Quasi-decadal periodicities in growth and production of the copepod Eodiaptomus japonicus in Lake Biwa, Japan, related to the Arctic Oscillation

Copepods are important secondary producers that support higher trophic levels in aquatic food webs. Large-scale climate events such as climate oscillations and global warming force on physical and chemical conditions in aquatic ecosystems might regulate copepod production through physiological and biochemical processes. We evaluated how large climatic and anthropogenic events impacted secondary production of copepods in Lake Biwa, the largest lake in Japan. We determined demographic traits such as body size, growth rate, biomass and production of the dominant copepod Eodiaptomus japonicus in this lake over four decades (1971-2010). To evaluate in situ food conditions and estimate growth and production for this omnivorous species, we firstly defined a size-based food index (f), that is the ratio of in situ body size to ideal body size in adult females. Values of f were mostly < 1 even during eutrophication (1970s to the early 1980s), suggesting that this copepod continuously suffered from a food shortage in this lake. Quasi-decadal periodicities were detected in f, growth and production (but not biomass) for this copepod throughout the study period. These periodicities were correlated with the Arctic Oscillation, implying that long-term trends in climate could regulate copepod food availability and production. This correlation weakened after 1990, which might be due to a regime shift in lake water temperature, which increased abruptly in the mid-1980s. Global warming might now be disrupting historical quasi-decadal periodicity in growth and production of copepods in Lake Biwa.