Grazing in tropical copepods, measured by gut fluorescence, in relation to seasonal upwelling in the Banda Sea (Indonesia)

Abstract

The amount of chlorophyll pigments in the gut was measured fluorometrically in adult females of 27 species of copepods sampled in the eastern Banda Sea during the Indonesian-Dutch Snellius-II Expedition. During the southeast monsoon (August 1984) and the northwest monsoon (February–March 1985) vertical net hauls were made near a subsurface drifter at four stations, covering several day and night periods and the depth strata 0 to 50, 50 to 150 and 150 to 250 m, with 250 to 500 m added at some stations. Diurnal feeding rhythms were frequently observed. At all stations, significantly higher values of gut fluorescence occurred at night in about 60% of the species, except for 22% at the August station in the Aru Basin, where upwelling was clearest. Reverse rhythms were not detected. Intraspecific differences reflected the vertical heterogeniety between stations: at the upwelling site 67% of the species had higher fluorescence levels in the upper 50 m; at the oligotrophic February stations, with a deep chlorophyll maximum at 50 to 70 m, this was only 5%. In many species the increase in gut fluorescence by night was not limited to the upper layer but extended to 150 to 250 m, which suggests rapid sinking of satiated animals. Carbon body weights were measured to determine the weight-specific gut fluorescence. This was at a maximum at the upwelling station, and there highest in Eucalanus and Temora, and only moderate in the upwelling species Calanoides. At the other stations no clear differences in weight-specific gut fluorescence occurred between species differing in feeding behaviour. In Cosmocalanus, Undinula, Euchaeta and Candacia the carbon weight of adult females was twice as high in the upwelling season as in the other season. Daily rations, estimated using a mean gut passage time of 30 min, were on average only 11% of body carbon. But at the upwelling station about 70%. Estimates of daily mesozooplankton grazing varied between 2 and 6% of the standing stock of chlorophyll, and between 5 and 26% of the primary production. Highest figures concern the upwelling site, where ambient HPLC-determined phaeophorbide concentrations were also at a maximum.