Importance of mesozooplankton feeding for the downward flux of biogenic carbon in the Gulf of St. Lawrence (Canada)

We tested the importance of mesozooplankton feeding and defecation for the downward flux of biogenic carbon (C) in the Gulf of St. Lawrence, a marine coastal environment characterized by high zooplankton abundance. Five stati ons were sampled over nine cruises between July 1992 and June 1994, The rat es of chlorophyll (Chl) ingestion and C defecation were determined for the major copepod species. Free-drifting, short-term (24 h) sediment traps were deployed at the sampled sites at 50 and 150 m, and the trap contents were analyzed for fecal pellets (microscopy), Chi and pheopigments (chromatograp hy). The flux at 50 m of C associated with fecal pellets ranged between 5 a nd 138 mg C m(-2) d(-1), while total POC flux varied between 78 and 302 mg C m(-2) d(-1). Of the total POC found in sediment traps at 50 m, close to 5 0%, on average, was composed of zooplankton fecal pellets. In contrast, dir ect algal (Chl) sinking was < 10%. The fecal pellet contribution varied sea sonally, with a maximum (> 73%) in June 1994 at all stations. This maximum did not necessarily coincide with the highest rate of production of fecal p ellets in the surface waters, indicating that other factors influenced the sinking of fecal pellets. A shallow mixed-layer depth seemed to favor fast removal from the surface. The average fecal pellet contribution to the tota l downward POC flux was slightly higher for stations 4 and 5 (52% compared with 39% for the other stations). These stations were characterized by zoop lankton communities that differed from those of the other stations, smaller copepods such as Temora longicornis frequently dominating at stations 4 an d 5, while Calanus spp. dominated at stations 1, 2 and 6. High fluxes of fe cal pellets frequently occurred when the index of herbivory (Chl ingestion : total C ingestion) was low, transferring downwards carbon of heterotrophi c origin. The contribution of C-transformed pheopigments to total POC flux was low (6% on average). It co-varied with the degree of herbivory in the o verlying waters. The production of fecal pellets in the top 50 m co-varied with Chi ingestion, but not with the index of herbivory, consistent with an algal food intake representing < 50% of total C ingested. Phytoplankton in gestion was highest when primary production (not algal biomass) was maximum . In consequence, mesozooplankton feeding did not exert a strong control ov er the phytoplankton biomass of the Gulf of St. Lawrence (at most 10%), but it was more closely tied to primary production. Phytoplankton biomass is m ore likely controlled by physical factors in this region. Mesozooplankton f ecal pellets contributed significantly to the downward flux of biogenic C a ll through the year in this environment, transferring alternatively C of au totrophic and heterotrophic origin. (C) 1999 Elsevier Science Ltd. All righ ts reserved.