Search

The unprecedented pace of warming of the Arctic Ocean affects a wide range of pelagic processes, from primary production to fish recruitment. Sequential sediment traps deployed on oceanographic moorings at the Beaufort Sea shelf break were used to investigate the impact of variations in physical conditions on the phenology of microalgae and herbivorous copepods. Water temperature, salinity, microalgal fluxes, and zooplankton fluxes were monitored over 6 of the 7 annual cycles from September 2009 to September 2016 with a hiatus from September 2013 to August 2014. Satellite-derived sea ice concentration and snow depth were retrieved for the same period. For 5 of the 6 cycles, a consistent interval between the vernal abundance of the Calanus species (C. glacialis and C. hyperboreus), the export of the ice alga Nitzschia frigida, and peak in nauplii abundance and diatom export, reflected a match between microalgae and herbivorous copepods. Drastic changes in water temperatures and ice conditions during the 2012–2013 cycle affected this synchronization. In 2012, the record sea ice minimum extent led to anomalously high upper water temperatures during autumn and winter that led to a shoaling of the vertical distribution of C. hyperboreus females and to the production of their nauplii at shallower depths several weeks before diatom export. Due to several oceanic and atmospheric forcing, the record sea ice minimum extent was followed in 2013 by a rebound in ice cover that delayed snowmelt, sea ice melt, and ice algae export. This delay resulted in a mismatch between C. glacialis females and N. frigida fluxes and in fewer C. glacialis nauplii in June and July 2013. These mismatch events likely had a negative impact on Calanus recruitment and on the subsequent transfer of energy to carnivorous zooplankton and fish. It is crucial to maintain mooring deployments to accurately assess the wide range of effects of climate warming on the marine ecosystems.

The shallow Pacific Arctic shelf has historically acted as an effective carbon sink, characterized by tight benthic pelagic coupling. However, the strength of the biological carbon pump in the Arctic has been predicted to weaken with climate change due to increased duration of the open-water period for primary production, enhanced nutrient limitation, and increased pelagic heterotrophy. In order and gain insights into how the biological carbon pump is functioning under the recent conditions of extreme warming and sea ice loss on the Pacific Arctic shelf, we measured sinking particulate organic carbon (POC) fluxes from drifting and moored sediment traps, as well as rates of primary production and particle-associated microbial respiration in June 2018. We measured high sinking POC fluxes similar to and/or higher than previous measurements from this region (up to 2.3 g C m-2 d-1), reaching flux levels among the highest ever documented in the global oceans. Furthermore, high export ratios averaging 82% and low rates of particle-associated microbial respiration also indicated negligible recycling of sinking POC in the water column during spring of 2018. These results highlight the extraordinary strength of biological carbon pump on the Pacific Arctic shelf during an unusually warm and low-sea ice year. While additional measurements and time are needed to confirm the ultimate trajectory of these fluxes in response to ongoing climate change, these results call into question the prevailing hypothesis that the strength of the biological carbon pump in the Pacific Arctic will weaken under these conditions.

Sea ice in the Arctic Ocean (AO) has been undergoing dramatic changes during the last two decades. In addition, the water temperature of the inflow of Atlantic water masses at the gateway Fram Strait has recently increased. Long-term data may help to evaluate the impact of these physical changes on the biological processes in surface waters. Over a 25-year period, and mostly in summer, water samples were collected at discrete depths within the uppermost 100 m of the Fram Strait and other regions of the AO to investigate chlorophyll a (Chl a) and particulate organic carbon (POC) standing stocks. Stations sampled from 1991 to 2015 were located in the Fram Strait, Barents Sea (BS), on the Eurasian shelf, and over the Nansen, Amundsen, and parts of the Amerasian basins (AB). Discrete Chl a and POC measurements obtained during 33 and 24 expeditions, respectively, were integrated over the upper 100 m of the water column to monitor spatial and interannual variations in distribution patterns of standing stocks. In general, the highest Chl a and POC standing stocks were observed in the eastern Fram Strait (EFS) and in the BS, while the lowest biomasses were observed in the heavily ice-covered regions of the central AO, mainly in the Amundsen and ABs. Whereas summertime Chl a stocks sharply decreased northward from the Fram Strait and Barents Sea toward high latitudes, the decline in POC standing stocks was less pronounced. Over the sampling period, summertime Chl a stocks slightly increased in the EFS but remained more or less constant in the central AO. In contrast to Chl a, standing stocks of POC eventually increased over the last 25 years in the central AO, possibly as an effect of increasing air temperatures, decreasing sea ice extent and thickness, and increasing light availability. Moreover, variations in riverine discharge and in sea ice export within the Transpolar Drift may have contributed to the enhanced POC stock in the central AO surface waters. Overall, the objective of the present study was to provide baseline datasets of Chl a and POC to better track the effects of environmental changes due to global warming on the Arctic pelagic system.

