Non-predatory mortality in Mediterranean coastal copepods

Abstract

Mortality affects the dynamics of zooplankton populations with important effects on trophic interactions and biogeochemical fluxes in marine environments, but is still one of the processes least investigated in the field. In the present study, the non-predatory mortality in copepod assemblages and species was investigated by applying the neutral red staining method to identify and quantify copepod carcasses throughout an annual cycle in a Mediterranean coastal site (station LTER-MC in the inner Gulf of Naples). Carcasses accounted on average for 10.3% (±9.7%) of total copepod abundance and were most abundant in spring, summer and autumn. Carcasses were represented predominantly by copepodites (78.9 ± 22.0%) and occurred more frequently and abundantly in calanoids than in other copepod orders, with interspecific differences in their abundance and temporal patterns. Using carcass abundances from field data and decomposition times from laboratory observations, we estimated non-predatory mortality rates of key calanoids that are common and abundant in Mediterranean coastal waters. Non-predatory mortality rates averaged 0.13 day−1 in Paracalanus parvus, 0.07 day−1 in Clausocalanus spp., 0.06 day−1 in Temora stylifera and 0.04 day−1 in Acartia clausi. Non-predatory mortality rates in these populations were not correlated with temperature, salinity or chlorophyll a.