Mesozooplankton carbon requirement in the southern Adriatic Sea: vertical distribution, diel and seasonal variability, relation to particle flux

Mesozooplankton plays a role in particulate organic carbon (POC) remineralisation from the pelagic sinking POC flux down the water column. The vertical distribution (to 1000 m depth), diel variability (during a 24 h cycle) and seasonal differences of the mesozooplankton carbon requirement (μg C g−1 d−1), estimated by measuring the activity of the electron transport system, were investigated at a fixed station in the southern Adriatic Sea during November 2006 and April 2007. To estimate the quantitative role of zooplankton in the carbon loss that occurs during organic particle sinking, the zooplankton carbon requirement was compared with organic carbon vertical fluxes measured using a sediment trap. Zooplankton abundance, biomass and community composition were investigated along with the diel vertical distribution of euphausiid migrating groups and the relative importance of crustaceans and gelatinous taxa to the potential community carbon requirement. Day/night zooplankton carbon requirement differences in the upper 100 m were shown in both sampling periods. They were not at all correlated to the vertical distribution pattern of euphausiids but were attributable to the amount of non-living specimens and/or the ratio between gelatinous and crustacean taxa. The mean metabolic requirement in the sampled layers of the water column was lower in November than in April, ranging from 0.02 to 9.10% and from 0.07 to 31.86%, respectively, of the measured carbon losses. In the epi- and mesopelagic zones, the total percent contributions of mesozooplankton respiration to organic carbon losses were 0.18 and 18.45%, respectively, in November and 1.41 and 87.27%, respectively, in April. The higher remineralisation percentages detected in April compared to November could be linked to seasonal differences that involve higher primary production, zooplankton biomass, vertical amount of sinking POC flux and more intense heterotrophic metabolism, which take place mainly in the meso pelagic layer in each period.