A multi-institutional initiative was created in a context of potential oil exploration at the Old Harry site to monitor the marine ecosystem and manage resources of the Gulf of St. Lawrence (GSL). As part of this initiative, two sediment traps were deployed at depths of 34 m and 100 m at a site northwest of Cabot Strait in the Laurentian Channel to measure downward carbon fluxes from October 2014 to October 2015. Fluxes of particulate organic carbon (POC), biogenic silica (BSi), chloropigments, protists and zooplankton fecal pellets were used to evaluate vertical and temporal changes in the magnitude and composition of biogenic carbon fluxes and to provide baseline information prior to any exploration activities in the region. Peaks in POC, BSi, chloropigment and diatom fluxes at both depths at the end of April 2015 indicated the rapid export of a locally-produced bloom mainly composed of pennate diatoms. A coincident peak in fecal pellet carbon fluxes at 100 m indicated a match between the spring ascent of copepods and the diatom bloom. The distinct composition of protist fluxes at 34 and 100 m following the formation of the winter mixed layer highlighted the influence of the pycnocline as a physical barrier for the export of particles in the region. Among identifiable components, dinoflagellates and fecal pellets, followed by diatoms and foraminifera, contributed most to the annual POC flux at 100 m, indicating a balance between lateral advection of slow-sinking protists and rapid export of fast-sinking diatoms and fecal pellets in the supply of POC toward the seafloor in the GSL. These measurements of biogenic carbon export provide insight into the fate of large and small particles in the Cabot Strait region to consider in the potential development of oil and gas activities.

Time-series sediment traps were deployed at 4 depths in the eastern Fram Strait from July 2007 to June 2008 to investigate variations in the magnitude and composition of the sinking particulate matter from upper waters to the seafloor. Sediment traps were deployed at 196 m in the Atlantic Water layer, at 1296 and 2364 m in the intermediate and deep waters, and at 2430 m on a benthic lander in the near-bottom layer. Fluxes of total particulate matter, particulate organic carbon, particulate organic nitrogen, biogenic matter, lithogenic matter, biogenic particulate silica, calcium carbonate, dominant phytoplankton cells, and zooplankton fecal pellets increased with depth, indicating the importance of lateral advection on fluxes in the deep Fram Strait. The lateral supply of particulate matter was further supported by the constant fluxes of biomarkers such as brassicasterol, alkenones, campesterol, β-sitosterol, and IP25 at all depths sampled. However, enhanced fluxes of diatoms and appendicularian fecal pellets from the upper waters to the seafloor in the presence of ice during spring indicated the rapid export (15–35 days) of locally-produced large particles that likely contributed most of the food supply to the benthic communities. These results show that lateral supply and downward fluxes are both important processes influencing the transport of particulate matter to the seafloor in the deep eastern Fram Strait, and that particulate matter size dictates the prevailing sinking process.

Microalgal cells collected with moored sediment traps deployed during three to five annual cycles at three sites in the Beaufort Sea were identified to investigate variations in the timing, abundance and composition of microalgal fluxes in relation to snow and sea ice cover. The investigation period encompassed two extremes in snow and sea ice conditions: a delayed melt due to an ice rebound in 2013 and a premature snowmelt and sea ice breakup that led to an ice-free Beaufort Sea in 2016. Diatoms dominated the microalgal fluxes, with the pelagic centric diatoms Thalassiosira spp. and the ice-associated pennate diatoms Fragilariopsis spp. consistently collected at the three sites. The export of the ice-obligated algae Nitzschia frigida indicated the release of sea ice algae at the onset of snowmelt. Early snowmelt and sea ice breakup in 2016 contributed to an early start of ice algae release accompanied with early peaks in diatom fluxes and higher diatom and phytoplankton carbon (PPC) fluxes during spring and summer. Conversely, delayed sea ice algae release, low diatom fluxes, and low PPC fluxes were observed when snowmelt and sea ice breakup occurred late over the Mackenzie shelf break. The amount of diatoms exported at ~100–300 m also likely depended on a match or mismatch between algal production and zooplankton grazing. Variations in the snow and sea ice regimes at the regional scale therefore directly impact the timing and magnitude of microalgal export and its contribution to particulate organic carbon flux in the Arctic Ocean. With global warming, the ongoing sea ice reduction in the Arctic Ocean may increase PPC fluxes to the seafloor and potential carbon sequestration at depth.

