominant physical controls on allochthony, but add that it sharply declines at high elevation due to changes in terrestrial primary production near or above tree line. I also found that primarily rain-fed rivers undergo large seasonal temperature fluctuations that closely track air temperature – high coupling – while snow-fed rivers tend to be more weakly, and in some cases inversely, coupled with air temperature fluctuation due to influx of meltwater. However, variation in coupling among snow-fed rivers is high and disproportionately influenced by artificial reservoirs, which appear to magnify the buffering effect of melting snow and glacial ice on riverine thermal regimes in summer.