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A Larger Area of Arctic Seafloor is Exposed to Sunlight

Most of the sunlight reaching the Arctic Ocean is reflected by sea ice, shielding ocean ecosystems from light. As Arctic sea ice continues to melt, larger areas of the ocean and seafloor become exposed to sunlight, potentially allowing more photosynthesis to occur and making the Arctic Ocean more productive. However, this does not seem to be occurring uniformly across the Arctic Ocean.

By Birgitte Svennevig, , 3/5/2024

Over the past 25 years, the amount of summer Arctic sea ice has diminished by more than 1 million square kilometers. As a result, vast areas of the Arctic Ocean are now, on average, ice free in summer. Scientists are closely monitoring how this impacts sunlight availability and marine ecosystems in the far north.

- Many questions arise when such large areas become ice-free and can receive sunlight. A prevailing paradigm suggests that the Arctic Ocean is rapidly becoming more productive as sunlight becomes more abundant in the marine environment. However, it is unclear how ecosystems will evolve in response to increasing sunlight availability and how different parts of the marine ecosystem will be affected, says Karl Attard, a marine scientist and Associate Professor at the Department of Biology.

Attard has led an international research team investigating sunlight availability and photosynthetic production on the understudied Arctic seafloor. Their study has been published in the scientific journal Proceedings of the National Academy of Sciences (PNAS).

The Uncharted Arctic Ocean

The Arctic Ocean is the smallest and shallowest of the five major world oceans. Its vastness and remoteness mean that basic ocean measurements, such as underwater depth, remain few and far between. Understanding Arctic Ocean ecosystems against a backdrop of rapid environmental change remains a challenge. 

The research team focused on the vast shelf regions of the Arctic Ocean - portions of the seafloor that are relatively shallow, rarely exceeding 200 meters in depth. These vast ocean regions, accounting for around half of the Arctic Ocean area, have experienced dramatic changes in sunlight availability in recent decades.

Sunlight is a crucial energy source for life forms that grow through photosynthesis; they require water, carbon dioxide, nutrients, and sunlight. Scientists refer to these organisms as primary producers, and they thrive within the sea ice, within the water column, and on the seafloor.

These organisms serve as the foundation for a broader food web in the ocean, supporting commercially important fisheries and large predators like polar bears. The main primary producers on the seafloor are microalgae, seaweeds, and seagrasses.

Cloudy meltwater flow into Arctic Ocean (©Karl Atttard)

Microalgae on bottom of Arctic Ocean (© Amalia Al Habahbeh)

Arctic seagrass (©Peter Bondo Christensen)

No overall increase in sunlight reaching the Arctic seafloor

- It might seem reasonable to assume that the abundance of primary producers on the seafloor in the shallower regions of the Arctic Ocean would increase as more sunlight reaches the bottom. In fact, our research suggests that since 2003, the seafloor area exposed to sunlight has been increasing rapidly at around 47000 square kilometers per year. Curiously, however, we do not see an increase in the total amount of sunlight reaching the Arctic seafloor, says Karl Attard, referring to the research team’s models based on 20 years of satellite data from the Arctic Ocean.

This surprising result is seemingly due to water transparency, which has decreased in many parts of the Arctic Ocean.

- Sunlight reaching the ice-free ocean is rapidly absorbed by phytoplankton, sediments, and dissolved substances in the water, preventing much of the sun’s rays from reaching the seafloor. Our models predict increases in primary production in some regions but not in others, even though all these regions are now ice-free and receive sunlight at the ocean surface, explains Karl Attard.

Water gets murky from flowing rivers

Some of the world’s largest rivers pour into the Arctic Ocean, delivering murky waters laden with sediment particles and dissolved substances. These murky waters can cloud substantial portions of the coastal Arctic Ocean- regions where we would expect most of the seafloor primary producers to occur.

These rivers originate as far away as Mongolia or central North America, draining thousands of cubic kilometers of murky waters into the Arctic Ocean each year.

In the researchers’ models, there are regional variations in the amount of biomass produced at the bottom of the new ice-free areas. The models indicate an increase in primary production in several places along Greenland and Canada’s extensive coastlines. Conversely, there is a decline in primary production on much of the Russian continental shelf.

Seaweed and eelgrass coming to the Arctic Ocean

- The question then arises: why does sunlight availability and primary production increase in some areas while diminishing in others? Unfortunately, our models do not provide a clear answer as to what specifically is driving this change, and obtaining this information necessitates investigating individual regions and validating our models with more observational data. The latest models suggest that seaweeds and eelgrass will establish themselves on the shallow coastal seafloor and will expand into the Arctic Ocean as the ice further diminishes and water temperature increases”, says Karl Attard, adding: “Here too, more observations are needed to test the uncertainties in the models.

Traditionally, eelgrass and seaweeds struggle in ice-filled waters because the ice reduces sunlight availability and grinds and damages their tissues. However, in increasingly ice-free waters, extensive fields of underwater vegetation may emerge, serving as habitats for fish fry and other organisms.

The primary production calculated by researchers for the Arctic Ocean seafloor is only a part of the total marine primary production, which also occurs in the water column and within sea ice.

We may lose what is special for the Arctic

- We have estimated seafloor primary production by microalgae, eelgrass, and seaweeds, and it is significant. We estimate that it is four times larger than the production in sea ice, which has received greater attention and is better represented within our understanding of Arctic ecosystems. Including all components of marine primary production is crucial for understanding what is happening within Arctic marine ecosystems, says Karl Attard.

The Arctic Ocean has been undergoing rapid environmental change over the past decades, and he expects these transformations to continue.

- Our study suggests that the impacts of climate change on sunlight availability and primary production in the Arctic Ocean are complex. Additionally, as the Arctic Ocean continues to warm, we may witness more species migrating from lower latitudes, potentially leading to a more productive marine environment than what exists today—at the cost of losing what is special for the Arctic, he says.

Meet the researcher

Karl Attard is Associate Professor of Marine Science at the Department of Biology, and a Fellow at the Danish Institute for Advanced Study. He researches marine ecosystems and ocean productivity. Attard has participated in over 30 international expeditions, including to the Arctic, Antarctica, and remote islands in the Pacific and Atlantic. His research is supported by the Independent Research Fund Denmark through a Sapere Aude project.

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Editing was completed: 05.03.2024