Seasonal cycles of primary production and freshwater run-off contribute to elevated particulate matter fluxes of marine and terrestrial origin in fjords. In Norwegian fjords, aquaculture operations and jellyfish carcasses may further enhance these particulate matter fluxes. In this study, sediment traps were deployed in four Norwegian fjords (Lurefjorden, Masfjorden, Hardangerfjorden and Kaldfjorden) to quantify seasonal variations in downward particle fluxes and to investigate the contribution of aquaculture operations to these fluxes. Sediment traps were deployed in Lurefjorden, a fjord heavily populated by the deep-sea coronate medusae Periphylla periphylla, in Masfjorden, a nearby fjord without mass occurrence of P. periphylla, in Hardangerfjorden, a fjord with high density of salmon farms, and in Kaldfjorden, a northern fjord also hosting aquaculture activities. Measurements of total particulate matter (TPM), particulate organic carbon (POC), chlorophyll a (chl a), and diatom fluxes indicated the export of a diatom bloom starting in March or April and a large input of land-derived inorganic matter during rain and snowmelt events in all fjords sampled. The highest annual POC flux was recorded in the nearly landlocked Lurefjorden. In Hardangerfjorden, export fluxes suggested a local impact of aquaculture operations but freshwater run-off and water exchange had a larger influence on export fluxes than the release of aquaculture waste. In Kaldfjorden, nearly absent TPM, POC, chl a, and diatom fluxes in late spring-early summer suggested that strong stratification prevented the export of particulate matter to depth. Overall, sediment trap-derived fluxes reflected the spatial and temporal variations in the sources of particulate matter in Norwegian fjords.

A long-term sediment trap was deployed at 100 m (station AN03; 55°24′N; 77°55′W; water depth = 136 m) from October 2005 to August 2007 to monitor downward carbon export in southeastern Hudson Bay in the Canadian subarctic region. Elevated particulate organic carbon (POC) and total fluxes during fall 2005 and 2006 were the result of wind-induced resuspension of settled POC. The absence of a significant increase in POC fluxes in summer 2006 in contrast to summer 2007 suggests variation in the processes regulating carbon export in southeastern Hudson Bay. Large quantities of the holoplanktonic jellyfish Aglantha digitale were collected in the sediment trap during fall 2006 and summer 2007. The large collection of A. digitale during fall 2006 corresponds to the seasonal descent of the population, while the large abundance during summer 2007 remains unexplained. The increased POC fluxes in summer 2007 and the large occurrence of A. digitale in 2006 and 2007 may be early signs of environmental changes in southeastern Hudson Bay, however more sampling is needed to capture the variability of the ecosystem.

Calanoid copepods dominate mesozooplankton biomass in the Arctic Ocean. Variations in C content, C:N ratio and stable isotope composition (δ 13 C, δ 15 N) of Calanus hyperboreus, Calanus glacialis and Metridia longa collected from January to August 2008 in the southeast Beaufort Sea provided insights into their metabolism, feeding and reproduction. Seasonal differences in the C―prosome length relationships and C:N ratios were driven by distinct spawning strategies and changes in lipid content. Relatively high copepod biomass over the study period (2.6―9.7 g C m ―2 ) corresponded to favorable growth conditions in 2007―2008. The mean 15 N enrichment of copepods (+2.8―4.7‰) relative to particulate organic nitrogen values recorded at the ice bottom and at the chlorophyll maximum indicated a primarily herbivorous diet. In all species, δ 13 C and δ 15 N decreased markedly in April, reflecting the feeding onset on ice algae, but a rapid transition to feeding on phytoplankton occurred as a pelagic bloom was triggered by the early ice melt in May. A second decline in the δ 13 C and δ 15 N of copepods was recorded in June―July, coincident with a second increase in phytoplankton production. The two isotope depletion events in copepods were both followed by a return to high values and an increase in their C:N as a consequence of previous C fixation and nitrate limitation in phytoplankton and the likely formation of body reserves/tissue. Our results illustrate that Arctic calanoids respond quickly to any increase in primary production and can cope with changes in its nature and timing.

