Sediment trap observations from the Gulf of St. Lawrence and the continental margin of eastern Canada

Abstract

Sediment trap samples have provided the first direct observations of the sinking particles that account for the export of material out of the photic zone in the Gulf of St. Lawrence, as well as their relationship to variations in the trophic regime. Particles were collected at several sites over 24-h periods using 0.03 m2 (collecting surface) and 0.5 m2 free-drifting sediment traps at 50 and 150 m. Total mass flux varied widely (80-1500 mg/m2/d), as did carbon flux (16-300 mg C/m2/d). Small cylinders consistently oversampled with respect to big cylinders, regardless of depth or drifter design. Also, 6-month time series were obtained with a moored, 0.125 m2 trap at two sites. In the Anticosti Gyre, time-series fluxes were consistent with those obtained from the big drifting trap (means: 480 mg dry wt/m2/d; 39 mg C/m2/d), and with independently measured sediment accumulation rates. Numeric fluxes of phytoplankton cells were similar to moderately productive ocean margins during the April 1994 bloom, but otherwise resembled those from oligotrophic regimes. Fecal pellet numeric fluxes, in contrast, were always high, similar to other continental margins. The composition of the material collected by the small and big traps is a good indicator of the changing trophic regime in the water column. Relative numeric abundances suggest three distinctly different periods. A "bloom" period (represented by April 1994, but including a weaker late-fall bloom over a shelf valley), when a variety of centric and pennate diatom cells made up 70-95% of the particle numbers; a transitional or "post-bloom" period (June 1994), when phytoplankton were less abundant, pennate forms were scarce and a single species dominated the centric diatoms; and a "non-bloom" period (May to December, 1993) when fecal pellets and microzooplankton accounted for greater numbers than the phytoplankton cells, including abundant dinoflagellates. The time-series Anticosti Gyre trap showed continued large-particle settling throughout the winter with total mass and carbon fluxes similar to the ice-free seasons. The most frequent fecal pellets were 50-109 [mu]m diameter compact and loose rods, produced by the dominant calanoid copepods. Large macrozooplankton fecal pellets occurred only sporadically. Many pellets <49 [mu]m were collected in December 1993, probably produced by Microcalanus, which was unusually abundant at this time. Oval pellets occurred over a broad range of diameters, suggesting multiple origins. The bulk of the settling material produced by the pelagic food web in the Gulf appears to be of zooplanktonic origin (mainly fecal pellets and abundant microzooplankton). Diatom frustules were the most frequently encountered particles, numerically, but phytoplankton rarely made up more than 35% of the 2D projected area of all particles. Much of the organic matter produced photosynthetically must thus be transformed by heterotrophs before escaping from the surface and intermediate waters in this region.