Scalable Climate Forcing Optical Indices: How Effective Are They in Alpine Lakes?
Tuesday, 16 December 2014: 8:45 AM
As the world gets warmer and wetter, it is important to identify sentinel systems to resolve the response of ecosystems to climate change across drivers and complex landscapes. Recently developed climate forcing optical indices (CFOI) related to the quality and quantity of dissolved organic carbon (DOC) have been shown to be scalable from short-term storm events to interdecadal periods in diverse lakes in lowland landscapes. Are these CFOI metrics effective in alpine ecosystems, which are often viewed as more responsive sentinels of climate change than lowland ecosystems? DOC-related optical data from a series of alpine lakes in North America are used here to address this question. The response of these CFOI metrics followed the same tight relationship in alpine lakes as in lowland lakes, with a seasonal increase in spectral slope between 275 and 295 nm (S275-295) and a decrease in DOC-specific absorbance at 320 nm (a*320) as well as in the ratio of these two indices (a*320/S275-295, a composite climate forcing index, CF). These changes are likely driven largely by photobleaching of terrestrial DOC during the ice-free season. Closer analysis of five years of data for Lake Oesa in the Canadian Rockies revealed marginally significant relationships between precipitation and the CFOI metrics. Specifically, S275-295 decreased with increasing cumulative precipitation for the 15 day period preceding water sample collection. a*320 increased with cumulative precipitation for the 45 day period preceding water sample collection. Relationships between CFOI metrics and precipitation at other time scales were not significant. While the DOC-quality related CFOI metrics were responsive to precipitation, DOC concentration alone was not. The time scale of precipitation effects on CFOI metrics in Lake Oesa was shorter than those previously observed for lowland lakes. This contrasting response is likely related to differences in the characteristics of the hydrology and catchments of lowland lakes vs. that of Oesa which is largely covered by ice, rock, and glacial till rather than the extensive vegetation found at lower elevations. These optical changes in alpine lakes in response to precipitation may have important ecological implications for the differences in how alpine vs. lower elevation systems respond to climate change.