Surface concentrations and vertical fluxes of particulate organic carbon (POC) were assessed in the Amundsen Gulf (southeastern Beaufort Sea, Arctic Ocean) over the years 2004 to 2006 by using ocean color remote-sensing imagery and sequential sediment traps moored over the ca. 400 m isobath. Environmental conditions (sea ice, wind) and oceanographic variables (temperature, salinity, fluorescence and currents) were investigated to explain the variability of POC data. Annual downward POC fluxes in 2004, 2005 and 2006 cumulated, respectively, to 3.3, 4.2 and 6.0 g C m −2 yr −1 at ∼100 m depth, and to 1.3, 2.2 and 3.3 g C m −2 yr −1 at ∼210 m depth. The fraction of settling POC attributable to autochthonous processes occurring at or next to ice break-up was estimated to be 75–84% of the 100 m annual fluxes and to be 61–75% of the 210 m fluxes. Over the three ice-reduced seasons, distinct scenarios between ice conditions, surface POC pools and vertical POC export at 100 m were identified: (1) in 2004, despite a normal ice break-up, a weak primary production was measured and low vertical fluxes were collected as old ice moved across the region; (2) in 2005, a lengthened ice-free period allowed an extended season of surface POC production near-shore, while an intermediate increase of vertical fluxes was recorded offshore; and (3) in 2006, a late ice melt gave rise to a pulsed ice edge bloom and to large vertical fluxes also associated with extra ice-flushed material. Linear regressions of vertical POC fluxes against satellite-derived surface POC concentrations suggested that the pelagic POC retention in the upper 100 m of the Amundsen Gulf ranged from ca. 70% to 90% depending on the timing of ice cover melt. Regardless of the inter-annual variability, the estimated fraction of the surface POC reservoir reaching the 210 m water depth was reduced to ∼5%. Therefore, as the Arctic Ocean warms up, our results support the expectation that the increasing extent of the seasonal ice zone will promote the POC pathways that benefit pelagic webs rather than benthic communities.

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.

Large-volume sampling of 234Th was conducted to estimate particulate organic carbon (POC) export in conjunction with drifting sediment trap deployments in the northern Barents Sea in July 2003 and May 2005. 234Th-derived POC fluxes averaged 42.3 ± 39.7 mmol C m− 2 d− 1 in 2003 and 47.1 ± 30.6 mmol C m− 2 d− 1 in 2005. Sediment trap POC fluxes averaged 13.1 ± 8.2 mmol C m− 2 d− 1 in 2003 and 17.3 ± 11.4 mmol C m− 2 d− 1 in 2005, but better reflected the transient bloom conditions that were observed at each station within a season. Although 234Th fluxes agreed within a factor 2 at most stations and depths sampled, sediment trap POC fluxes were lower than large-volume POC flux estimates at almost every station. This may represent an under-collection of POC by the drifting sediment traps or, conversely, an over-collection of POC by the large-volume sampling of 234Th. It is hypothesized that the offset between the two methods is partly due to the presence of the prymnesiophyte Phaeocystis pouchetii, which potentially causes a large variation in > 53-μm POC/234Th ratios. Due to the large proportion of dissolved carbon or mucilage released by P. pouchetii, and because it is thought that P. pouchetii does not contribute significantly to the vertical export of biogenic matter in the Barents Sea, the application of large-volume sampling of 234Th may yield relatively high, and possibly inaccurate POC/234Th ratios. Hence, POC fluxes derived from 234Th sampling may be inappropriate and drifting sediment traps might be a more reliable method to measure the vertical export of biogenic matter in regions that have recurrent P. pouchetii blooms, such as the Barents Sea.

Drifting sediment traps were deployed at 9 stations in May-June (ice-covered conditions) and July-August (ice-free conditions) 2004 in the Chukchi Sea to investigate the variability in export fluxes of biogenic matter in the presence and absence of sea ice cover. Measurements of chlorophyll- a (Chl- a ), particulate organic carbon (POC), particulate nitrogen (PN), phytoplankton, zooplankton fecal pellets, and the stable carbon isotope composition ( δ 13 C) of the sinking material were performed along Barrow Canyon (BC) and a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. POC export fluxes were similarly high in the presence (378±106 mg C m −2 d −1 ) and in the absence of ice cover (442±203 mg C m −2 d −1 ) at the BC stations, while fluxes were significantly higher in the absence (129±98 mg C m −2 d −1 ) than in the presence of ice cover (44±29 mg C m −2 d −1 ) at the EHS stations. The C/N ratios and δ 13 C values of sinking organic particles indicated that POC export fluxes on the Chukchi continental shelf were mostly composed of freshly produced labile material, except at the EHS stations under ice cover where the exported matter was mostly composed of refractory material probably advected into the EHS region. Chl- a fluxes were higher under ice cover than in ice-free water, however, relatively low daily loss rates of Chl- a and similar phytoplankton carbon fluxes in ice-covered and ice-free water suggest the retention of phytoplankton in the upper water column. An increase in fecal pellet carbon fluxes in ice-free water reflected higher grazing pressure in the absence of ice cover. Elevated daily loss rates of POC at the BC stations confirmed other indications that Barrow Canyon is an important area of carbon export to the basin and/or benthos. These results support the conclusion that there are large spatial and temporal variations in export fluxes of biogenic matter on the Chukchi continental shelf, although export fluxes may be similar in the presence and in the absence of ice cover in highly productive regions.

