Feeding, respiration and egg production rates of copepods during austral spring in the Indian sector of the Antarctic Ocean: role of the zooplankton community in carbon transformation

During the austral spring period of 1996, the composition, age structure and physiological activity of zooplankton were studied in the Indian sector of the Southern Ocean. Zooplankton biomass ranged from less than 1 g m−2 in the Northern Polar Front Zone (PFZ) to 16 g m−2 near the ice edge in the Seasonal Ice Zone (SIZ). Zooplankton communities were dominated by copepods associated with euphausiid larvae. At all stations, species composition of copepods was dominated in number by small species (Oithona spp, Ctenocalanus citer). Northern stations were characterized by Calanus simillimus and Metridia lucens. Southern stations showed high abundance of Calanoides acutus, Calanus propinquus and Rhincalanus gigas. Stage distribution was analyzed for the four main contributors to the copepod biomass (Calanus simillimus, Calanoides acutus, Calanus propinquus and Rhincalanus gigas). gment content and gut transit time showed a strong day–night periodicity. Gut transit times were usually high with values ranging from 1 h (Calanus propinquus) to 1 h 30 min (Rhincalanus gigas). Maximum ingestion rates were recorded for Calanus propinquus and Pleuromamma robusta. Respiration rates were measured for 13 species of copepods and varied from 0.5–0.6 μl O2 ind−1 day−1 for smaller species to 20–62 μl O2 ind−1 day−1 for the larger ones. The impact of the copepod population was estimated from the CO2 produced per m−2 and per day, which showed a release of 4.2–4.5 mmol. It corresponded to a minimum ingestion of 41.4% in the Permanent Open Ocean Zone (POOZ) and 22.6% in the SIZ of the daily primary production. The budget between carbon ingestion and respiratory requirements appears to be nearly balanced, but with the exception of Calanus propinquus, cannot accommodate the addition of the cost of egg production, which only partially relies on food intake. During austral spring, the population studied appeared to rely mostly on phytoplankton as food, though additional use of internal energy reserves and intake of protozoan cells was likely needed to fully balance the energy budget.