bedford institute of oceanography 2001 in review - Région des ...

SCIENCE ACTIVITIES / Ocean Sciencesdeeper. It plays a critical role in the uptake and long-term sequestrationof anthropogenic CO 2 .Less well known is the role of deep convective areas such as theLabrador Sea in the sequestration of CO 2 through the biologicalpump. The biological pump is the transfer of carbon from thesurface to the deep ocean through biological processes. It beginswith the production of organic carbon from dissolved CO 2 throughphotosynthesis. A fraction of that organic material escapes theocean’s surface, effectively removing CO 2 from the atmosphereoceansystem. In most of the world’s oceans, this export of organiccarbon is due mostly to the settling of detritus (sinking flux) and, toa lesser extent, to the vertical motions of marine biota (verticalmigration flux). Thus, in most of the world’s oceans, organic carbonmoves through the water column to reach the depths. In convectiveareas, however, such as the Labrador Sea, dissolved organic carbon(DOC) can be transported with the water to the depths where it canbe sequestered. A collaborative project with Memorial University inNewfoundland was formed to explore the strength of this DOC fluxrelative to other export fluxes and its sensitivity to climate usingecosystem models and physical information on the climate of theLabrador Sea.The project uses an ecosystem model that simulates the flows ofnitrogen among various ecosystem compartments and tracks thevertical motions of organic nitrogen. Nitrogen is used because it isassumed to limit biological activity in the ocean. The associatedcarbon flows are calculated using mostly fixed ratios. In some cases,we introduce simple rules that essentially say that organisms prefer toretain nitrogen in their bodies, because it is in short supply, and expelcarbon. This model has been tested extensively in the Gulf of St.Lawrence. When applied to the Labrador Sea, the model suggests thatthe DOC flux is as large as the sinking flux and is substantially largerthan the vertical migration flux (Figure 1). The model also suggeststhat the DOC flux is particularly efficient in moving organic carbonto depth, while retaining nitrogen in the surface ocean. This decouplingof carbon and nitrogen fluxes enhances the biological pumpwhen nutrients are limiting.We also examined the impact of variations in the strength ofwinter convection on carbon export (Figure 2). To study this effect,the ecosystem model was forced with data on the physical structureand climate of the Labrador Sea from periods when convection wasweak (the late 1960s and early 1970s) and when it was strong (early tomid-1990s). The results indicate that the sinking flux is relativelyinsensitive to the variations in ocean climate. In contrast, the DOCflux is very sensitive and is the main contributor to climate-relatedvariations in total export. This goes against prevailing views that thesinking flux is the main driver behind export variability. More to thepoint, this gives clues as to the potential impact of climate change onorganic carbon cycling in the Labrador Sea if climate warming doesresult in a reduction of winter convection. Lower organic carbonexport would leave more CO 2 in surface waters and lower the ocean’suptake of CO 2 .Relative change(100 is average convection)200150100500ShallowAverage DeepMigrationSinkingDOMExportFigure 2. Relative changes in export fluxes simulated by the physical-ecosystemmodel under shallow (< 200 m), average (300-1000 m) and deep (> 1500 m)maximum depth of winter convection in the central Labrador Sea.Figure 1. Relative importance of export fluxes in the Labrador Sea simulated by aphysical-ecosystem model. C export is the total downward organic carbon fluxacross the 200 m isobath (moles C/yr). C:N export is the carbon-to-nitrogen ratio ofthe exported organic matter (moles C/moles N); a C:N higher than 6.6, the averageC:N of organic matter, indicates that C is preferentially exported. C export and C:Nexport averaged across a number of simulations are given at the top of each barand are broken down into contributions from dissolved organic matter (DOM),particle sinking, and vertical migration.These results are preliminary and subject to change as the workevolves. Nevertheless, they are stimulating further research. Oneongoing effort is to develop and refine an ecosystem model thatcouples carbon and nitrogen flows across all compartments, asopposed to only some of the compartments in this model. A relatedeffort is to investigate the implications of the DOC flux for climatevariations at a global scale by coupling the improved ecosystemmodel to an atmosphere-ocean climate model. This work will be partof a new international research initiative (SOLAS or Surface OceanLower Atmosphere Study) that DFO has joined along with a networkof Canadian universities and other government laboratories. As partof a team working to understand the future of the ocean’s carbonsink, our goal is to learn more about potential interactions betweenDOC flux and climate.22 / BIO-2001 IN REVIEW