As part of the HAUSGARTEN long-term observatory, sediment trap deployments were carried out before, during, and after the anomalously warm Atlantic Water inflow observed from 2005 to 2007 in the eastern Fram Strait. Downward export of particulate organic carbon (POC), zooplankton fecal pellet carbon (FPC), and biogenic particulate silica (bPSi) were measured from August 2002 to June 2003 and from July 2004 to July 2008 to indirectly assess the impact of the warm anomaly on phytoplankton and zooplankton communities in the region. Lower and less frequent bPSi fluxes were observed during most of the warm anomaly period, reflecting a shift in phytoplankton community composition towards dominance of small-sized phytoplankton under warmer conditions. Lower FPC fluxes observed concurrently with the lower bPSi fluxes may indicate a decrease in fecal pellet production due to changing feeding conditions. In addition, the export of smaller fecal pellets in fall 2005 and spring 2006 suggests a dominance of smaller zooplankton during the warm anomaly. Nonetheless, bPSi and FPC export always increased in the presence of ice cover in the area above the sediment trap, even during the warm anomaly period, suggesting that sea ice is a key factor influencing the frequency of export events in the eastern Fram Strait. The scarcity of ice over the sampling area in 2005 and 2006 may partly be due to the warm anomaly, although solar radiation and ice drift due to wind stress also govern ice cover extent in the region. Overall, the warm anomaly resulted in a shift in the composition of the export fluxes when associated with an absence of ice cover in the eastern Fram Strait.

Through the analysis of observational mooring data collected at the northeastern Laptev Sea continental slope in 2004-2007, we document a hydrographic seasonal signal in the intermediate Atlantic Water (AW) layer, with generally higher temperature and salinity from December-January to May-July and lower values from May-July to December-January. At the mooring position, this seasonal signal dominates, contributing up to 75% of the total variance. Our data suggest that the entire AW layer down to at least 840 m is affected by seasonal cycling, although the strength of the seasonal signal in temperature and salinity reduces from 260 m (±0.25°C and ±0.025 psu) to 840 m (±0.05°C and ±0.005 psu). The seasonal velocity signal is substantially weaker, strongly masked by high-frequency variability, and lags the thermohaline cycle by 45-75 days. We hypothesize that our mooring record shows a time history of the along-margin propagation of the AW seasonal signal carried downstream by the AW boundary current. Our analysis suggests that the seasonal signal in the Fram Strait Branch of AW (FSBW) at 260 m is predominantly translated from Fram Strait, while the seasonality in the Barents Sea branch of AW (BSBW) domain (at 840 m) is attributed instead to the seasonal signal input from the Barents Sea. However, the characteristic signature of the BSBW seasonal dynamics observed through the entire AW layer leads us to speculate that BSBW also plays a role in seasonally modifying the properties of the FSBW. Copyright 2009 by the American Geophysical Union.

[1] Particulate organic carbon (POC) export fluxes were estimated in the shelf-slope region of the Chukchi Sea using measurements of 234Th−238U disequilibria and the POC/234Th ratio in large (>53-μm) particles. These export fluxes were used in conjunction with rates of primary productivity and benthic carbon respiration to construct a POC budget for this shelf-slope region. Samples were collected along a series of shelf-basin transects in the spring (May–June) and summer (July–August) of 2004. These stations were previously occupied during the ice covered (spring) and open water (summer) seasons of 2002, allowing for an interannual comparison of export flux. In contrast to 2002, when open water POC fluxes were significantly higher than in the ice-covered period, POC export fluxes in 2004 were similar during the spring (average = 19.7 ± 24.8 mmol C m−2 d−1) and summer (average = 20.0 ± 14.5 mmol C m−2 d−1). The high POC fluxes measured during the spring are attributed to a plankton bloom, as evidenced by exceptionally high rates of primary productivity (average = 124.4 ± 88.1 mmol C m−2 d−1). The shelf-slope budget of particulate organic carbon indicates that 10–20% of primary productivity was exported below 50 m but was not consumed during benthic carbon respiration or burial and oxidation in underlying sediments. Furthermore, a water column−sediment budget of 234Th indicates that particulate material is retained in shelf sediments on a seasonal basis.