Cladoceran Responses to Environmental Change in High Elevation Andean Lakes

Abstract

Andean lakes are hotspots of biodiversity and serve as critical freshwater resources for millions of people. However, little information is available regarding their ecological responses to anthropogenic climate change, which is occurring rapidly in the Andes. Recent paleolimnological work has revealed marked shifts in diatom species in relation to enhanced water column stratification, caused by warming air temperatures and declining wind speeds. However, the response of higher trophic order consumers in the lakes is not well understood. A paleolimnological examination of Cladocera from four lakes in Cajas National Park, southern Ecuador, recorded relative increases in Daphnia and decreases in Bosmina during the period of recent warming. Cajas lakes were formerly believed to be polymictic, however many now thermally stratify by several degrees for large portions of the year. The deployment of temperature data loggers in four Cajas lakes revealed maximum thermal stratification between November to May, when air temperatures were highest and wind speeds were lowest. The mechanism driving cladoceran assemblage shifts is likely a combination of direct thermal response, as well as indirect bottom-up (food supply) and top-down (predation) factors. In a further examination of three shallow (< 5 m) non-stratified Cajas lakes, climate-related cladoceran shifts did not occur, though long-term assemblage changes appeared to track size of the littoral area, perhaps reflecting water level shifts caused by catchment alteration and/or precipitation shifts. To determine climate change impacts elsewhere in the Andes, three non-stratified lakes were examined from the Cordillera Vilcanota, Peru. The cladoceran assemblages from the two deepest lakes showed marked shifts in the past (i.e., > 150 years ago), possibly related to glacier activity and water level changes. However, none of the three lakes experienced marked cladoceran shifts over the past century, despite climate stations ~60–70 km away recording temperature increases. The relatively stable assemblages over the last century may be due to the lack of thermal stratification, caused by a climate-moderating effect from nearby glaciers. Collectively, this thesis provides among the first paleolimnological cladoceran data from the Andes and highlights the potential importance of lake stratification and related limnological changes in driving cladoceran assemblage composition.