Algal abundance and warming: an empilical analysis using monitoring data of artificial resevoirs along the latitudal gradient

Background/Question/Methods: The global average air temperature has increased by 0.76 °C since the mid-19th century and is predicted to rise an additional 1.0-4.0 °C by the end of this century. Although lake water temperatures are affected by various factors, a number of studies have shown that lake temperatures have indeed been increasing for the past several decades. Since algal growth rate is temperature-dependent, their abundance is likely increased under the warming. However, response of the lake ecosystems to warming may be complex since a variety of organisms are involved in shaping the ecosystems and a number of physical and chemical processes affect the biological interactions. Experimental approach has been often used to examine the potential effects of warming on lake communities. The approach is useful to uncover some mechanisms of the effects. However, the mechanisms may play a limited role in given lakes due to other factors that override or mask the effects. Another approach examining the effects of warming is empirical studies, i.e. comparison of different lakes along latitudinal gradients. Although it is less powerful to uncover actual mechanisms of the effects, the empirical approach can provide a general picture of the effects that are operating in nature. In Japan, a large number of artificial lakes (reservoirs) have been constructed. Since 1990s, limnological parameters including temperature, chl-aand TP are periodically monitored in several reservoirs (>100) as the National Survey for River and Riparian Environment. Since these reservoirs are distributed from the south (Kyushu Is.) to the north regions (Hokkaido Is.), the monitoring data provide a chance to examine response of the lake ecosystems to warming empirically by comparing the reservoirs along the latitude.

Results/Conclusions: If warming has general effects on algal abundance in lake ecosystems, we can expect that there is a trend in the abundance along the latitude. Indeed, the algal abundance increased towards the reservoirs located at the southern regions of Japan. However, detailed statistical analyses showed that warming have been affecting on the algal abundance indirectly but not directly. In this talk, with these monitoring results, I will discuss how we should adapt to warming for conserving and managing water quality in lake and reservoir ecosystems.