A modelling study of temporal and spatial <i>p</i>CO₂ variability on the biologically active and temperature-dominated Scotian Shelf

Continental shelves are thought to be affected disproportionately by climate change and are a large contributor to global air–sea carbon dioxide (CO2) fluxes. It is often reported that low-latitude shelves tend to act as net sources of CO2, whereas mid- and high-latitude shelves act as net sinks. Here, we combine a high-resolution regional model with surface water time series and repeat transect observations from the Scotian Shelf, a mid-latitude region in the northwest North Atlantic, to determine what processes are driving the temporal and spatial variability of partial pressure of CO2 (pCO2) on a seasonal scale. In contrast to the global trend, the Scotian Shelf acts as a net source. Surface pCO2 undergoes a strong seasonal cycle with an amplitude of ∼ 200–250 µatm. These changes are associated with both a strong biological drawdown of dissolved inorganic carbon (DIC) in spring (corresponding to a decrease in pCO2 of 100–200 µatm) and pronounced effects of temperature, which ranges from 0 ∘C in the winter to near 20 ∘C in the summer, resulting in an increase in pCO2 of ∼ 200–250 µatm. Throughout the summer, events with low surface water pCO2 occur associated with coastal upwelling. This effect of upwelling on pCO2 is also in contrast to the general assumption that upwelling increases surface pCO2 by delivering DIC-enriched water to the surface. Aside from these localized events, pCO2 is relatively uniform across the shelf. Our model agrees with regional observations, reproduces seasonal patterns of pCO2, and simulates annual outgassing of CO2 from the ocean of +1.7±0.2 mol C m−2 yr−1 for the Scotian Shelf, net uptake of CO2 by the ocean of -0.5±0.2 mol C m−2 yr−1 for the Gulf of Maine, and uptake by the ocean of -1.3±0.3 mol C m−2 yr−1 for the Grand Banks.

Enhanced chlorophyll-<i>a</i> concentration in the wake of Sable Island, eastern Canada, revealed by two decades of satellite observations: a response to grey seal population dynamics?

Elevated surface chlorophyll-a (chl-a) concentration ([chl-a]), an index of phytoplankton biomass, has been previously observed and documented by remote sensing in the waters to the southwest of Sable Island (SI) on the Scotian Shelf in eastern Canada. Here, we present an analysis of this phenomenon using a 21-year time series of satellite-derived [chl-a], paired with information on the particle backscattering coefficient at 443 nm (bbp(443), a proxy for particle suspension) and the detritus/gelbstoff absorption coefficient at 443 nm (adg(443), a proxy to differentiate water masses and presence of dissolved organic matter) in an attempt to explain some possible mechanisms that lead to the increase in surface biomass in the surroundings of SI. We compared the seasonal cycle, 8 d climatology and seasonal trends of surface waters near SI to two control regions located both upstream and downstream of the island, away from terrigenous inputs. Application of the self-organising map (SOM) approach to the time series of satellite-derived [chl-a] over the Scotian Shelf revealed the annual spatio-temporal patterns around SI and, in particular, persistently high phytoplankton biomass during winter and spring in the leeward side of SI, a phenomenon that was not observed in the control boxes. In the vicinity of SI, a significant increase in [chl-a] and adg(443) during the winter months occurred at a rate twice that of the ones observed in the control boxes, while no significant trends were found for the other seasons. In addition to the increase in [chl-a] and adg(443) within the plume southwest of SI, the surface area of the plume itself expanded by a factor of 5 over the last 21 years. While the island mass effect (IME) explained the enhanced biomass around SI, we hypothesised that the large increase in [chl-a] over the last 21 years was partly due to an injection of nutrients by the island's grey seal colony, which has increased by 200 % during the same period. This contribution of nutrients from seals may sustain high phytoplankton biomass at a time of year when it is usually low following the fall bloom. A conceptual model was developed to estimate the standing stock of chl-a that can be sustained by the release of nitrogen (N) by seals. Comparison between satellite observations and model simulations showed a good temporal agreement between the increased abundance of seal on SI during the breeding season and the phytoplankton biomass increase during the winter. We found that about 20 % of chl-a standing stock increase over the last 21 years could be due to seal N fertilisation, the remaining being explained by climate forcing and oceanographic processes. Although without in situ measurements for ground truthing, the satellite data analysis provided evidence of the impact of marine mammals on lower trophic levels through a fertilisation mechanism that is coupled with the IME with potential implications for conservation and fisheries.

Ocean Regime Shift is Driving Collapse of the North Atlantic Right Whale Population

Ocean warming linked to anthropogenic climate change is impacting the ecology of marine species around the world. In 2010, the Gulf of Maine and Scotian Shelf regions of the Northwest Atlantic underwent an unprecedented regime shift. Forced by climate-driven changes in the Gulf Stream, warm slope waters entered the region and created a less favorable foraging environment for the endangered North Atlantic right whale population. By mid-decade, right whales had shifted their late spring/summer foraging grounds from the Gulf of Maine and the western Scotian Shelf to the Gulf of St. Lawrence. The population also began exhibiting unusually high mortality in 2017. Here, we report that climate-driven changes in ocean circulation have altered the foraging environment and habitat use of right whales, reducing the population’s calving rate and exposing it to greater mortality risks from ship strikes and fishing gear entanglement. The case of the North Atlantic right whale provides a cautionary tale for the management of protected species in a changing ocean.

