Gulf of St. Lawrence Field Program |
Studies in the Gulf of St. Lawrence are characterizing the biologically mediated export of carbon from the euphotic zone and exploring the relationship between export and hydrodynamic processes. In addition, heterotrophic processes that affect carbon transport in the mid-water column are being identified and characterized in order to better understand how transfer of carbon to the sediments is affected by trophic structure.
A large part of the carbon dioxide flux towards the deep ocean is mediated by the photosynthetic fixation of inorganic carbon into organic matter, followed by export from the photic zone. Legendre and Le Fevre (1989) proposed a model which specifies five major bifurcations resulting in different pathways: sedimentation, herbivory, detritory, "baleen whale" feeding and marine snow. In this model the production of large cells favours export from the photic zone, while that of small cells results in high recycling (microbial food web) and correspondingly high losses of biogenic carbon through respiration. The above mentioned bifurcations are controlled by hydrodynamical singularities such as pycnoclines, ice-water interface, frontal and upwelling zones, and temporal transitions in the vertical stability of the water column with periodicities ranging from annual (e.g. spring blooms) to a few days (e.g. storms), to a few hours (e.g. semidiurnal tides), to even a few minutes (e.g. Langmuir circulation). The production of large cells is generally favoured by hydrodynamical singularities, because they provide a combination of high nutrient concentrations (required by large cells) with relative strong vertical mixing (necessary for retaining large nonmotile organisms like diatoms in the upper part of the water column; Margalef 1978). These ideas lead to the following working hypothesis: Exportable large cells account for a proportion of primary production that increases with higher rates of upward diffusion of the limiting nutrient (generally nitrate). Thus one of the objectives of the project is to link production export to hydrodynamics, thereby contributing to a significant decrease in variance in estimates of production export and a reduction in the number of degrees of freedom in export models.
Besides the production and export of organic carbon from the euphotic zone, the fact of organic matter in the whole water column is studied. A large part of the organic carbon produced by photosynthesis is consumed by heterotrophic organisms (bacteria, microzooplankton (see Demers) and mesozooplankton). These heterotrophic processes can both retain ingested carbon in the water column and accelerate the downward flux of carbon by the evaluation of their impact on the carbon export from the photic zone is one of the main objectives of the project. Specific tasks include the evaluation of the coupling between the primary producers and the different heterotrophic processes and its geographic, seasonal and diurnal variations; the evaluation of the geographic, seasonal and diurnal variations of feces production by mesozooplankton; and the determination of the relative importance of particle sinking versus grazing in the carbon export by characterization of particles collected in sediment traps placed below the euphotic zone (in collaboration with Silverberg, see Boudreau).
Part of the organic carbon is eventually remineralized to CO2 by respiration. This process proceeds throughout the water column and a fraction of photosynthetically produced organic matter is respired at depth. This difference between the vertical distributions of photosynthesis and respiration drives the biological pump. The magnitude of the mismatch, possibly regulated by the physical structure, reflects different potentials for downward carbon flow. Therefore, one of the objectives is to characterize the vertical distribution of respiration for comparison with that of photosynthesis and with the vertical physical structure. An analogous mismatch phenomena takes place over the size spectrum of planktonic organisms contained in a particular volume of water. Photosynthesis and respiration do not necessarily proceed over the same size ranges and discrepancies between their size distributions reflect different carbon flows up and down the size spectrum and different potentials for export out of the microbial community. Another objective is to characterize the size distribution of respiration in the upper layer for comparison with the size distributions of biomass, photosynthesis and nutrient flows.
As a long term objective, the different results of the project will be integrated into a 1-dimensional (vertical) description of carbon flows and of their relationship to physical structure.
Seven cruises in the Gulf region in two years have resulted in extensive data sets that include nearly all of the core measurements. Work within this group and collaboration with other projects has extended results to include benthic processes, the microbial loop and the effect of trophic structure on carbon fluxes. Integration of these results will provide a much better understanding of the influence of physical processes on production, recycling and carbon export.
Measurements made at each station:
Ships position with time of deployment and recovery
time release messenger etc.: no. but this is not important since we are not working in
deep waters.
Meteorology: all 8 standard observations are made.
Temperature: CTD
Salinity: CTD
Oxygen: bottles, no titrations on board of ship
O2 sensor on CTD since cruise 4
Nutrients: NO2, NO3, NH4, PO4, SiO2 and urea, generally measured directly on board of the
ship (Technicon)
Carbonate measurement: Total DIC (collaboration with Mucci) and pH
Pigments: chlorophyll a by fluorometer all sampled depths; Size fractionated in the photic
zone except from cruises 5 and 6
HPLC analysis from surface, chla max, 50 and 100m
HPLC analysis of copepod gut pigments, samples collected 4 times during 24 h
HPLC analysis and chl a by fluorometer from sediment trap samples
POC and PON from cruise 2 on, also on sediment trap samples
Seston from cruise 2 on, also on sediment trap samples
DOC: not yet measured
Primary production by C-14: incubations not in situ but simulated in situ (deck
incubators), with blue and neutral filters for simulation of the in situ light. Size
fractionated except for cruises 5 and 6.
Bio-optical measurements: PAR profiles are collected with every CTD profile since cruise
5, during cruises 1-4 the secchi disk was used; Surface PAR measurements during cruise 3
and 6
Fluorescence: CTD
Transmission: CTD
Bacterial abundance, biomass and production since cruise 4
Picoplankton cell counts and cynobacterial abundance since cruise 4
Phytoplankton cell counts
ETS (size fractionated)
Direct plankton respiration, since cruise 5, size fractionated
Mesoplankton abundance and grazing: 3 depths (surface, mid and deep layers) at midnight
and noon; defecation rates and ingestion rates
Microzooplankton abundance, since cruise 4 (only in selected depths)
Microzooplankton grazing: see Demers
Floating sediment traps: at 50 m, at all stations, since cruise 4, and 150 m (or 80 m),
only at deep stations, cruises 1,5,6,7,etc. (collaboration with Silverberg, see Boudreau)
Moored sediment traps at stations 1 and 6 (collaboration with Silverberg, see Boudreau)
ADCP, till 250 m cruises, 1,2 and 3 and till 75 m since cruise 5
Turbulence profiler, cruise 5 and from cruise 7 on
Standard depths for sampling : 100, 50, 25, 15, 10, 1, and 0.1% light depth, 50, 75, 100,
150, 200, 250, 300, 350 m and close to bottom (depending on the depth and the station and
the photic zone).
All samples are prefiltered through 200 µm mesh.
Size fractionation by filtration through 5 µm meshes and GF/F filters.