The rapid physical changes affecting the Arctic Ocean alter the growth conditions of primary producers. In this context, a crucial question is whether these changes will affect the composition of ...phytoplankton communities, augment their productivity, and eventually enhance food webs. We combined satellite and model products with in situ datasets collected during fall and provide new insights into the response of phytoplankton biomass and production in the Canadian Arctic by comparing an interior shelf (Beaufort Sea) and an outflow shelf (Baffin Bay). Correlation analysis was used to distinguish between seasonal and interannual variability and revealed that most biological variables are responding to the interannual pressures of climate change. In southeast Beaufort Sea, a change in phytoplankton community composition occurred, with a significant increase in diatoms from 2% (2002) to 37% (2010–2011) of the total protist abundance. In 2011, photosynthetic picoeukaryotes were twice as abundant as in 2002. For these two phytoplankton groups, abundance was correlated with the duration of the open-water period, which also increased and affected vertical stratification and sea-surface temperature. In contrast, there was a sharp decline in centric diatom abundance as well as in phytoplankton biomass and production in northern Baffin Bay over the years considered. These decreases were linked to changes in seasonal progression and sea-ice dynamics through their impacts on vertical stratification and freshwater input. Overall, our results highlight the importance of stratification and the duration of the open-water period in shaping phytoplankton regimes—either oligotrophic or eutrophic— in marine waters of the Canadian Arctic.
The first quasi‐annual time series of nutrients and chlorophyll fluorescence in the southeast Beaufort Sea showed that mixing, whether driven by wind, local convection, or brine rejection, and the ...ensuing replenishment of nutrients at the surface were minimal during autumn and winter. Anomalously high inventories of nutrients were observed briefly in late December, coinciding with the passage of an eddy generated offshore. The concentrations of NO3− in the upper mixed layer were otherwise low and increased slowly from January to April. The coincident decline of NO2− suggested nitrification near the surface. The vernal drawdown of NO3− in 2004 began at the ice‐water interface during May, leaving as little as 0.9 μM of NO3− when the ice broke up. A subsurface chlorophyll maximum (SCM) developed promptly and deepened with the nitracline until early August. The diatom‐dominated SCM possibly mediated half of the seasonal NO3− consumption while generating the primary NO2− maximum. Dissolved inorganic carbon and soluble reactive phosphorus above the SCM continued to decline after NO3− was depleted, indicating that net community production (NCP) exceeded NO3− ‐based new production. These dynamics contrast with those of productive Arctic waters where nutrient replenishment in the upper euphotic zone is extensive and NCP is fueled primarily by allochthonous NO3−. The projected increase in the supply of heat and freshwater to the Arctic should bolster vertical stability, further reduce NO3− ‐based new production, and increase the relative contribution of the SCM. This trend might be reversed locally or regionally by the physical forcing events that episodically deliver nutrients to the upper euphotic zone.
The Canadian Beaufort Sea has been categorized as an oligotrophic system with the potential for enhanced production due to a nutrient‐rich intermediate layer of Pacific‐origin waters. Using under‐ice ...hydrographic data collected near the ice‐edge of a shallow Arctic bay, we documented an ice‐edge upwelling event that brought nutrient‐rich waters to the surface during June 2008. The event resulted in a 3‐week long phytoplankton bloom that produced an estimated 31 g C m−2 of new production. This value was approximately twice that of previous estimates for annual production in the region, demonstrating the importance of ice‐edge upwelling to the local marine ecosystem. Under‐ice primary production estimates of up to 0.31 g C m−2 d−1 showed that this production was not negligible, contributing up to 22% of the daily averaged production of the ice‐edge bloom. It is suggested that under‐ice blooms are a widespread yet under‐documented phenomenon in polar regions, which could increase in importance with the Arctic's thinning ice cover and subsequent increase in transmitted irradiance to the under‐ice environment.
The head of the Laurentian Channel is a very dynamic region of exceptional biological richness. To evaluate the impact of freshwater discharge, tidal mixing, and biological activity on the pH of ...surface waters in this region, a suite of physical and chemical variables was measured throughout the water column over two tidal cycles. The relative contributions to the water column of the four source-water types that converge in this region were evaluated using an optimum multiparameter algorithm (OMP). Results of the OMP analysis were used to reconstruct the water column properties assuming conservative mixing, and the difference between the model properties and field measurements served to identify factors that control the pH of the surface waters. These surface waters are generally undersaturated with respect to aragonite, mostly due to the intrusion of waters from the Upper St. Lawrence Estuary and the Saguenay Fjord. The presence of a cold intermediate layer impedes the upwelling of the deeper, hypoxic, lower pH and aragonite-undersaturated waters of the Lower St. Lawrence Estuary to depths shallower than 50 m.