Two new species of encrusting sponge (Porifera, family Crellidae) from eastern Canada

Two new species of Crellidae Dendy, 1922 from the east coast of Canada are described. The first is Crella (Pytheas) cutis sp. nov., a massively encrusting species of Crella (Pytheas) collected from depths of 84 to 249 m in the Gulf of St. Lawrence and on the Scotian Shelf. The second is Crellomima mehqisinpekonuta sp. nov., a thinly encrusting sponge found at diving depths in the Bay of Fundy. We also report the first records of Crellomima derma Hentschel, 1929 from outside the type locality (Barents Sea). All known species of Crellomima are reviewed based on type material.

Variability of the circulation in the western North Atlantic and its impact on deep-water oxygen concentrations in the St. Lawrence Estuary on the western continental shelf: evidence from observations

&lt;p&gt;Oxygen concentrations in the deep waters of the Lower St. Lawrence Estuary, in eastern Canada, have decreased by 50% over the past century, reaching hypoxic levels. To study the causes of this deoxygenation, we applied a mixing model (an extended multi-parameter analysis - eOMP) to data collected in the St. Lawrence Estuary since the 1970s and from the late 1990s to 2018. This method accounts for diapycnal mixing and can distinguish between the physical and biogeochemical causes of deoxygenation. The eOMP reveals that, in recent years, most of the deoxygenation of deep waters of the St. Lawrence Estuary is due to a change in the circulation pattern in the western North Atlantic. Since 2008, the Slope Sea and the deep waters of the St. Lawrence Estuary are fed by an increasing amount of oxygen-poor North Atlantic Central Waters (NACW), transported by the Gulf Stream, at the expense of oxygen-rich Labrador Current Waters (LCW). The oxygenation level of the St. Lawrence Estuary therefore reflects what is happening in the western North Atlantic. In contrast, the eOMP shows that, from the 1970s to the late 1990s, biogeochemical changes such as local eutrophication and variations in oxygen consumption rates in the North Atlantic dominated the deoxygenation.&amp;#160;&lt;/p&gt;&lt;p&gt;Further analyses suggest that the variability in the LCW:NACW ratio in the Slope Waters is mainly controlled by the Scotian Shelf-break Current, an extension of the Labrador Current, and not by the position or strength of the Gulf Stream, as often suggested. When the Labrador Current is strong, little of the southward flowing Labrador Current waters follow the coast all the way to the Scotian Shelf, and most of these waters are deviated east towards the North Atlantic. The opposite is true when the Labrador Current is weak. We will present some analysis of LCW trajectories in different conditions and discuss their potential drivers, based on a high resolution model. Overall, our results highlight the primary role of the Labrador Current in determining (i) the oxygen concentration and other water properties on the western North Atlantic continental shelf and slope, and (ii) the advection of fresh Labrador Current Water into the subpolar North Atlantic, with possible implications on the thermohaline and gyre circulation.&lt;/p&gt;

A modeling study of temporal and spatial <i>p</i>CO₂ variability on the biologically active and temperature-dominated Scotian Shelf

Continental shelves are thought to be affected disproportionately by climate change and are a large contributor to global air-sea carbon dioxide (CO2) fluxes. It is often reported that low-latitude shelves tend to act as net sources of CO2 whereas mid- and high-latitude shelves act as net sinks. Here, we combine a high-resolution regional model with surface water time-series and repeat transect observations from the Scotian Shelf, a mid-latitude region in the northwest North Atlantic, to determine what processes are driving the temporal and spatial variability of partial pressure of CO2 (pCO2). In contrast to the global trend, the Scotian Shelf acts as a net source. Surface pCO2 undergoes a strong seasonal cycle associated with both a strong biological drawdown of Dissolved Inorganic Carbon (DIC) in spring, and pronounced effects of temperature, which ranges from 0 °C in the winter to near 20 °C in the summer. Throughout the summer, events with low surface-water pCO2 occur nearshore associated with coastal upwelling. This effect of upwelling on pCO2 is also in contrast to the general assumption that upwelling increases surface pCO2 by delivering DIC-enriched water to the surface. Aside from these localized events, pCO2 is relatively uniform across the shelf. Our model agrees with regional observations, reproduces seasonal patterns of pCO2, and simulates annual outgassing of CO2 from the ocean of +1.9 ± 0.2 mol C m−2 yr−1 for the Scotian Shelf, net neutral CO2 flux of −0.09 ± 0.16 mol C m−2 yr−1 for the Gulf of Maine and uptake by the ocean of −0.88 ± 0.4 mol C m−2 yr−1 for the Grand Banks.