Stoichiometric effects of pCO_2 on aquatic communities

本研究旨在阐明：(1)流域土地利用/覆被对湖泊生态系统pCO2的影响；(2)pCO2的变化如何通过浮游藻类的物种组成、丰度和化学计量学的变化影响浮游动物群落；(3)局地和全球环境变化如何通过pCO2的化学计量学效应影响湖泊水生群落。为此，我们首先对日本东北地区48个湖泊的湖泊生物学特征进行了研究，并对这些湖泊的流域进行了GIS分析。结果表明，湖泊的pCO2变化幅度超过5个数量级，且与流域植被覆盖、水深和食物网结构高度相关，表明土地利用和大气pCO2变化对湖泊碳代谢的影响因流域形态和生物群而异。为了研究pCO2对水生群落的影响，以浮游藻类为主要生产者，水蚤为消费者，对水体中pCO2的影响进行了实验研究。实验表明，藻类的竞争相互作用因二氧化碳分压和养分供应而异。在任何单一藻类物种的实验中，二氧化碳分压的增加增加了藻类的丰度，但由于藻类食物的特定营养成分含量的减少，降低了消费者的生长速度。然而，在多种藻类的实验中，pCO2的增加也增加了藻类的丰度，但并没有降低消费者的生长速度，这可能是由于藻类营养的互补作用。这些结果表明，藻类物种多样性具有减轻pCO2增加对湖泊生态系统群落结构和传质效率的负面影响的功能。

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.