The Canadian Arctic Shelf Exchange Study (CASES) included the overwintering deployment of a research platform in Franklin Bay (70°N, 126°W) and provided a unique seasonal record of bacterial dynamics ...in a coastal region of the Arctic Ocean. Our objectives were (1) to relate seasonal bacterial abundance (BA) and production (BP) to physico‐chemical characteristics and (2) to quantify the annual bacterial carbon flux. BA was estimated by epifluorescence microscopy and BP was estimated from 3H‐leucine and 3H‐thymidine assays. Mean BA values for the water column ranged from 1.0 (December) to 6.8 × 105 cells mL−1 (July). Integral BP varied from 1 (February) to 80 mg C m−2 d−1 (July). During winter‐spring, BP was uncorrelated with chlorophyll a (Chl a), but these variables were significantly correlated during summer‐autumn (rs = 0.68, p < 0.001, N = 38), suggesting that BP was subject to bottom‐up control by carbon supply. Integrated BP data showed three distinct periods: fall‐winter, late winter–late spring, and summer. A baseline level of BB and BP was maintained throughout late winter–late spring despite the persistent cold and darkness, with irregular fluctuations that may be related to hydrodynamic events. During this period, BP rates were correlated with colored dissolved organic matter (CDOM) but not Chl a (rs BP.CDOM∣Chl a = 0.20, p < 0.05, N = 176). Annual BP was estimated as 6 g C m−2 a−1, implying a total BP of 4.8 × 1010 g C a−1 for the Franklin Bay region. These results show that bacterial processes continue throughout all seasons and make a large contribution to the total biological carbon flux in this coastal arctic ecosystem.
Abstract
The North Water polynya, the largest polynya in the world, forms annually and recurrently in Smith Sound in northern Baffin Bay. Its formation is governed in part by the formation of an ice ...bridge in the narrow channel of Nares Strait below Kane Basin. Here, the widely used elastic–viscous–plastic elliptical rheology dynamic sea ice model is applied to the region. The idealized case is tested over a range of values for e = 1.2, 2.0 and initial ice thicknesses from 0.75 to 3.5 m, using constant northerly winds over a period of 30 days, to evaluate long-term stability of different rheological parameterizations. Idealized high-resolution simulations show that the formation of a stable ice bridge is possible for e ≤ 1.8. The dependence of the solution in terms of grid discretization is studied with a domain rotated 45°. A realistic domain with realistic forcing is also tested to compare time-variant solutions to actual observations. Cohesion has a remarkable impact on if and when the ice bridge will form and fail, assessing its importance for regional and global climate modeling, but the lack of observational thickness data during polynya events prevents the authors from identifying an optimal value for e.
As part of the Canadian Arctic Shelf Exchange Study (CASES), we assessed the importance of new production and resuspension in determining the nature and magnitude of the deep (210 m) particulate ...organic carbon (POC) flux from October 2003 to September 2004 in central Franklin Bay. In spring and summer, phytoplankton production was nutrient‐limited in the stratified surface layer and the initial spring bloom evolved into a subsurface chlorophyll maximum (SCM) at the nutricline. Large herbivorous calanoid copepods intercepted little of the initial bloom but grazed intensely on the SCM. The phytoplankton and fecal pellet fluxes culminated simultaneously in July–August (24 and 23 mg C m−2 d−1, respectively). The detrital POC flux peaked in September (52 mg C m−2 d−1), coincident with wind‐induced resuspension of recently settled POC. In the fall, detrital POC fluxes increased again to 22 mg C m−2 d−1, following the off‐shelf transport of terrigenous POC carried by the Mackenzie River plume and POC resuspended by wind on the shelf. In winter, the relatively weak POC fluxes (2–7 mg C m−2 d−1, detrital at 90%) resulted from the settling down of resuspended sediments. We propose a conceptual model in which the ecosystem of Franklin Bay shifts from an algal to a detrital mode according to seasonal changes in the relative importance of fresh and old POC supplies. On the basis of this model, the ecosystem of southeastern Beaufort Sea could evolve toward a less productive equilibrium dominated by sediment resuspension in response to the ongoing reduction of the ice cover.
