In the North Atlantic, subpolar currents shape Arctic ecosystems and productivity

08/10/2024

6 minutes

oceans and climate

Biogeochemical transport in the sub-polar North Atlantic (below the polar regions) plays a crucial role in feeding the marine ecosystems of the Arctic and the north-west European continental shelf.These flows of nutrients, influenced by ocean currents, support the production of phytoplankton, which is at the base of the marine food chain, and help to absorb carbon. However, variations in currents and long-term changes in the chemical composition of water are still poorly understood.

By Laurie Henry

A recent study by researchers from the Scottish Association for Marine Science and the National Oceanography Centre in Southampton, published in the Journal of Geophysical Research: Oceans, has provided previously unpublished data on the transport of nutrients and carbon in the subpolar Atlantic between 2017 and 2020, highlighting the potential impacts on Arctic and European ecosystems.

The subpolar Atlantic, a hotbed of primary productivity

The sub-polar Atlantic is an important region for understanding the ocean dynamics that influence marine and terrestrial ecosystems. This area, between the northern temperate regions and the southern Arctic, acts as a crossroads where nutrient-rich waters from the north-east Atlantic feed the Arctic and the north-western European continental shelf. The latter is a vast shallow submarine area that includes the waters around the UK, Ireland, Norway and western France.

The growing interest in this region can be explained by the major ecological implications that these nutrient flows have for marine productivity, notably via the Atlantic Meridional Overturning Circulation (AMOC). Primary productivity, i.e. the capacity of marine ecosystems to produce biomass through photosynthesis, is directly linked to the availability of nutrients transported by currents, but also to carbon sequestration. In recent decades, the productivity of some areas of the Arctic has increased, probably due to the increase in nutrients in these zones. However, this increase is fragile and could be threatened by the global decline in nutrient transport.

Indeed, data shows a drop in nutrient and carbon concentrations in other parts of the Arctic, due to a decrease in the intensity of the subpolar gyre, a vast system of ocean currents circulating in this region. Research shows that this decline could have long-term consequences, not only for marine biodiversity, but also for global climate processes.

However, despite the importance of these biogeochemical flows, most research to date has focused on the physical aspects of ocean circulation, neglecting the variability and impact of nutrient and carbon transport. Understanding these complex upward and downward dynamics is essential for anticipating changes in marine and terrestrial ecosystems, particularly in the context of climate change.

Intensive monitoring of the sub-polar north-east Atlantic

A study led by Clare Johnson and her team has made it possible to rigorously monitor biogeochemical transport in the subpolar north-east Atlantic. By deploying a series of moorings during campaigns at sea between 2017 and 2020, the researchers were able to observe, at high resolution, seasonal and interannual variations in nutrient and carbon fluxes in this key region. The moorings, equipped with biogeochemical sensors measuring parameters such as dissolved oxygen (DO), pH and nutrient concentrations (nitrate, phosphate), as well as automatic water samplers, were used to collect data on levels of dissolved inorganic carbon (DIC) and total alkalinity (TA). Total alkalinity measures the capacity of water to neutralise acids, regulating pH and the ocean carbon cycle.

The chart shows the position of anchorages WB1, WB2, EB1 and ADCP1. Also shown are the North-West European Shelf, the Greenland-Scotland Ridge, Rockall Trough and ocean currents. In (b), sensors and samplers are represented by circles and a star, while hydrographic stations are marked by white triangles © C. Johnson et al., 2024

These instruments, positioned at 63 m and 750 m in the Rockall Trough, an essential transit zone for ocean circulation, recorded data every 12 hours. This intensive monitoring, made possible by a mooring installed since 1975, provided a detailed view of vertical water dynamics, capturing variations in biogeochemical properties in the water column.

The graph shows dissolved oxygen (DO) concentrations at different depths on the EB1 mooring. The filled circles indicate the mean concentration with the error bars representing one standard deviation. The black lines show the dissolved oxygen profiles measured in May 2017, July 2018 and October 2020 © C. Johnson et al., 2024

To complement these in situ observations, the researchers used multiple linear regression methods, a statistical approach that makes it possible to predict nutrient and carbon concentrations as a function of physical variables such as temperature, salinity and pressure. By combining these time series with volume transport models developed as part of the OSNAP*(Overturning in the Subpolar North Atlantic Programme) project, the team was able to estimate biogeochemical fluxes at different depths and times of year.

The graph shows silicate transport in Rockall Trough, separating the effect of variations in volume (blue) and silicate concentrations (pink). The thin lines represent data every 12 hours, and the thick lines the average over 90 days. The graph in (c) shows normalized transport by 20-metre depth increments for volume (blue), nitrate (orange), phosphate (yellow), silicate (purple) and dissolved inorganic carbon (green) over the first 1000 metres. The circles indicate the depth of maximum transport for each element.  © C. Johnson et al., 2024

The transport of nutrients and carbon is northwards, but varies greatly depending on the currents. It is interesting to note that the transport of nutrients, such as nitrate and phosphate, was 15% and 19% lower respectively in the 2000s, when circulation in the subpolar gyre was weaker, bringing nutrient-poor water masses.

These results have major implications for future biological productivity, particularly in the Arctic, where the continuing decline in nutrient concentrations (such as nitrate and phosphate) could have a direct effect on primary productivity. This productivity, which relies on the availability of these nutrients to support phytoplankton growth, is a key driver of carbon sequestration through photosynthesis and the marine food chain. A reduction in primary productivity would therefore reduce the capacity of Arctic ecosystems to absorb and store carbon dioxide, which could exacerbate the effects of climate change. In addition, local communities that depend on fisheries for their food and livelihoods could see their resources dwindle, threatening their long-term food security and economic stability.

In conclusion, the study reveals that the variability of biogeochemical transports in the sub-polar Atlantic is having a major impact on marine ecosystems and Arctic productivity. These changes, linked to ocean currents, raise concerns about carbon sequestration and biodiversity. Further research is crucial to anticipate the effects of climate change.


Source : Johnson, Clare; Fraser, Neil; Cunningham, Stuart; Burmeister, Kristin; Jones, Sam; Drysdale, Lewis; Abell, Richard; Brown, Peter ; Dumont, Estelle; Fox, Alan; Holliday, N. Penny ; Inall, Mark; Reed, Sarah. 2024. “Biogeochemical properties and transports in the North East Atlantic”. Journal of Geophysical Research: Oceans, 129 (4)

* OSNAP : international research initiative to monitor and understand the Atlantic Meridional Overturning Circulation (AMOC) in the sub-polar North Atlantic, launched in 2014.

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