Plankton ecosystem response to freshwater-associated bulk turbidity in the subarctic Gulf of St. Lawrence (Canada): A modelling study

International audienceWe present a three-dimensional physical–biological modelling study aiming to infer the effect of freshwater-associated bulk turbidity on the Gulf of St. Lawrence (Canada) plankton ecosystem. Bulk turbidity is parameterised using an inverse relationship derived from an extensive in situ dataset linking salinity to the diffuse attenuation coefficient of photosynthetically available radiation (PAR) due to nonchlorophyllous matter. Embedding bulk turbidity in the model led to shallowing of the photic zone in the estuarine plume in accordance with coincident observations and allowed a better discrimination between Case 1 (chlorophyll-mediated variability of the photosynthetic available radiation attenuation) and Case 2 waters. The spring bloom was delayed, and primary and secondary production rates as well as the export of biogenic matter at depth decreased in the freshwater-influenced subregions. Comparisons with literature and coincident in situ measurements showed that nitrates were over- and underestimated in the run with and without bulk turbidity, respectively. A sensitivity analysis was performed with a relatively simple but robust parameterisation of photoacclimation, i.e. the adjustment of the phytoplankton photosynthetic efficiency to local underwater light conditions. Photoacclimation allowed simulated chlorophyll and nitrate concentrations as well as lateral fluxes of nitrate to achieve the best agreement with coincident measurements and literature estimates, respectively. This study showed that accounting for the freshwater-associated bio-optical variability and phytoplankton response in terms of photosynthetic efficiency improved the model's ability to predict the plankton ecosystem dynamics and associated biogeochemical fluxes in the river-influenced Gulf of St. Lawrence

The fate of riverine nutrients on Arctic shelves

Present and future levels of primary production (PP) in the Arctic Ocean (AO) depend on nutrient inputs to the photic zone via vertical mixing, upwelling and external sources. In this regard, the importance of horizontal river supply relative to oceanic processes is poorly constrained at the pan-Arctic scale. We compiled extensive historical (1954–2012) data on discharge and nutrient concentrations to estimate fluxes of nitrate, soluble reactive phosphate (SRP), silicate, dissolved organic carbon (DOC), dissolved organic nitrogen (DON), particulate organic nitrogen (PON) and particulate organic carbon (POC) from 9 large Arctic rivers and assess their potential impact on the biogeochemistry of shelf waters. Several key points can be emphasized from this analysis. The contribution of riverine nitrate to new PP (PP_new) is very small at the regional scale (< 1% to 6.7%) and negligible at the pan-Arctic scale (< 0.83%), in agreement with recent studies. By consuming all this nitrate, oceanic phytoplankton would be able to use only 14.3% and 8.7–24.5% of the river supply of silicate at the pan-Arctic and regional scales, respectively. Corresponding figures for SRP are 28.9% and 18.6–46%. On the Beaufort and Bering shelves, riverine SRP cannot fulfil phytoplankton requirements. On a seasonal basis, the removal of riverine nitrate, silicate and SRP would be the highest in spring and not in summer when AO shelf waters are nitrogen-limited. Riverine DON is potentially an important nitrogen source for the planktonic ecosystem in summer, when ammonium supplied through the photoammonification of refractory DON (3.9 × 10⁹ mol N) may exceed the combined riverine supply of nitrate and ammonium (3.4 × 10⁹ mol N). Nevertheless, overall nitrogen limitation of AO phytoplankton is expected to persist even when projected increases of riverine DON and nitrate supply are taken into account. This analysis underscores the need to better contrast oceanic nutrient supply processes with the composition and fate of changing riverine nutrient deliveries in future scenarios of plankton community structure, function and production in the coastal AO

Modelling the impact of riverine DON removal by marine bacterioplankton on primary production in the Arctic Ocean

The planktonic and biogeochemical dynamics of the Arctic shelves exhibit a strong variability in response to Arctic warming. In this study, we employ a biogeochemical model coupled to a pan-Arctic ocean–sea ice model (MITgcm) to elucidate the processes regulating the primary production (PP) of phytoplankton, bacterioplankton (BP), and their interactions. The model explicitly simulates and quantifies the contribution of usable dissolved organic nitrogen (DON) drained by the major circum-Arctic rivers to PP and BP in a scenario of melting sea ice (1998–2011). Model simulations suggest that, on average between 1998 and 2011, the removal of usable riverine dissolved organic nitrogen (RDON) by bacterioplankton is responsible for a ~ 26% increase in the annual BP for the whole Arctic Ocean. With respect to total PP, the model simulates an increase of ~ 8% on an annual basis and of ~ 18% in summer. Recycled ammonium is responsible for the PP increase. The recycling of RDON by bacterioplankton promotes higher BP and PP, but there is no significant temporal trend in the BP : PP ratio within the ice-free shelves over the 1998–2011 period. This suggests no significant evolution in the balance between autotrophy and heterotrophy in the last decade, with a constant annual flux of RDON into the coastal ocean, although changes in RDON supply and further reduction in sea-ice cover could potentially alter this delicate balance

