Dead heat: copepod carcass occurrence along the Japanese coasts and implications for a warming ocean

Abstract

Abstract Understanding global warming effects on marine zooplankton is key to proper management of marine resources and fisheries. This is particularly urgent for Japan where the coastal water temperature has been increasing faster than the global average over the past decade. Conventional sampling and monitoring programmes, by ignoring the in situ vital status of the zooplankton, produce incomplete information about the state of the ecosystem. We showed that marine copepod carcasses were ubiquitous along a latitudinal gradient of 34–39°N of the Japanese coasts. On average, 4.4–18.1% of the individuals of the main copepod genera (Acartia, Paracalanus, Oithona, and Pseudocalanus) were carcasses, equivalent to 19–250 µg C m−3. Higher fractions of dead copepods tended to occur at higher water temperatures, implicating temperature-dependent non-predation mortality. Carcass occurrence may represent a loss of copepod production for the traditional predation-based food chain. On average, 49.5% of the carcass carbon would be remineralized in the water column via bacteria respiration, with the remainder potentially exported to the seafloor. Continuous warming in the Japanese coasts is expected to accelerate non-predation copepod mortality, with unknown consequences for the local marine food web.