Bio-optics and fate of terrestrial CDOM as key issues in biogeochemical modelling of Arctic and subarctic coastal seas

Le Fouest^1* V., B. Zakardjian², F. J. Saucier³, Z.-P. Mei³, D. Lefaivre⁴, S. Bélanger⁵, and M. Babin^6
1Scottish Association for Marine Science (SAMS), Dunstaffnage Marine Laboratory, Oban, Scotland
²Laboratoire de Sondages Électromagnétiques de l'Environnement Terrestre (LSEET), CNRS/USTV, La Garde, France
³Institut des Sciences de la Mer de Rimouski (ISMER), Rimouski (QC), Canada
⁴Fisheries and Oceans Canada, Maurice-Lamontagne Institute, Mont-Joli (QC), Canada
⁵Universite du Québec à Rimouski (UQAR), Rimouski (QC), Canada
⁶Observatoire Océanologique de Villefranche, Laboratoire d'Océanographie de Villefranche, CNRS/UPMC, Villefranche-Sur-Mer, France
*corresponding author: vincent.lefouest@sams.ac.uk

Faced with the challenges of global change and sustainable development, modern oceanography has seen the emergence of new observation and predictive tools. In the past two decades, remote sensing and numerical modelling have evolved considerably to infer oceanic variability with a high spatial and temporal resolution hard to achieve with field surveys alone. This is a key issue for understanding the carbon cycle, especially in productive coastal waters which are impacted by human induced eutrophication. Little has been done, however, about the impact of CDOM-dominated waters on plankton production modelling in the coastal ocean often constrained by complex bio-optical properties.
Here we present some quantitative elements about such constraints based on recent modelling efforts within the Gulf of St. Lawrence (GSL), Canada. The GSL is a highly dynamic subarctic marginal sea receiving the second largest freshwater runoff (~700 km³ yr^-1) of North America from the St. Lawrence River and many tributaries. The high concentrations of nonchlorophyllous material (mainly terrestrial CDOM) drained from the continent cause the St. Lawrence estuarine waters to be of case 2 type¹. They impact both quantitatively and qualitatively the underwater light field experienced by phytoplankton over a large part of the GSL. The results presented here were produced from a high resolution coupled physical/biological model^{2, 3} accounting for freshwater-associated turbidity. An extensive in situ dataset was used to parameterize the model through an inverse relationship linking seawater salinity with a diffuse attenuation coefficient of PAR associated to nonchlorophyllous material (k_p) (Fig. 1).

Figure 1 - Scatter plot showing the linear relationship between seawater salinity and the diffuse attenuation coefficient due to nonchlorophyllous material (k_p) derived from in situ measurements. The linear regression gives the equation k_p=0.0364 Salinity+1.1942 with a correlation coefficient r2=0.71. (From Le Fouest et al., 2006)

Comparisons of model outputs for 1998 with coincident SeaWiFS ocean colour data and extensive nitrate and Chl-a measurements suggested CDOM-dominated waters largely contributed to SeaWiFS-derived Chl-a², causing its overestimation in the western GSL. Nevertheless, the striking agreement between the simulated CDOM-dominated plume and SeaWiFS patterns made it possible to validate the regional estuarine circulation and associated mesoscale variability (Fig. 2), highlighting the farfield effects of the plume over the western part of the Gulf.

Figure 2 - Comparisons of SeaWiFS-derived Chl-a (right panels) in respect with simulated surface (0-10 m depth-averaged) Chl-a (upper left panel) and diffuse attenuation coefficient due to nonchlorophyllous material (lower left panel) for the 2-8 July 1998 period. Arrows overlaid on the lower left panel are the simulated mean surface (0-10 m depth-averaged) currents. Note the irregular scale for the SeaWiFS-derived and simulated Chl-a panels. (From Le Fouest et al., 2006)

Figure 3 illustrates the impact of the plume on primary production by comparing model runs with and without the parameterized bulk turbidity⁴. The impact came primarily from differences in the euphotic depths with a marked shallowing along the estuarine plume from the Estuary towards the southwestern GSL (Fig. 3, upper panels). This difference translated into a delayed spring bloom and lower primary production rates in river-influenced areas (Fig. 3, middle panels), with a consequent effect on nutrient dynamics over half the GSL (Fig. 3, lower panels). Comparisons with literature estimates and coincident in situ data suggested nutrients were under- and overconsumed in simulations with and without riverine CDOM shading, respectively. A further sensitivity analysis (not shown) was completed by including photoacclimation, that is, the adjustment of the photosynthetic efficiency of phytoplankton to local underwater light conditions. Including photocclimation allowed simulated nutrient concentrations and lateral fluxes to get closer to observations, emphasizing the key role of this process in the plankton dynamics and nutrient budget of the GSL.
Modelling the freshwater-associated bio-optical variability and phytoplankton response in terms of photosynthetic efficiency are key issues in predicting plankton productivity and nutrient budgets in river-influenced coastal systems like the GSL. Changes in phytoplankton productivity is of major concern for the coastal Arctic Ocean, which is predicted to experience global warming sooner and to a greater extent than elsewhere. Remobilisation of thawing permafrost carbon stocks together with an increasing river discharge might result in more CDOM reaching the coastal Arctic waters, with a consequent impact on the ecosystem productivity. In this perspective, modelling the fate and bio-optical properties of terrestrial CDOM poses as an inescapable but challenging topic in the biogeochemical modelling of changing Arctic and subarctic coastal waters.

References
1. Babin M., J.-C. Therriault, L. Legendre, and A. Condal, Variations in the specific absorption coefficient for natural phytoplankton assemblages: impact on estimates of primary production, Limnol. Oceanogr. 38, 154-177, 1993.
2. Le Fouest V., B. Zakardjian, F. J. Saucier, and S. A. Cizmeli, Application of SeaWIFS- and AVHRR-derived data for mesoscale and regional validation of a 3-D high-resolution physical-biological model of the gulf of St. Lawrence (Canada), J. Mar. Syst.,  60(1-2), 30-50, 2006.
3. Le Fouest V., B. Zakardjian, F. J. Saucier, and M. Starr, Seasonal versus synoptic variability in planktonic production in a high-latitude marginal sea: the gulf of St. Lawrence (Canada), J. Geophys. Res. (Ocean), 110, C099012, doi:10.1029/2004JC002423, 2005.
4. Le Fouest V., B. Zakardjian, and F. J. Saucier, Impact of CDOM-dominated waters on phytoplankton in a highly dynamic shelf sea: a modelling study in the Gulf of St. Lawrence (Canada), in prep.

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