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)

International audienceWe present here a first attempt to validate a regional three-dimensional (3-D) physical–biological coupled model of the Gulf of St. Lawrence with coincident Advanced Very High Resolution Radiometer (AVHRR)-derived sea surface temperature (SST) and Sea-viewing Wide Field-of-view Sensor (SeaWIFS)-derived Chlorophyll-a (Chl-a) data. The analysis focused on comparisons between remotely sensed data and simulated as well as in situ temperature, salinity, Chl-a, and nitrate. Results show that the simulated and AVHRR-derived fields of SST were qualitatively and quantitatively in agreement with in situ measurements. By contrast, marked differences were found between the simulated and SeaWIFS-derived fields of Chl-a, the latter comparing better with the freshwater-associated turbidity simulated by the model. Simulated temperature, salinity, nitrate, and Chl-a data compared well with coincident in situ measurements, and it is then suggested that freshwater-associated turbidity related to the river discharges largely contributed to the Chl-a retrievals by SeaWIFS in the Gulf's waters when using the standard OC4v.4 algorithm and atmospheric correction. Nevertheless, the striking agreement between SeaWIFS-derived ocean colour data and the simulated freshwater-associated turbidity allowed to validate the regional estuarine circulation and associated mesoscale variability. This result brings support to the model's ability to simulate realistic physical and biogeochemical fields in the Gulf of St. Lawrence

Seasonal versus synoptic variability in planktonic production in a high-latitude marginal sea: The Gulf of St. Lawrence (Canada)

International audienceThe Gulf of St. Lawrence (Canada) is a subarctic marginal sea characterized by highly variable hydrodynamic conditions that generate a spatial heterogeneity in the marine production. A better understanding of physical-biological linkages is needed to improve our ability to evaluate the effects of climate variability and change on the gulf's planktonic production. We develop a three-dimensional (3-D) eddy permitting resolution physical-biological coupled model of plankton dynamics in the Gulf of St. Lawrence. The planktonic ecosystem model accounts for the competition between simplified herbivorous and microbial food webs that characterize bloom and post-bloom conditions, respectively, as generally observed in temperate and subarctic coastal waters. It is driven by a fully prognostic 3-D sea ice-ocean model with realistic tidal, atmospheric, and hydrological forcing. The simulation shows a consistent seasonal primary production cycle, and highlights the importance of local sea ice dynamics for the timing of the vernal bloom and the strong influence of the mesoscale circulation on planktonic production patterns at subregional scales

Potential impact of photoinhibition on microphytobenthic primary production on a large intertidal mudflat

Microphytobenthos (MPB) are a key primary producer of intertidal mudflats. MPB face strong variability in incident irradiance during low tides. Despite photoprotection and photoacclimation, such variations can translate into photoinhibition of MPB cells. This study explores the effect of photoinhibition on MPB primary production (PP) over a large and productive temperate mudflat (Brouage mudflat, NW France). We used a regional and high-resolution tri-dimensional hydrodynamic model coupled to a MPB model with or without photoinhibition. Photoinhibition leads to a 20% (-0.79 t C) decrease of the simulated MPB PP over the entire mudflat. As the upper shore is exposed to light more frequently and longer than the lower shore, the decrease of MPB PP is higher on the upper shore (-29%) than on the lower shore (-5%). With the highest photosynthetically active radiation cumulated over the mudflat, the decrease of MPB PP due to photoinhibition is the highest during spring and spring tides (-22% and -23%, respectively). The model suggests MPB photoinhibition is sensitive to the photoacclimation status of MPB cells through the light saturation parameter. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP. As such, assessing the MPB photosynthetic response to the highly variable environmental conditions that prevail in large and productive intertidal mudflats is a real challenge for quantifying MPB PP from a synoptic to inter-annual time scale. Plain Language Summary Benthic micro-algae or microphytobenthos (MPB) inhabiting the surficial sediment sustain the high biological production of intertidal mudflats. MPB achieve photosynthesis by aggregating into a dense biofilm at the mud surface during daytime low tides. As MPB can be exposed to short-term variations and high light levels, they change their short-term physiology and position within the sediment to protect themselves. However, such strategies can be outbalanced by a too long stressful light exposure. In this study, we explore with a numerical model the impact of photoinhibition on MPB primary production (PP) over a large and very productive mudflat (NW France). The model suggests that photoinhibition can strongly impact MPB PP. With photoinhibition, the yearly PP decreases by 20 % over the whole mudflat. The model suggests MPB PP is sensitive to the photoacclimation status of MPB cells, i.e. their light use efficiency at a given light level. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP

