The power of marine plants in the Arctic

The rate at which the Arctic Ocean is being transformed by global warming is staggering. The accelerated melting of sea ice is profoundly altering marine ecosystems in a region that used to be frozen for much of the year. But while the seabed is more exposed to light, making the process of photosynthesis more favourable for certain marine species, the overall effects on ocean productivity are complex. An international team of researchers has published a study that challenges current knowledge of primary production in the Arctic, and the results are startling.

by Laurie Henry

Traditionally, scientists considered phytoplankton and ice algae (microalgae living at the interface between sea ice and ocean water) to be the main producers of marine biomass. However, this new research reveals that benthic plants, such as macroalgae and seagrass beds, play a much more important role than previously thought in capturing carbon and supporting Arctic food webs.

This discovery calls into question models of marine productivity and calls for a re-examination of carbon budgets in a rapidly changing region, where more than a million km² of sea ice has already disappeared over the last 25 years.

Benthic producers: a key source of energy

Phytoplankton and ice algae, the main primary producers in the Arctic, capture carbon dioxide and generate biomass, which feeds the marine food chain. But a study led by the CNRS and published recently in the Proceedings of the National Academy of Sciences reveals the undoubtedly underestimated importance of benthic primary producers (BPP), which includes microalgae, macroalgae and seagrass beds.

These marine plants, which cover around 3 million km² of the Arctic shelf, contribute between 20 and 35% of the region’s annual carbon production. This high percentage underlines the vital role they play in Arctic ecosystems and their ability to adapt to climate change.

Phanerogam (Zostera marina) bed, Nuuk, south-west Greenland © Peter Bondo Christensen

According to the new study, these BPP capture almost 100 million tonnes of carbon each year, transforming it into biomass. This figure represents almost half of the carbon production attributed to phytoplankton. More specifically, macroalgae and seagrass beds each contribute around 45% of this total production, while benthic microalgae play a more modest role. These results show that these benthic plants are not just secondary players, but essential pillars of productivity and carbon capture in the polar regions.

Unexpected effects of melting ice and exposure to light

The accelerated melting of ice in the Arctic is exposing more and more areas of the seabed to sunlight, a condition favourable to the photosynthesis of benthic primary producers (BPP). Every year since 2003, around 47,000 km² more of the seabed has become accessible to light, potentially allowing microalgae, macroalgae and seagrass beds to grow.

In theory, this increased access to light should lead to an overall increase in primary production in these ecosystems. However, this increase is neither uniform nor proportional to the expansion of the lighted zone. Primary production varies greatly from region to region, due to complex environmental and geophysical factors.

Microphytobenthos sur du sédiment, Young Sound, nord-est Groenland.© Amalia Al-Habahbeh

One of the main reasons for this variation is the reduction in water transparency. As Karl Attard, from the University of Southern Denmark and co-author of the publication, explains, the massive input of suspended particles from melting glaciers and discharges from the major rivers flowing into the Arctic reduces the penetration of light. These particles, originating from sources as far away as Mongolia and North America, darken Arctic waters by carrying organic and inorganic sediments, limiting the amount of light reaching the seabed.

Estimates for the month of August for (A) benthic photosynthetic radiation, (B) microalgal primary production rates, (C) water turbidity trend (KdPAR), and (D) microalgal production trend. Data calculated from MODIS-Aqua observations (NASA). Source : Attard and al., 2024

This phenomenon has had contrasting consequences: in some regions, such as the coasts of Greenland and Canada, benthic production has increased thanks to light; in others, such as the Russian continental shelf, it has fallen as a result of high water turbidity. These results demonstrate the complexity of the interactions between light and marine ecosystems in a rapidly changing Arctic region.