PS 5-44
Impacts of seasonal variation in trophic interactions and climate on carbon dioxide flux from San Diego reservoirs

Monday, August 11, 2014
Exhibit Hall, Sacramento Convention Center
Emily M. Adamczyk, Ecology, Behavior, and Evolution, University of California-San Diego, La Jolla, CA
Jonathan B. Shurin, Ecology, Behavior and Evolution, University of California, San Diego, La Jolla, CA
Background/Question/Methods

While many lakes export CO2 to the atmosphere, the ecological and environmental factors that determine whether manmade reservoirs emit or sequester CO2 emissions are poorly known. As atmospheric CO2 concentrations continue to increase and more reservoirs are created for hydroelectric power, it is important to determine if reservoirs are sources or sinks for CO2 and how we can improve their management.  We sampled three reservoirs in semi-arid San Diego, CA, all of which receive most of their water from the Colorado River. We sampled weekly for one year to measure seasonal variation in partial pressure of CO2 (pCO2), water chemistry (pH, conductivity, nutrients, ions, dissolved organic carbon [DOC]), and food web structure, specifically bacteria, chytrids, phytoplankton, and zooplankton. These data were used to determine seasonal trends and patterns of covariation in biomass and community composition. pCO2 was used to estimate COflux. 

Results/Conclusions

San Diego reservoirs are consistently undersaturated with CO2 and vary seasonally in both pCO2 and different trophic level abundances. We observed consistent seasonal trends in both chytrid fungi and bacteria abundances; biomasses were lowest during the summer and highest during winter months, in contrast with phytoplankton biomass which did not vary seasonally. Each water body showed a distinct seasonal pattern of CO2 concentrations that were largely unrelated to the biomass of phytoplankton.  pCO2 rose during the winter in two of the three reservoirs at the same time that abundances of bacteria and fungi were highest.  Bacteria abundance and pCO2 were not correlated with DOC concentrations, suggesting that the increase in bacteria biomass and pCO2 during the winter were results of a factor other than the input of allochthonous terrestrial organic matter or in situ primary production. With these data, we can better determine how the seasonal variation in ecological processes and species interactions influence the release or absorption of atmospheric carbon in low vegetated reservoirs and what that means for long-term changes in the global carbon cycle.