Selected Publications

Losa, S.N., A. Vézina, D. Wright, Y. Lu, K. Thompson and M. Dowd. 2006.

3D ecosystem modelling in the North Atlantic: Relative impacts of physical and biological parameterizations. Journal of Marine Systems 61 (2006) 230-245. Abstract   (download pdf file @ 1.56MB).

Vézina, Alain F. 2004.

Ecosystem modelling of the cycling of marine dimethylsulfide: a review of current approaches and of the potential for extrapolation to global scales. Can. J. Fish. Aquat. Sci. 61: 845-856 (2004). Abstract   (download pdf file @ 296k).

Tian, R. C., Don Deibel, Richard B. Rivkin, Alain F. Vézina 2004.

Biogenic carbon and nitrogen export in a deep-convection region: simulations in the Labrador Sea. Deep-Sea Research I 51 (2004) 413-437 Abstract   (download pdf file @ 454).

Savenkoff, C., M. Castonguay, A.F. Vézina, S.-P. Despatie, D. Chabot, L. Morissette and M.O. Hamill. 2004.

Inverse modelling of trophic flows through an entire ecosystem: the northern Gulf of St. Lawrence in the mid-1980s. Can. J. Fish. Aquat. Sci. 61: 2194-2214. (download pdf file @ 554k)

Vézina, A.F., F. Berreville and S. Losa. 2004.

Inverse reconstructions of ecosystem flows in investigating regime shifts: impact of the choice of objective function. Prog. Oceanogr., 60: 321-341. (download pdf file @ 637k)

Le Clainche, Yvonnick, Maurice Levasseur, Alain Vézina, John W.H. Dacey, François J. Saucier. 2004.

Behaviour of the ocean DMS(P) pools in the Sargasso Sea viewed in a coupled physical-biogeochemical ocean model. Can. J. Fish. Aquat. Sci., 61: 788-803. (download pdf file @ 980k)

Alain F. Vézina, Markus Pahlow 2003

Reconstruction of ecosystem flows using inverse methods: how well do they work? Journal of Marine Systems 40-41 (2003) 55-77. Abstract   (download pdf file @ 657k)

Pahlow, M. and A. F. Vézina. 2003.

Adaptive model of DOM dynamics in the surface ocean, Journal of Marine Research 61: 127--146, 2003. (download pdf file @ 210k).

Tian, R. C., A. F. Vézina, D. Deibel, and R. B. Rivkin 2003

Sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a deep-water formation region. Global Biogeochem. Cycles, 17(4), 1090, 2003. Abstract   (download pdf file @ 179k.)

Irz P, Laurent A, Messad S, Pronier O, Argillier C. 2002

Influence of site characteristics on fish community patterns in French reservoirs. Ecology of Freshwater Fish, 2002, Vol. 11 Issue 2, p123-136. (download pdf file @ 703k.)

Lefèvre, M., A. Vézina, M. Levasseur and J. Dacey. 2002.

A model of DMS production and dynamics in the subtropical North Atlantic. Deep-Sea Res. 49(12) 2221-2239. (download pdf file @ 504k)

Tian, R., Vézina, A.F., Starr, M. and F.-J. Saucier. 2001.

Seasonal dynamics of coastal ecosystems and export production at high latitudes: a modeling study. Limnol. Oceanogr., 46: 1845-1859. (download pdf file @ 560k)

Savenkoff,Claude and Alain F. Vézina, and Alida Bundy 2001.

Inverse analysis of the structure and dynamics of the whole Newfoundland Labrador Shelf ecosystem. Canadian Technical Report of Fisheries and Aquatic Sciences 2354 (download pdf file @ 410k)

C. Savenkoff, A.F. Vézina, S, Royb, B, Klein, C, Lovejoy, J.-C. Therriault, L, Legendre, R, Rivkind, C, Berube, J.-C.. Tremblay, N, Silverberg 2000

Export of biogenic carbon and structure and dynamics of the pelagic food web in the Gulf of St. Lawrence. Part 1, Seasonal variations. Deep-Sea Research II 47 (2000) 585-607. Abstract   (download pdf file @ 410k)

