Biomass, feeding and metabolism of mesozooplankton in the equatorial Pacific along 180°

Mesozooplankton biomasses, grazing rates, and metabolism (respiration, ammonium and phosphate excretion) were measured in the central Pacific (180°; 8°S–8°N) during the EBENE cruise, in October–November 1996. In the HNLC zone, the highest mesozooplankton biomass occurred between 6°S and the equator. The decrease of biomass at the northern part of the HNLC zone was accounted to the passage of a tropical instability wave, carrying poorer zooplankton concentrations from the north. Within the HNLC zone, most of the biomass (60–96%) was found above 100m in accordance with the upper distribution of phytoplankton and heterotrophic protists. No significant day–night differences were observed in mesozooplankton biomass. These results suggest a strong and permanent coupling between mesozooplankton and the first links of the food chain. Respiration and phosphate excretion rates were steady at the different latitudes, but at the equator, grazing rates were the lowest, and ammonium excretion the highest. These variations were accounted to latitudinal differences in microzooplankton density and in particles C/N ratios, suggesting that more nitrogen was available for food at the equator. Moreover, among the dominant copepod species, higher proportions of carnivorous were found at the equator. Higher particle grazing and ammonium excretion rates occurred at the equator during the day, while the chlorophyll gut content of copepods did not show any significant diel cycle. These results suggest a strong trophic link between mesozooplankton feeding activity, on the one hand, and cyclic variations of phytoplankton production and of of their main predators (microzooplankton protists), on the other hand. From biomass estimations of phytoplankton and protists in the euphotic layer and results of dilution experiments, the production and grazing mortality of the main autotrophic taxa (from picoplankton to microplankton) were computed at the equator. Mesozooplankton, which was mainly involved in the catch of the largest (>8μm) food particles, such as diatoms or dinoflagellates, accounted for only a minor part (22%) in total phytoplankton grazing, compared to microzooplankton. As the chlorophyll ration was insufficient to balance its metabolism expenditures, mesozooplankton had to to complete its diet with other sources of food such as microzooplankton. The low level of zooplankton metabolic O:N ratios, suggesting the use of a food with a large protein content, supports of this hypothesis.