The Effect of an Extreme Rain Event on the Biogeochemistry and Ecosystem Metabolism of an Oligotrophic High-Elevation Lake

We characterized the effect of an extreme rain event on the biogeochemistry and ecosystem metabolism of an oligotrophic Sierra Nevada (California) lake. During a 10 hour period of an autumn season rainfall event, lake discharge increased from <1.0 L s(-1) to over 3000 L s(-1), reaching a rate one order of magnitude higher than found during peak snowmelt. Large quantities of terrestrial particulate and dissolved organic matter were washed into the lake. An entire season of autochthonous dissolved organic carbon (DOC) was flushed and replaced by allochthonous DOC as light attenuation increased by >300%. The resulting truncation of the photic zone, reduction of water column chlorophyll-a, and increase in particulate and dissolved organic matter available to microbes resulted in a 47% reduction in whole lake gross primary production and 30% increase in respiration relative to average autumn values. As a consequence, the lake went from being slightly autotrophic to strongly heterotrophic. If rain events increase in frequency, as many climate change models predict, increased terrestrial inputs to Sierran lakes may result in more frequent periods of reduced primary production, increased periods of hypoxia and anoxia, and an ecosystem shift toward net heterotrophy during the ice-free season.