Selected Abstracts

C. Savenkoff, A.F. Vézina, S, Royb, B, Klein, C, Lovejoy, J-C, Therriault, L, Legendre, R, Rivkind, C, Berube, J.-C. Tremblay, N, Silverberg 2000
Export of biogenic carbon and structure and dynamics of the pelagic food web in the Gulf of St. Lawrence
Part 1, Seasonal variations
Deep-Sea Research II 47 (2000) 585-607
Abstract
The seasonal changes in photosynthetic production, respiration, sinking flux of organic carbon, and food web structure are described in the Gulf of St. Lawrence over a two-year period during the Canadian Joint Global Ocean Flux Study (JGOFS) program. The results show contrasts in net metabolism between periods of low (winter and spring) and high (summer and Fall) vertical stability. The winter-spring period was associated with an autotrophic pelagic food web: predominance of large phytoplankton cells, large zooplankton, and high herbivorous potential transfers towards the zooplankton. The stratified summer-fall period was associated with a heterotrophic food web: dominance of small phytoplankton cells, replacement of the size class occupied by large phytoplankton with large heterotrophic dinoflagellates and ciliates, smaller zooplankton, and dominance of omnivorous transfers towards the zooplankton. Despite differences in algal size and composition as well as in size structure of the trophic compartments between winter-spring and summer-fall, the particulate organic carbon fluxes observed at 50 m depth was quantitatively similar during these two periods. Even though winter photosynthetic production was relatively low, the high chlorophyll a concentration, the size structure of the trophic compartments, and the high contribution of large phytoplankton cells (mainly diatoms) to biological activity were similar to those observed during the spring and could explain the high heterotrophic biomass observed during winter.

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Tian, R. C., A. F. Vézina, D. Deibel, and R. B. Rivkin 2003
Sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a deep-water formation region
Global Biogeochem. Cycles, 17(4), 1090, 2003.
Abstract
We used a physical-biogeochemical model to examine the sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a subpolar, deep-water formation region. Documented changes in winter mixed layer depth between the late 1960s and the mid-1990s were used to construct scenarios of weak, moderate, and strong winter convection that drive the biogeochemical model. The model simulations suggest that the total biogenic carbon export (particle sinking flux + DOC export) is higher under strong winter convection (eg. during the early 1990s) than under weak winter convection (eg. during the late 1960s), by -70% axcross the 200-m isobath and nearly double at 500 m and 1000 m depth. These large variations in total biogenic carbon export are essentially due to the response of DOC export to ocean climate conditions. Sensitivity analyses indicate that the variations in DOC export from the euphotic zone are due to the impact of the convection regime on the development of the microbial food web and on the bacterial consumption of DOC in surface waters. Although DOC downward fluxes within the mesopelagic zone (below -500 m) are largely controlled by physical processes, the effect of convection on microbial dynamics can potentially amplify the year-to-year variations in the transport of DOC to the deep ocean due to convection.

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Alain F. Vézina, Markus Pahlow 2003
Reconstruction of ecosystem flows using inverse methods: how well do they work?
Journal of Marine Systems 40-41 (2003) 55-77
Abstract
Inverse modelling is used to synthesize multivariate observations from marine and freshwater ecosystems into descriptions of mass flows among ecological and biogeochemical components. However, the conditions that affect the accuracy of these analyses remain poorly understood. In particular, it is suspected that the steady-state assumption often used in these analyses and the flow minimization principle that underlie inverse modelling introduce distortions into the reconstructions of ecosystem flows, but these potential biases have not been quantitatively investigated. Simulated inverse experiments were conducted to shed some light on these issues. In these experiments, inverse analyses are run on 'artificial' observations generated from a mechanistic ecological-biogeochemical model. The simulated experiments indicate that the steady-state assumption has little impact on the accuracy of inverse reconstructions of ecosystem flows. Inverse analyses run on observations from simulations of transient states are as accurate as analyses run on observations from simulations at steady state. The accuracy of inverse reconstructions is related to structural and dynamic features of the ecosystem. Inverse reconstructions on simulated ecosystems with weak nutrient recyclsng (dependent mostly on external nutrients) or with simple food webs show little bias. Reconstructions of simulated ecosystems with strong recycling or complex food webs show significantly more bias, with a tendency to overestimate small flows and to underestimate large flows: Despite these biases, inverse reconstructions were successful at detecting changes in flow structure associated with changes in simulated ecosystem properties. The simulations also indicate that the inverse analyses based on simultaneous accounting of more than one currency (e.g. carbon+ nitrogen) should be preferred over analyses based on balancing only one currency (e.g. carbon or nitrogen).

