First observations of the nature of sedimenting particles in The Gulf of St. Lawrence, Canada

Abstract

SEDIMENT TRAP SAMPLES HAVE PROVIDED THE FIRST DIRECT OBSERVATIONS OF THE SINGKING PARTICLES THAT ACCOUNT FOR THE EXPORT OF MATERIAL OUT OF THE PHOTIC ZONE IN THE GULF OF ST. LAWRENCE AND THEIR RELATIONSHIP TO VARIATIONS IN THE TROPHIC REGIME. PARTICLES WERE COLLECTED AT SEVERAL SITES OVER 24-HOUR PERIODS USING 0.03 m2 (COLLECTING SURFACE) AND 0.5 m2 FREE-DRIFTING SEDIMENT TRAPS AT 50 AND 150 m. TOTAL MASS FLUX VARIED WIDLEY (80-1500 mg/m2/d), AS DID CARBON FLUX (16-300 mgC/m2/d). SMALL CYLINDERS CONSISTENTLY OVER SAMPLED WITH RESPECT TO BIG CYLINDERS, REGARDLEES OF DEPTH OF 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 LARGE DRIFTING TRAP (MEANS: 480 mg DRY wt/m2/d; 39 mgC/m2/d), AND WITH INDEPENDENTLY MEASURED SEDIMENT ACCUMULATION RATES. NUMERIC FLUXES OF PHYTOPLANKTON CELLS WERE SIMILAR TO THOSE FOUND IN 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 LARGE 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; AN A "NON-BLOOM" PERIOD (MAY TO DEC., 1993) WHEN FECAL PELLETS AND MICROZOOPLANKTON ACCOUNTED FOR GREATER NUMBERS THAN THE PHYTOPLANKTON CELLS, INCLUIDING ABUNDANT DINOFLAGELLATES. THE TIME-SERIES ANTICOSTI GYRE TRAP SHOWED CONTINUED LARGE-PARTICLE SETTLING THROUGHOUT THE WINTER WITH TOTAL MASS AND CARBON FLUXES SIMILAR TO THOSE FOUND IN THE ICE-FREE SEASON. THE MOST FREQUENT FECAL PELLETS WERE 50-109 um DIAMETER COMPACT AND LOOSE RODS, PRODUCED BY THE DOMINANT CALANOID COPEPODS. LARGE MACROZOOPLANKTON FECAL PELLETS OCURRED ONLY SPORADICALLY. MANY PELLETS <49 um WERE COLLECTED IN DECEMBER 1993, PROBABLY PRODUCED BY Microcalanus, WICH WAS UNUSUALLY MULTIPLE ORIGINS. THE PARTICULARE ORGANIC CARBON (POC) AND THE C CONTRIBUTION OF THE DIFFERENT TYPES OF PARTICLES HAVE BEEN ANALYZED FOR FREE.DRIFTING SEDIMENT TRAPS. TWO TRAP MODELS ONLY WERE USED IN 1993-1994: SMALL TRAPS AT 50 m AN AND LARGER TRAPS AT 50 AND 150 m. ABSOLUTE MEAN FLUXES OF POC ( 42- 149 mg C m-2 d-1 ON LARGE AND SMALL TRAPS RESPECTIVELY), OF C ATTRIBUTED TO FECAL PELLETS (6 -60 mg C m2 d-1) AND OF C ATTRIBUTED TO PHYTOPLANKTON (3.2 - 42.9 MG C m-2 d-1) WERE ALL IN THE SAME RANGE AS THOSE ENCOUNTERED IN REGIONS OF MODERATE PRODUCTIVITY. FECAL PELLETS WERE THE MAJOR COMPONENT OF T HIS FLUX WITH AN IMPORTANT CONTRIBUTION OF MICROZOOPLANKTON, PARTICULARLY DURING THE SUMMER OF 1994, THE PHYTOPLANKTON CONTRIBUTION TO POC FLUX WAS LIGHTLY SMALLER THAN THE FECAL PELLETS CONTRIBUTION. THE IDENTIFICATION OF ALGAL GROUPS THAT ARE PART OF THIS FLUX LED TO RECOGNITION OF THE THREE TROPHIC REGIMES ALREADY IDENTIFIED FROM WATER COLUMN STUDIES AND FROM THE NUMERIC FLUXES IN THIS STUDY: (1) A "BLOMM" PERIOD, WHEN THE DIATOMS WERE DOMINANT DURING SPRING, (2) "NON-BLOMM" PERIOD, A LONGER INTERVAL, WICH WAS DOMINATED BY DINOFLAGELLATES AND (3) A TRANSITION "POST-BLOOM" PERIOD DURING THE SUMMER. ALTHOUGH THE CONTRIBUTION OF MARINE SNOW WAS ESTIMATED, ITS REAL IMPORTANCE IN THE VERTICAL FLUX OF POC REMAINS UNCERTAIN. 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).. MUCH OF THE ORGANIC MATTER PRODUCED PHOTOSYNTHETICALLY MUST THUS BE TRANSFORMED BY HETEROTROPHOS BEFORE ESCAPING FROM THE SURFACE AND INTERMEDIATE WATERS IN THIS REGION. THE VERTICAL FLUX OF RECOGNIZABLE C AT THE GSL IS DOMINATED BY COMPONENTS OF ANIMAL ORIGIN.