Modeling plankton ecosystem functioning and nitrogen fluxes in the oligotrophic waters of the Beaufort Sea, Arctic Ocean: a focus on light-driven processes

The Arctic Ocean (AO) undergoes profound changes of its physical and biotic environments due to climate change. In some areas of the Beaufort Sea, the stronger haline stratification observed in summer alters the plankton ecosystem structure, functioning and productivity, promoting oligotrophy. A one-dimension (1-D) physical–biological coupled model based on the large multiparametric database of the Malina project in the Beaufort Sea was used (i) to infer the plankton ecosystem functioning and related nitrogen fluxes and (ii) to assess the model sensitivity to key light-driven processes involved in nutrient recycling and phytoplankton growth. The coupled model suggested that ammonium photochemically produced from photosensitive dissolved organic nitrogen (i.e., photoammonification process) was a necessary nitrogen source to achieve the observed levels of microbial biomass and production. Photoammonification directly and indirectly (by stimulating the microbial food web activity) contributed to 70% and 18.5% of the 0–10 m and whole water column, respectively, simulated primary production (respectively 66% and 16% for the bacterial production). The model also suggested that variable carbon to chlorophyll ratios were required to simulate the observed herbivorous versus microbial food web competition and realistic nitrogen fluxes in the Beaufort Sea oligotrophic waters. In face of accelerating Arctic warming, more attention should be paid in the future to the mechanistic processes involved in food webs and functional group competition, nutrient recycling and primary production in poorly productive waters of the AO, as they are expected to expand rapidly

Towards an assessment of riverine dissolved organic carbon in surface waters of the western Arctic Ocean based on remote sensing and biogeochemical modeling

Future climate warming of the Arctic could potentially enhance the load of terrigenous dissolved organic carbon (tDOC) of Arctic rivers due to increased carbon mobilization within watersheds. A greater flux of tDOC might impact the biogeochemical processes of the coastal Arctic Ocean (AO) and ultimately its capacity to absorb atmospheric CO₂. In this study, we show that sea-surface tDOC concentrations simulated by a physical–biogeochemical coupled model in the Canadian Beaufort Sea for 2003–2011 compare favorably with estimates retrieved by satellite imagery. Our results suggest that, over spring–summer, tDOC of riverine origin contributes to 35 % of primary production and that an equivalent of ∼&thinsp;10 % of tDOC is exported westwards with the potential of fueling the biological production of the eastern Alaskan nearshore waters. The combination of model and satellite data provides promising results to extend this work to the entire AO so as to quantify, in conjunction with in situ data, the expected changes in tDOC fluxes and their potential impact on the AO biogeochemistry at basin scale

The effect of tides on dense water formation in Arctic shelf seas

Ocean tides are not explicitly included in many ocean general circulation models, which will therefore omit any interactions between tides and the cryosphere. We present model simulations of the wind and buoyancy driven circulation and tides of the Barents and Kara Seas, using a 25 km × 25 km 3-D ocean circulation model coupled to a dynamic and thermodynamic sea ice model. The modeled tidal amplitudes are compared with tide gauge data and sea ice extent is compared with satellite data. Including tides in the model is found to have little impact on overall sea ice extent but is found to delay freeze up and hasten the onset of melting in tidally active coastal regions. The impact that including tides in the model has on the salt budget is investigated and found to be regionally dependent. The vertically integrated salt budget is dominated by lateral advection. This increases significantly when tides are included in the model in the Pechora Sea and around Svalbard where tides are strong. Tides increase the salt flux from sea ice by 50% in the Pechora and White Seas but have little impact elsewhere. This study suggests that the interaction between ocean tides and sea ice should not be neglected when modeling the Arcti