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Markus Pahlow and Alain F. Vézina 2003
Adaptive model of DOM dynamics in the surface ocean
Journal of Marine Research, 61, 127-146, 2003
Abstract
The bulk of marine dissolved organic matter (DOM) forms a large, old, refractory carbon pool in the deep ocean, yet a small fraction in the surface ocean is actively involved in the global carbon cycle and may contribute significantly to the biological pump. We argue that present models of plankton and DOM in the surface ocean are incompatible with current knowledge of marine DOM dynamics. We present a plankton model with an adaptive formulation of bacteria-DOM interactions which is more consistent with observations. Our model reproduces net accumulation of DOM and is the first to reconcile the prevailing reports of net consumption of inorganic nitrogen by bacteria with commonly found DOC:DON ratios in the surface ocean.
Our model predicts that factors governing DOM production by phytoplankton and zooplankton have little influence on DOM accumulation in the surface ocean. Longterm accumulation, eventual export of DOM, and hence its contribution to the biological pump appear to be primarily controlled by characteristics of bacterial DOM utilization. The model implies a negative relation between temperature and DOM accumulation, which can be obscured, however, by the impact of temperature on water-column stability. On longer time scales, this negative relation could indicate a positive feedback between temperature and CO₂. DOM can accumulate independently of inorganic nutrient content of the surface ocean. Therefore, the predicted positive feedback is potentially very strong and could help explain the large variations in atmospheric CO₂ between glacial and interglacial periods.

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Pahlow, M. and U. Riebesell.
Long-term trends in deep ocean Redfield ratios
Science 287: 831-833, 2000.
The Redfield ratio [ carbon:nitrogen:phosphorus (C:N:P)] of particle flux to the deep ocean is a key factor in marine biogeochemical cycling. Changes in oceanic carbon sequestration have been linked to variations in the Redfield ratio on geological time scales, but this ratio generally is assumed to be constant with time in the modern ocean. However, deep-water Redfield ratios in the northern hemisphere show evidence for temporal trends over the past five decades. The North Atlantic Ocean exhibits a rising N:P ratio, which may be related to increased deposition of atmospheric nitrous oxides from anthropogenic N emissions. In the North Pacific Ocean, increasing C:N and C:P ratios are accompanied by rising remineralization rates, which suggests intensified export production. Stronger export of carbon in this region may be due to enhanced bioavailability of aeolian iron. These findings imply that the biological part of the marine carbon cycle currently is not in steady state.

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Vézina, A.F. 2004
Ecosystem modelling of the cycling of marine dimethylsulfide: a review of current approaches and of the potential for extrapolation to global scales, Can. J. Fish. Aquat. Sci.61, 845 - 856
Abstract
There is rising interest from oceanic and atmospheric scientists in the potential role of dimethylsulphide (DMS) in regulating global climate. The increased availability of field observations of DMS and related compounds ( DMS(P)) and of their transformation rates in the ocean has stimulated the development of ecosystem models of marine sulfur cycling. The models cover a wide range of complexity levels and spatial/temporal scales, from zero-dimensional local simulations spanning a few days to regional/global simulations driven by ocean general circulation models. The degree of complexity required to model DMS(P) dynamics, particularly the differentiation into phyto plankton species or groups, remains an important open question. First attempts to drive these models with vertically resolved turbulence models suggest interesting interactions between DMS(P) dynamics and fine-scale ocean mixing that can modify fluxes of DMS to the atmosphere. Recent models also bring into focus the strong affinities between the cycling of DMS(P) and that of dissolved organic carbon in the surface ocean. Formal parameter estimation techniques, which are increasingly used in ecosystem modelling of carbon and nitrogen dynamics, should play a stronger role in the development of DMS sulfur modelling. Extrapolation of DMS cycling and fluxes to the global scale presently relies largely on empirical approach. A semiempirical approach, based on a simple ecosystem model, is shown to reproduce gross features of the global distribution of DMS in the surface ocean. This shows promise for the continuing development of ecosystem models for global modelling of marine sulfur fluxes to the atmosphere.

