Fecal carbon production by zooplankton in Santa Monica Basin: The effects of body size and carnivorous feeding

Abstract

Measurement of carbon:pigment (C:Pig) ratios in fresh fecal pellets of two size classes of marine zooplankton in Santa Monica Basin indicated that the source for much of the carbon came from ingestion of animal matter. When these C:Pig ratios (in conjunction with data on gut pigment concentrations, gut evacuation rates, and biomasses of the two size classes) were used to predict daily fecal carbon production rates (mgC m −2d −1) for 10 cruise periods, it was found that carnivory contributed over 70% of the egested carbon on 5 of those cruise periods, and over 50% on all but one of them. Daily fecal production rates by 500–1000μm zooplankton were usually slightly higher than those for 1000–2000μm zooplankton, largely because of greater gut pigment concentrations per unit of body weight and slightly greater gut evacuation rates for the smaller size group, in the face of usually greater biomass in the larger size group. Fecal production at night always exceeded that during daylight, for both size groups. No measurements of fecal C:Pig ratios in pellets of 200–500μm animals were possible, but relatively large size-specific gut pigment concentrations and gut evacuation rates suggested that, there have been any carnivory at all in this size group, daily fecal carbon production by these tiny zooplankton would have been even greater than fecal production by the 500–1000μm size group. Assuming that measured daily fecal carbon production by the 500–1000 and 1000–2000μm groups, plus daily fecal carbon production by the 200–500μm group (assuming no carnivory), together represented potential carbon export from the photic zone during each cruise period, the daily production rates were calculated as percentages of total trap-measured carbon flux out of the photic zone. These percentages ranged from about 18% (January/February 1990) to 199% (May 1986). When just the pellet production rates from 500–2000μm zooplankton were considered (presuming that tiny pellets from 200–500μm animals would all be recycled in the photic zone), percentages of trap-measured fluxes ranged from about 10% (October 1985, 1986 and 1987) to about 172% (May 1986). The percentage in May 1986 greater than 100% indicated that some of the pellets from 500–2000μm animals could not have settled into the traps at the base of the photic zone. However, the potential contribution of fecal carbon to total carbon flux was very great in spring in Santa Monica Basin, and least in autumn months. The fact that zooplankton can on occasion be the primary agents for the physical and chemical transformation of the suspended particle field into the sinking flux, and that the transformations are in part dependent on animal size and degree of carnivory, suggests that study of these “intermediary processors” is important in the context of global ocean flux.