Warming and chlorophyll-a abundance in reservoirs: empirical analysis using a latitudinal gradient in Japan

• Published in: Japanese Journal of Limnology (Rikusuigaku Zasshi) Vol. 84; no. 3; pp. 187 - 201
• Main Authors: URABE, Jotaro, MARUOKA, Natsumi, ENOMOTO, Megumi, TAKANO, Kohei Takenaka, ICHIYANAGI, Hidetaka, OGURO, Michio, ISHIGOOKA, Yasushi, NAKASHIZUKA, Toru
• Format: Journal Article
• Language: Japanese; English
• Published: The Japanese Society of Limnology 25.09.2023
• Subjects: dam reservoir; land cover land use; total phosphorus; warming; water quality
• ISSN: 0021-5104; 1882-4897
• DOI: 10.3739/rikusui.84.187

Chlorophyll-a (chl-a), which reflects algal abundance, is an important determinant of water quality in lakes and reservoirs. In this study, we analyzed the mean chl-a abundance from 1990 to 2010 in 91 dam reservoirs located in southern and northern Japan. Although no significant annual trend in chl-a abundance was detected in all reservoirs during the observation period, we found a significant positive relationship between chl-a abundance and warmth index (or mean temperature) across the reservoirs. Using this relationship, we projected the chl-a abundance for 2076–2100 in each reservoir using a global climate model under the RCP 8.5 scenario. The results showed that the chl-a abundance in 2076–2100 was 2.2 times higher than the maximum abundance in the study period and, on an average, 1.5 times higher than that in 2000. To examine whether this increase in chl-a abundance could be attributed to the direct effects of warming, we further analyzed chl-a abundance using a structural equation model that included the indirect effects of warming through changes in land use/land cover in the reservoir watersheds. When indirect effects were considered, the direct effect of warming on chl-a abundance was no longer significant. Instead, the chl-a abundance was significantly related to the indirect effects of warming through changes in land use/land cover components in the watersheds. Based on these results, we discuss the importance of land use/land cover policies in managing the water quality in reservoirs under the influence of global warming.