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A. F. Vézina and F. Berreville and S. Loza 2004
Inverse reconstructions of ecosystem flows in investigating regime shifts: impact of the choice of objective function
Progress in Oceanography 60, 321 - 341
Abstract
Inverse analysis is increasingly used in ecosystem modelling to objectively reconstruct a large number of unknown flows or interactions from a small number of observations. This type of analysis may be useful in relating observed regime shifts in ecosystem structure to underlying processes. Inversions of ecosystem flow networks currently constrained least-squares solution which at the same time minimizes the squared norm (the sum of squares) of the reconstructed flows. This minimum norm (MN) inversion is thought to be a parsimonious solution to the ecosystem flow inverse problem, but it may well not reflect how ecosystems are organised. It has been proposed instead that ecosystems evolve to maximize energy/mass flows or that they maximize the information content of the network weighted by ecosystem flows (ascendancy). We used simulated inverse experiments, where inverse analyses are applied to simulations of flow networks, to explore objective functions different than the MN generally used. We could not compute inverse solutions that maximize ascendancy because the objective function is unbounded. We could calculate inversions that maximize flows; however, these generally overestimated the simulated flows, even though the simulations were designed to maximize flows. It appears that the ecosystem flow inverse problem is too under-determined (too few data relative to the number of unknowns) to allow the use of these maximizing goa functions. We introduce a new minimization that simultaneously minimizes the squared flows and the squared differences between flows. This smoothing minimization makes the inverse flows as even as possible and it helps with some technical issues with MN inversions. The simulated inverse experiments indicated that this smoothed norm (SM) is the most robust in comparative analyses of contrasting ecosystem states, such as those that can be associated with regime shifts. Like the MN inversion, the SN inversion has no ecological basis. However, it is a conservative nor that is less likely to produce false differences between the dynamics of regimes.

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C. Savenkoff and M. Castonguay and A.F. Vézina and S.-P. Despatie and D. Chabot and L. Morissette and M.O. Hamill 2004
Inverse modelling of trophic flows through an entire ecosystem: the northern Gulf of St. Lawrence in the mid-1980s
Can. J. Fish. Aquat. Sci.61, 2194-2214
Abstract
Mass-balance models using inverse methodology have been constructed for the northern Gulf of St. Lawrence ecosystem in the mid-1980s, before the groundfish collapse. The results highlight the effects of the major mortality sources (fishing, predation, and other sources of mortality) on the fish and invertebrate communities. Main predators of fish were large cod (Gadus morhua) followed by redfish (Sebastes spp.), capelin (Mallotus villosus), and fisheries. Large cod were the most important predator of small cod, with cannibalism accounting for at least 44% of the mortality of small cod. The main predators of large cod were harp (Phoca groenlandica) and grey (Halichoerus grypus) seals. However, predation represented only 2% of total mortality on large cod. Mortality other than predation dominated the mortality processes at 52% of the total, while the fishery represented 46%. Tests were performed to identify possible sources of this unexplained mortality. The only way to significantly reduce unexplained mortality on large cod in the model was to increase landings of large cod above those reported. This suggests that fishing mortality was substantially underestimated in the mid-1980s, just before the demise of a cod stock that historically was the second largest in the northwest Atlantic.

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Tian, R. and D. Deibel and R. Rivkin and A. Vézina 2004
Biogenic carbon and nitrogen export in a deep convection region: simulations in the Labrador Sea
Deep-Sea Research 51, 413-437
Abstract
The Labrador Sea is a major sink of anthropogenic CO₂ due to deep-water formation in winter. To investigate the relative importance of different forms of export flux, we used a physical-biogeochemical model to simulate the vertical fluxes of particulate and dissolved biogenic carbon as a function of winter convection, food web dynamics and zooplankton vertical migration. The C:N ratio of these export fluxes was simulated based on trophic dynamics and bacterial activity. The model was run using winter convection and seasonal mixed layer evolution extracted from multi-year physical data collected in the central Labrador Sea. Comparisons between model output and data from the Labrador Sea and other systems indicate that the model provides a realistic picture of carbon and nitrogen pools and fluxes. Our results suggest that on an annual basis, dissolved organic carbon (DOC) export by deep, vertical convection is greater than that of the sinking flux of POC. Furthermore, the C:N ratio of exported dissolved organic matter (DOM) is higher than that of the particle sinking flux, resulting in 23% more carbon exported than would be estimated if predictions were made from the Redfield ratio (e.g., 11.4 vs. 7.0 for DOM and particulate organic matter, respectively, at the bottom of the euphotic zone and 17.2 vs. 9.3 at 1000 m depth). The active export of carbon by the respiration and mortality of migrating zooplankton amounts to 19% of sinking flux annually, but only 6% of total carbon export because of the high rates of DOC export in deep-water formation regions. Our model simulations indicate that non-Redfield ratio DOC export characterizes the function of the biological pump in deep-water formation regions.

