Seasonal contributions of phytoplankton and fecal pellets to the organic carbon sinking flux in the North Water (northern Baffin Bay)

The downward export of organic material was assessed during the productive period (late spring to early autumn) in the North Water polynya area (NOW; northern Baffin Bay). Free-drifting short-term particle interceptor traps were deployed at 3 depths below the euphotic zone (50, 100 and 150 m) for 12 to 24 h at 19 stations from June to July in 1998 and at 12 stations during August and September in 1999. The sinking material was analyzed for particulate organic carbon (POC) and pigments (chlorophyll a and pheopigments), and for phytoplankton and fecal pellet enumerations, POC and pigment sinking fluxes at 50 m averaged 337 and 8 mg m(-2) d(-1), respectively. The maximum sinking fluxes of organic matter (ca. 554 mg C m(-2) d(-1) and 14 mg pigments m(-2) d(-1)) were observed in June, after which they decreased and remained relatively constant (ca. 219 to 240 mg C m(-1) d(-1) and 3.9 to 4.1 mg pigments m(-2) d(-1)). Phytoplankton carbon flux decreased while fecal carbon flux increased during the study. In June, intact diatom cells and fecal pellets made up 50 and 4% of the total POC sinking flux, respectively. During the following months, they made up less than or equal to10% (diatoms) and 30 to 50% (fecal pellets) of the total POC flux. A rapid increase in sinking cell numbers and fecal pellets at the end of the sampling season points to a late summer-early autumn bloom in the NOW. The dominant pathway of carbon cycling changed during the study, from export in spring to retention afterwards. From July on, the retention of particulate organic matter in the upper 100 m of the water column was favored by the low sinking velocities of POC and chlorophyll a, which averaged less than or equal to1.3 m d(-1). Over the whole sampling period, 35% of the particulate phytoplankton production was exported through sinking, while 65% was recycled or remained suspended in the euphotic zone. The organic carbon produced in the euphotic zone was, thus, mainly available to pelagic grazers rather than being exported to the benthic community. These results are in agreement with the abundance of higher trophic level pelagic organisms observed in this region.