Stoichiometric effects of pCO_2 on aquatic communities

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The present study was performed to clarify (1) how land use/cover in the watershed affects on pCO2 in lake ecosystems, (2) how changes in pCO2 affects zooplankton communities through changes in species composition, abundance and stoichiometry of planktonic algae, and then (3) how local and global environmental changes influence on aquatic communities in lakes through these stoichiometric effects of pCO2. For these, we first examined limnological and biological characteristics of 48 lakes located at east-north area in Japan with GIS analyses on the watershed of these lakes. The analyses revealed that pCO2 in the lakes varied over 5 orders magnitude and highly related with land cover of vegetation in the watershed, water depth and food web structures, suggesting that effects of changes in land use and atmospheric pCO2 on lake carbon metabolisms differ depending on the basin morphometry and biota.To examine changes of pCO2 to aquatic communities, laboratory experiments with manipulation of pCO2 in water were made using several taxonomic group of algal plankton as primary producers and Daphnia as consumers. The experiments showed that algal competitive interactions differed depending on pCO2 and nutrient supplies. In any experiments with single algal species, increase of pCO2 increased algal abundance but decreased growth rate of consumers due to decrease in specific nutrient contents of the algal food. However, in experiments with multiple algal species, increase of pCO2 also increased algal abundance but did not decrease growth rate of consumers, probably due to complementary effects in nutrition of algae. These results suggest that algal species diversity functions to mitigate negative effects of increased pCO2 on community structures and mass transfer efficiency in lake ecosystems.