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Tian, Ru Cheng and Alain F. Vézina and Michel Starr and François Saucier 2001
Seasonal dynamics of coastal ecosystems and export production at high latitudes: A modeling study
Limnology and Oceanography 46(8), 1845-1859
Abstract
Export of organic matter from the surface to deeper waters often shows much smaller seasonal variations than primary production or nitrate-based new production in mid- to high-latitude marine systems. The mechanisms underlying this pattern remain poorly understood, but seasonal shifts in food web structure and dynamics have been implicated. We report here on an ecosystem modeling analysis of a high-resolution (biweekly) time series of biomass, production, and export flux (sediment trap) measurements conducted in 1991 in Bonne Bay (Newfoundland). This time series shows the classical pattern of a spring bloom followed by a summer low biomass period, yet export is bimodal, with maxima during spring and late summer. The ecosystem model was forced by diagnostic vertical mixing calculations based on temperature and salinity records taken every 3 d and hourly wind data. The physical analysis indicated that the nitrate flux into the euphotic zone during summer was equivalent to that during the spring and fall seasons and accounted for half of the summer export. Statistical adjustments of the parameters of the ecosystem model indicated that strong production of dissolved organic carbon during the spring bloom, high temperature dependence of microbial activity, and physico-chemical particle aggregation played key roles in explaining the remainder of the summer export. Seasonal changes in trophic pathways between spring and summer, such as a shift from a herbivorous to a microbial food web, played a comparatively smaller role. Our modeling analysis suggests that physical mixing processes and physico-chemical aggregation processes are at least as important as shifts in food web trophic pathways in explaining the postbloom export flux in mid- to high-latitude marine systems.

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Lefèvre, M. and A. Vézina and M. Levasseur and J. Dacey 2002
A model of DMS production and dynamics in the subtropical North Atlantic
Deep-Sea Research 49, 2221-2239
Abstract
Dimethylsulfide (DMS) is a volatile sulfur compound produced by the marine biota. The flux of DMS to the atmosphere may act on climate via aerosol formation. It is therefore important to improve our understanding of the processes that regulate sea surface DMS concentrations for eventual inclusion into climate models. In order to simulate the dynamics of DMS concentrations in the mixed layer, a model of DMS production was developed and calibrated against a 1 year time-series of DMS and DMSP (dissolved and particulate) data collected in the Sargasso Sea at Hydrostation 'S'. The model reproduces the observed divergence between the seasonal cycles of particulate DMSP, the DMS precursor produced by algae, and DMS produced through the microbial loop from the cleavage of dissolved DMSP. DMSPp (particulate) reaches its maximum in the spring whereas DMSPd (dissolved) and DMS reach maximum concentrations in summer. Several parameters had to vary seasonally and with depth in order to reproduce the data, pointing out the importance of physiological and structural changes in the plankton food web. These parameters include the intracellular S(DMSp):N ratio, the C:Chl ratio and the sinking rates of phytoplankton and detritus. For the Sargasso Sea, variations in the solar zenithal angle, which co-vary with the seasonal variations in the depth of the mixed layer, proved to be a convenient signal to drive the seasonal variation in the structure and dynamics of the plankton. Variations of the temperature and photosynthetically active radiation also help to reprodce the short-term variability of the annual S cycle. Results from a sensitivity analysis show that variations in DMSPp are dependent mostly on parameters controlling phytoplankton biomass, whereas DMS is dependent mostly on variables controlling phytoplankton productivity.