The ongoing regression of sea ice cover is expected to significantly affect the fate of organic carbon over the Arctic continental shelves. Long-term moored sediment traps were deployed in 2005–2006 ...in the Beaufort Sea, Northern Baffin Bay and the Laptev Sea to compare the annual variability of POC fluxes and to evaluate the factors regulating the annual cycle of carbon export over these continental shelves. Annual POC fluxes at 200
m ranged from 1.6 to 5.9
g
C
m
−2
yr
−1 with the highest export in Northern Baffin Bay and the lowest export over the Mackenzie Shelf in the Beaufort Sea. Each annual cycle exhibited an increase in POC export a few weeks before, during, or immediately following sea ice melt, but showed different patterns over the remainder of the cycle. Enhanced primary production, discharge of the Lena River, and resuspension events contributed to periods of elevated POC export over the Laptev Sea slope. High POC fluxes in Northern Baffin Bay reflected periods of elevated primary production in the North Water polynya. In the Beaufort Sea sediment resuspension contributed to most of the large export events. Our results suggest that the outer shelf of the Laptev Sea will likely sustain the largest increase in POC export in the next few years due to the large reduction in ice cover and the possible increase in the Lena River discharge. The large differences in forcing among the regions investigated reinforce the importance of monitoring POC fluxes in the different oceanographic regimes that characterize the Arctic shelves to assess the response of the Arctic Ocean carbon cycle to interannual variability and climate change.
Depth profiles of heterotrophic bacteria abundance were measured weekly over a 6-month period from December to May in Franklin Bay, a 230 m-deep coastal Arctic Ocean site of the southeastern Beaufort ...Sea. Total bacteria, low nucleic acid (LNA) and high nucleic acid (HNA) bacteria abundances were measured using flow cytometry after SYBR Green I staining. The HNA bacteria abundance in surface waters started to increase 5–6 weeks after phytoplankton growth resumed in spring, increasing from 1
×
10
5 to 3
×
10
5 cells mL
−
1
over an 8-week period, with a net growth rate of 0.018 d
−
1
. LNA bacteria response was delayed by more than two months relative to the beginning of the phytoplankton biomass accumulation and had a lower net growth rate of 0.013 d
−
1
. The marked increase in bacterial abundance occurred before any significant increase in organic matter input from river discharge (as indicated by the unchanged surface water salinity and DOC concentrations), and in the absence of water temperature increase. The abundance of bacteria below the halocline was relatively high until January (up to 5
×
10
5 cells mL
−
1
) but then decreased to values close to 2
×
10
5 cells mL
−
1
. The three-fold bacterial abundance increase observed in surface waters in spring was mostly due to HNA bacteria, supporting the idea that these cells are the most active.
Water column samples from the Mackenzie Shelf, the Beaufort Sea, and the Amundsen Gulf were obtained during 2003–2004 to investigate nutrient dynamics in an Arctic ecosystem influenced by a large ...river and flaw lead polynya. Nutrient inventories in the upper water column showed a significant seasonal drawdown of nitrate (NO3−) and silicic acid (Si(OH)4) (1:1.75 mol/mol) and subsequent accumulation of nitrite (NO2−) and ammonium (NH4+). Dissolved organic nitrogen and phosphorus concentrations (DON and DOP) were elevated in surface waters during the time of peak phytoplankton growth (bloom and postbloom phases) and covaried in a 17:1 molar ratio. Vertical profiles showed the typical middepth maxima in NO3−, PO43−, and Si(OH)4 at salinity 32–33.1. The concentration of DOP (0.76 ± 0.27 μmol P L−1) was also 2 times higher in this layer compared to the surface average and inversely correlated with the water mass tracer N* N* = (NO3− − 16 × PO43− + 2.9) × 0.87, suggesting that Pacific‐derived waters were a source of the enrichment. Below the mixed layer, DON was generally constant with depth, although averaged profiles (like those of urea) suggested the presence of subsurface maxima at 50 m and between 250 and 300 m. Regeneration ratios varied with depth and were approximately 9.0 mol −O2/mol NO3− and 122.5 mol −O2/mol PO43− in shallow Pacific‐derived waters (halocline layer) and 17.4 and 193.5 in deep Atlantic waters (Atlantic layer), respectively. Deep waters of the Amundsen Gulf contained an excess of 1.7 μmol NO3− L−1, 0.12 μmol PO43− L−1, and 6.2 μmol Si(OH)4 L−1 and a deficit in O2, relative to waters of similar density in the Beaufort Sea, in proportions consistent with the remineralization ratios derived from oxygen‐nutrient regressions. Enhanced export of particulate matter from the overlying polynya and subsequent remineralization at depth are hypothesized to create this nutrient enrichment.
PDF
