Vertical distribution of copepods and the utilization of the chlorophyll a-rich layer within Concepcion Bay, Chile

Abstract

The potential factors contributing to the development of vertical distribution patterns of two copepod species, Calanus chilensis and Calanoides patagoniensis, were studied in Concepcion Bay, Chile, during three 24-h cruises in late winter, mid-spring 1984 and early winter 1986. Results show that both species tend to aggregate at the depth of maximum chlorophyll a in the water column and that neither minimum oxygen concentrations (<1 ml O 2l −1) nor strong thermoclines play a determinant role in the depth of maximum copepod aggregation. Gut contents (chlorophyll a and phaeopigments) in copepods revealed that the peak in feeding activity occurred during the night for both C. chilensis in June and for C. patagoniensis in November. The C. chilensis and C. patagoniensis populations consumed about 70% and 50% of their total daily ingestion in the chlorophyll a-rich layers in June and November, respectively. In spite of the environmental and population abundance differences between June and November, both copepod populations presented similar values of food consumption: the C. chilensis population ingested up to 8·47 mg C m 2 day −1 in June while the C. patagoniensis population consumed 6·81 mg C m −2 day −1 in November. The daily ingestion carried out by each copepod species would represent less than 1% of the total daily primary productivity in both months. Our results suggest that a great amount of phytoplankton biomass is not directly utilized by zooplankton in Concepcion Bay.