Seasonal budgets

Based on different data on biomass, size structure, and metabolic processes (respiration, production, grazing, excretion, etc.) collected during the Canadian JGOFS program, we used inverse methodology to obtain seasonal mass-balance models that estimate carbon fluxes among different compartments or trophic groups in the euphotic layer. Our results show a seasonal global shift in the size structure of the trophic plankton food web, the carbon cycle, and the potential export in the Gulf of St. Lawrence between the winter-spring and summer-fall periods that is related to different oceanographic regimes.

 

Winter-spring period

November, December, and April here is generally characterized by a mean surface temperature near 0°C, weak vertical stratification (surface and intermediate layers well mixed), and high nutrient concentrations (phosphate: 0.9 µM, nitrate: 6.0 µM, and silicate: 8.1 µM). The net metabolism is autotrophic (ratio of respiration to primary production < 1). The chlorophyll a concentration, primary production, and community respiration are dominated by large-celled phytoplankton. Bacterial biomass and production are relatively low (biomass: 96 mg C m-2, production: 42 mg C m-2 d-1). The zooplankton are mainly herbivorous.

This pathway is more efficient for exporting a large amount of organic matter to deep waters. Total export (residual flux and sedimentation flux) out of the euphotic layer is equivalent to 72% of the total primary production during the winter-spring period.

Trophic fluxes (organic matter consumed in milligrams of carbon per square meter per day; mg C m-2 d-1) estimated by inverse modelling in the euphotic layer during the winter-spring period.

Trophic fluxes estimated by inverse modelling


 

Summer-fall period

The summer-fall period (May to October) is generally characterized by a mean surface temperature > 10°C, a well-stratified water column (surface and intermediate layers well distinguished), and low nutrient concentrations (phosphate: 0.4 µM, nitrate: 0.5 µM, and silicate: 2.1 µM). The net metabolism is heterotrophic (ratio of respiration to primary production > 1). Small-celled phytoplankton dominate the chlorophyll a concentration, primary production, and community respiration. Bacterial biomass and production are relatively high (biomass: 254 mg C m-2, production: 145 mg C m-2 d-1). The zooplankton are mainly omnivorous and graze non-chlorophyllous prey such as heterotrophic dinoflagellates and ciliates (protozoa).

This pathway is very efficient for recycling and keeping carbon in the surface waters (total export). Summing the inflowing (residual flux) and outflowing (sedimentation flux) fluxes leaves a net export of only 6 mg C m-2 d-1, or 1% of the total primary production out of the euphotic layer during the summer-fall period.

Trophic fluxes (organic matter consumed in milligrams of carbon per square meter per day; mg C m-2 d-1) estimated by inverse modelling in the euphotic layer during the winter-spring period.

Trophic fluxes estimated by inverse modelling