Mesozooplankton carbon requirement in the Tyrrhenian Sea: its vertical distribution, diel variability and relation to particle flux

Mesozooplankton consumes an important fraction of particulate organic matter sinking down the water column in all oceans. We investigated the vertical distribution (down to 2000 m depth) and diel variability (during a 24 h cycle) of the mesozooplankton carbon requirement (μg C g−1 d−1), estimated by measuring the activity of the electron transport system (ETS), at a fixed station in the open southern Tyrrhenian Sea. To estimate the quantitative role of zooplankton in carbon losses occurring during organic particle sinking, zooplankton carbon demand was compared with organic carbon vertical flux (measured using the 234Th:238U disequilibrium method). Zooplankton abundance, biomass and community composition were also investigated, as were the vertical distribution of diel migrant species and the relative importance of crustaceans and gelatinous taxa. A distinct day-night variation of carbon demand in the upper 300 m of the water column was observed, with the highest values encountered during dark hours. The high values estimated in sunset samples were most likely due to the presence or dominance of the calanoid copepod Pleuromamma gracilis. Mesozooplankton were responsible for 13.2% and 8.8% of carbon losses in the mesoand bathypelagic zones, respectively. These results reflect the trophic status of the southern Tyrrhenian Sea, which is typically less oligotrophic than the central and eastern Mediterranean Sea.