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Yvonnick Le Clainche, Maurice Levasseur, Alain Vézina, John W.H. Dacey, and François J. Saucier 2004
Behaviour of the ocean DMS(P) pools in the Sargasso Sea viewed in a coupled physical-biogeochemical ocean model
Can. J. Fish. Aquat. Sci. 61, 788-803
Abstract
The dimethylsulfide (DMS) production model NODEM (Northern Oceans DMS Emission Model) was coupled with the water column ocean model GOTM (General Ocean Turbulence Model) that includes a two-equation κ-ε turbulence scheme. This coupled physical-biogeochemical ocean model represents a significant improvement over the previous uncoupled version of NODEM that was driven by a diagnostic vertical mixing scheme. Using the same set of biogeochemical parameters, the coupled model is used to simulate the annual cycles of 1992 and 1993 at Hydrostation S in the Sargasso Sea. The better reproduction of the turbulent mixing environment corrects some deficiencies in nitrogen cycling, especially in the seasonal evolution of the nutrient concentrations. Hence, the coupled model captures the late-winter chlorophyll- and DMS(P)-rich blooms. It is also more adept at reproducing the vertical distribution of chlorophyll and DMS(P) in summer. Moreover, the DMS pool becomes less dependent on parameters controlling the nitrogen cycle and relatively more sensitive to parameters related to the sulfur cycle. Finally, the coupled model reproduces some of the observed differences in DMS(P) pools between 1992 and 1993, the latter being an independent data set not used in calibrating the initial version of NODEM.

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Irz P, Laurent A, Messad S, Pronier O, Argillier C. 2002
Influence of site characteristics on fish community patterns in French reservoirs.
Ecology of Freshwater Fish 2002: 11: 123–136
Abstract
The objectives of this paper are to describe the fish assemblages in French reservoirs and to relate them to the sites’ characteristics. The results of 43 fishing surveys were collected and completed with environmental descriptors. Fish assemblages differ between salmonids-dominated mountain sites and lowland ones. The latter show higher species diversity and a distinction between rheophilic- and limnophilic-type communities. This distinction can be explained by the reservoir age, location in the catchment and depth. The response of fish communities to these variables was investigated by canonical correspondence analysis. It shows that rheophilic species are typically abundant in upper basin, deep and recently created reservoirs. The fish community response to the aging process corresponds to an addition of lowland standing waters species and an extinction of the native riverine ones. The structuring role of the sites’ depth, location and age is discussed, considering their relationship with the water body trophic status.

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Tian,R.C., Don Deibel, Richard B. Rivkin, Alain F. Vézina 2004
Biogenic carbon and nitrogen export in a deep-convection region: simulations in the Labrador Sea
Deep-Sea Research I 51 (2004) 413-437
Abstract
The Labrador Sea is a major sink of anthropogenic CO₂ due to deep-water formation in winter. To investigate the relative importance of different forms of export flux, we used a physical-biogeochemical model to simulate the vertical fluxes of particulate and dissolved biogenic carbon as a function of winter convection, food web dynamics and zooplankton vertical migration. The C:N ratio of these export fluxes was simulated based on trophic dynamics and bacterial activity. The model was run using winter convection and seasonal mixed layer evolution extracted from multiyear physical data collected in the central Labrador Sea. Comparisons between model output and data from the Labrador Sea and other systems indicate that the model provides a realistic picture of carbon and nitrogen pools and fluxes. Our results suggest that on an annual basis, dissolved organic carbon (DOC) export by deep, vertical convection is greater than that of the sinking flux of POC. Furthermore, the C:N ratio of exported dissolved organic matter (DOM) is higher than that of the particle sinking flux, resulting in 23% more carbon exported than would be estimated if predictions were made from the Redfield ratio (e.g., 11.4 vs. 7.0 for DOM and particulate organic matter, respectively, at the bottom of the euphotic zone and 17.2 vs. 9.3 at 1000m depth). The active export of carbon by the respiration and mortality of migrating zooplankton amounts to 19% of sinking flux annually, but only 6% of total carbon export because of the high rates of DOC export in deep-water formation regions. Our model simulations indicate that non- Redfield ratio DOC export characterizes the function of the biological pump in deep-water formation regions.

