Organic carbon losses measured by heterotrophic activity of mesozooplankton and CaCO3 flux in the bathypelagic zone of the Arabian Sea

Organic carbon requirements for metabolisms of mesozooplankton in the bathypelagic zone were calculated for two stations (Western Arabian Sea Sediment Trap, WAST, 16°N 60°E, and Central Arabian Sea Sediment Trap, CAST, 14°N 64°E) and two different intermonsoonal periods (October 1995 and April 1997) in the Arabian Sea. These requirements were compared with inputs of particulate organic carbon (POC) measured from sediment traps. The temporal variability of POC flux is compared to the CaCO3 flux made up by planktic foraminiferans and coccolithophorids. The potential oxygen requirement of mesozooplankton was measured by the electron transport system (ETS) activity and the organic carbon demand calculated using conversion factors from the literature. Particulate CaCO3 flux was calculated from multinet hauls and sediment trap samples. Mesozooplankton organic carbon demand in the zone between 1050 and 3000 m differed locally and seasonally. At WAST, the mesozooplankton required 1778 μg C m−2 d−1 in October and only 484 μg C m−2 d−1 in April. At CAST, the respective values were 997 and 211 μg C m−2 d−1. No differences between the stations and time periods were discernible for the zone between 3000 and 3900 m at CAST and between 3000 and 4000 m at WAST; the values ranged between 73 and 98 μg C m−2 d−1. The input measured by sediment traps was sufficient to cover the calculated requirements of the mesozooplankton. The flux of calcareous particles and the mesozooplankton carbon requirements showed a similar pattern of temporal variability. At CAST the planktic foraminiferal shell flux was 33.2 mg CaCO3 m−2 d−1 in October and only 5.9 mg CaCO3 m−2 d−1 at 3000 m depth in April. Flux data derived from multinet hauls are of the same order of magnitude as the sediment trap data and, as multinet data yield a high temporal and spatial resolution, they could serve as a measure for open-ocean particulate flux.