Seasonal variation in production, retention and export of zooplankton faecal pellets in the marginal ice zone and central Barents Sea

Abstract

Vertical distribution and sedimentation of faecal pellets (FPs) as well as the production rates of FPs by larger copepods were studied during three cruises to the Barents Sea in March and May 1998, and July 1999. Three to five 24-h stations were selected during each cruise, where at least one main station was located in Arctic water (ArW), one in the polar front region (PF) and one in Atlantic water (AW). A winter scenario was encountered in March with very low concentrations of FPs in the water column, most of the time well below 0.1 mg faecal pellet carbon (FPC) per cubic meter, and with sedimentation rates below 3 mg FPC m −2 day −1 at all depths and stations. Increased concentrations of FPs were observed in May and the maximum biomass of FPs was found in ArW (4.8 mg FPC m −3). This was reflected in high vertical flux of FPs in the ArW, just below the chlorophyll maximum (∼150 mg FPC m −2 day −1). FPC sedimentation explained ∼40% of the total particulate organic carbon (POC) export at 90 m depth at this station. Copepod FP production was moderate to high in May, reflecting favourable feeding conditions. Large spatial variation in the estimated retention potential of FPs was observed, ranging from 96% in AW to ∼40% in the PF region. The July scenario did not differ very much from that observed in May. The lowest suspended concentrations and vertical flux of FPs were again observed in AW, in spite of the high pellet-production rate. FPC explained 34% of the POC export out of the upper layer in ArW, 40% in the PF region, but only 8% in AW. The calculated retention potential of 70% of the produced copepod FPs in AW decreased to 60% and 47% in the PF region and ArW, respectively. Krill FPs comprised a significant fraction of both suspended and sedimented FPC throughout the central Barents Sea. The data show that spatial and temporal variations in the FP “retention filter” are extensive and evidently of importance for the patterns of vertical flux of organic matter and the regulation of pelagic–benthic coupling in the Barents Sea.