How human activity and environmental change affect long-term ecosystem dynamics has been a central ecological question. Since the late 20th century, human activity intensified due to human population increase and rapid economic growth. Previous studies have revealed that intensive human activity has severely affected ecosystems worldwide. Contrarily, we do not have enough knowledge about how extensive human activity and environmental change have affected ecosystems before human activity intensified. Here, we examined the environmental change since lake formation and its effect on lake ecosystems focusing on the dynamics of the cladoceran community, which is a keystone group in lake ecosystems. We focused on eutrophication, a major aquatic environmental change that has rapidly progressed due to intensive human activity during the 1950s, called as industrial eutrophication. We addressed these issues using paleolimnological analysis of varved sediments and subfossils of cladoceran. The studied lake was Lake Fukami-ike, Nagano Prefecture, Japan, from which we were able to obtain cores of varved sediments including a layer from the lake formation. We collected five sediment core samples, measured the concentration of total phosphorus and phytoplankton pigment, and counted the cladoceran and other invertebrate subfossils in each layer.
Results/Conclusions
In Lake Fukami-ike, rapid eutrophication has occurred twice in its history. In addition to the 1950s when the industrial eutrophication occurred, total phosphorus and phytoplankton pigment also temporarily increased in the 1850s. The initial eutrophication was likely to be caused by extensive human activity. Historical documents reported that a severe flood occurred in 1850, and the fertilized soil from the surrounding rice and mulberry fields should have flowed into the lake due to the heavy rain. Statistical analysis suggested that the first eutrophication event replaced a benthic cladoceran community with a pelagic one. This change was consistent with that caused by industrial eutrophication as previous studies reported. During the early 2000s, the cladoceran community diversified, and in more recent years, average body size of cladocerans became smaller. Statistical analysis suggested that the cladoceran community changes since the 2000s could not be explained by eutrophication alone. Instead, changes in dominant predators were likely to have affected the cladoceran community. Our results suggest that extensive agricultural activity in combination with natural disasters can cause rapid eutrophication even before the modern intensification of human activity and that such eutrophication can influence a cladoceran community to the same extent as does industrial eutrophication.