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Alain F. Vézina 2004
Can. J. Fish. Aquat. Sci. 61: 845-856 (2004)
Ecosystem modelling of the cycling of marine dimethylsulfide: a review of current approaches and of the potential for extrapolation to global scales
Abstract
There is rising interest from oceanic and atmospheric scientists in the potential role of dimethylsulphide (DMS) in regulating global climate. The increased availability of field observations of DMS and related compounds (DMS(P)) and of their transformation rates in the ocean has stimulated the development of ecosystem models of marine sulfur cycling. The models cover a wide range of complexity levels and spatial/temporal scales, from zerodimensional local simulations spanning a few days to regional/global simulations driven by ocean general circulation models. The degree of complexity required to model DMS(P) dynamics, particularly the differentiation into phytoplankton species or groups, remains an important open question. First attempts to drive these models with vertically resolved turbulence models suggest interesting interactions between DMS(P) dynamics and fine-scale ocean mixing that can modify fluxes of DMS to the atmosphere. Recent models also bring into focus the strong affinities between the cycling of DMS(P) and that of dissolved organic carbon in the surface ocean. Formal parameter estimation techniques, which are increasingly used in ecosystem modelling of carbon and nitrogen dynamics, should play a stronger role in the development of DMS sulfur modelling. Extrapolation of DMS cycling and fluxes to the global scale presently relies largely on empirical approach. A semiempirical approach, based on a simple ecosystem model, is shown to reproduce gross features of the global distribution of DMS in the surface ocean. This shows promise for the continuing development of ecosystem models for global modelling of marine sulfur fluxes to the atmosphere.

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B. Casault, A.F. Vézina and B. Petrie. 2003.
Can. Data Rep. Hydrogr. Ocean Sci. 164:v+306pp.
Atlas of surface mixed layer characteristics for the Scotian Shelf and the Gulf of Maine.
Abstract
The characteristics of the surface mixed layer, namely the depth and the mean temperature, salinity and density (expressed as sigma-t, σ_t), were calculated from temperature and salinity profiles collected over the Scotian Shelf and the Gulf of Maine areas and spanning the years 1900 - 2003. The results are presented for several subregions and include the surface mixed layer characteristics for each individual observed profile, the monthly statistics calculated over the entire time period as well as the spatial and temporal distributions of the observed profiles.

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A.F. Vézina and B. Casault. 2002.
AZMP Bulletin PZMA. 2:16-18.
Interannual variations in plankton dynamics at AZMP Sta. 2: first results from physical-biological modelling.
Abstract
Interannual variations in plankton dynamics are a critical feature of marine ecosystems and have been linked to variations in the recruitment of exploitable species. We report here on our first attempts to use a physical-biological model to simulate the observed dynamics at AZMP Sta. 2 from 1999 to 2001. These preliminary results indicate that the model is able to capture the mean seasonal cycle in plankton dynamics and also some features of the interannual differences in physical and plankton conditions. However, the interannual variations in the observations are stronger than in the model simulations and there are some critical differences between observed and predicted interannual differences. The AZMP data are an invaluable resource for pursuing the development and improvement of the physical-biological model and for increasing our capacity to predict impacts of climate change and climate variability on marine ecosystems.

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Losa, S.N., A. Vézina, D. Wright, Y. Lu, K. Thompson and M. Dowd. 2006.
Journal of Marine Systems 61: 230-245.
3D ecosystem modelling in the North Atlantic: Relative impacts of physical and biological parameterizations.
Abstract
A simple ecosystem model is coupled to a 3-dimensional general circulation model for the North Atlantic. The physical model is based on the Los Alamos Parallel Ocean Program (POP) and forced by climatological monthly mean data. Four biological components (phytoplankton, zooplankton, nutrients and detritus) are incorporated into POP as additional tracers with biological sources and sinks. The model solutions, obtained with different physical and biological parameterizations are compared against monthly mean SeaWiFS colour data averaged over the period 1997-2003 and Levitus's climatological nitrate data. A reference model solution, with constant biological model parameters over the whole basin, underestimates both the average chlorophyll level and its regional variability at mid- to high latitudes. Experiments with a different parameterization of heat and freshwater fluxes, which affects upper ocean mixing, indicate a strong impact of such parameterizations on nutrient supply to the surface layer at high latitudes, but with little impact on simulated chlorophyll. Other experiments where advection of the biological tracers is turned off show basically the same result: strong impact on regional nutrient patterns but a negligible impact on phytoplankton patterns. Only model runs with spatially variable biological parameters, obtained from a previous zero-dimensional ecosystem model calibration on CZCS ocean colour data, could reproduce regional scale patterns in the SeaWiFS imagery. We hypothesize that some of these patterns can be linked to coccolithophore blooms in areas influenced by the N. Atlantic Drift during summer and to effects of temperature on plankton loss rates during spring. Future work should focus on identifying the main factors responsible for these spatial patterns and developing the ecosystem models that can capture them.

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