Deep ocean warming as climate changes

Much of the “excess heat” stored in the subtropical North Atlantic is in the deep ocean (below 700m), new research suggests.

Oceans have absorbed about 90% of warming caused by humans. The study found that in the subtropical North Atlantic (25°N), 62% of the warming from 1850-2018 is held in the deep ocean.

The researchers – from the University of Exeter and the University of Brest – estimate that the deep ocean will warm by a further 0.2°C in the next 50 years.

Ocean warming can have a range of consequences including sea-level rise, changing ecosystems, currents and chemistry, and deoxygenation.

“As our planet warms, it’s vital to understand how the excess heat taken up by the ocean is redistributed in the ocean interior all the way from the surface to the bottom, and it is important to take into account the deep ocean to assess the growth of Earth’s ‘energy imbalance’,” said Dr Marie-José Messias, from the University of Exeter.

“As well as finding that the deep ocean is holding much of this excess heat, our research shows how ocean currents redistribute heat to different regions.

“We found that this redistribution was a key driver of warming in the North Atlantic.”

The researchers studied the system of currents known as the Atlantic Meridional Overturning Circulation (AMOC).

AMOC works like a conveyer belt, carrying warm water from the tropics north – where colder, dense water sinks into the deep ocean and spreads slowly south.

The findings highlight the importance of warming transferring by AMOC from one region to another.

Dr Messias said excess heat from the Southern Hemisphere oceans is becoming important in the North Atlantic – now accounting for about a quarter of excess heat.

The study used temperature records and chemical “tracers” – compounds whose make-up can be used to discover past changes in the ocean.

The paper, published in the Nature journal Communications Earth & Environment, is entitled: “The redistribution of anthropogenic excess heat is a key driver of warming in the North Atlantic.”

More information: Messias, MJ., Mercier, H. The redistribution of anthropogenic excess heat is a key driver of warming in the North Atlantic. Commun Earth Environ 3, 118 (2022). https://doi.org/10.1038/s43247-022-00443-4

Warming oceans are getting louder

FASTER SOUND TRANSMISSION IN THE OCEANS DUE TO CLIMATE CHANGE WILL CHANGE THE UNDERWATER SOUNDSCAPE MARINE ORGANISMS RELY ON FOR SURVIVAL AND REPRODUCTION IN COMING DECADES.

Climate change will significantly alter how sound travels underwater, potentially affecting natural soundscapes as well as accentuating human-generated noise, according to a new global study that identified future ocean “acoustic hotspots.” These changes to ocean soundscapes could impact essential activities of marine life.

In warmer water, sound waves propagate faster and last longer before dying away.

“We calculated the effects of temperature, depth and salinity based on public data to model the soundscape of the future,” said Alice Affatati, an bioacoustics researcher at the Memorial University of Newfoundland and Labrador in St. John’s, Canada, and lead author of the new study.

Warmer oceans mean sound will travel faster, impacting marine animals who depend on sounds to find each other and eat. The largest effect on the underwater speed of sound can be expected east of Greenland and off Newfoundland in the Atlantic, according to a new study in the AGU journal Earth’s Future.
Credit: NOAA

Two hotspots, in the Greenland Sea and a patch of the northwestern Atlantic Ocean east of Newfoundland, can expect the most change at 50 and 500 meter depths, the new study projected. The average speed of sound is likely to increase by more than 1.5%, or approximately 25 meters per second (55 miles per hour) in these waters from the surface to depths of 500 meters (1,640 feet), by the end of the century, given continued high greenhouse gas emissions (RCP8.5).

“The major impact is expected in the Arctic, where we know already there is amplification of the effects of climate change now. Not all the Arctic, but one specific part where all factors play together to give a signal that, according to the model predictions, overcomes the uncertainty of the model itself,” said author Stefano Salon, a researcher at the National Institute of Oceanography and Applied Geophysics in Trieste, Italy.

The ocean soundscape is a cacophony of vibrations produced by living organisms, natural phenomena like waves and cracking ice, and ship traffic and resource extraction. Sound speed at 50 meters depth ranges from 1,450 meters per second in the polar regions to 1,520 meters per second in equatorial waters (3,243 to 3,400 miles per hour, respectively).

Many marine animals use sound to communicate with each other and navigate their underwater world. Changing the sound speed can impact their ability to feed, fight, find mates, avoid predators and migrate, the authors said.

CHANGING SOUNDSCAPES

In addition to the notable hotpots around Greenland and in the northwestern Atlantic Ocean, the new study found a 1% sound speed increase, more than 15 meters per second, at 50 m in the Barents Sea, northwestern Pacific, and in the Southern Ocean (between 0 and 70E), and at 500 m in the Arctic Ocean, Gulf of Mexico, and southern Caribbean Sea.

Temperature, pressure with increasing depth and salinity all affect how fast and how far sound travels in water. In the new study, the researchers focused on hotspots where the climate signal stood out clearly from the model uncertainty and was larger than seasonal variability.

The new study also modeled common vocalisations, under the projected future conditions, of the North Atlantic right whale, a critically endangered species inhabiting both north Atlantic acoustic hotspots. The whales’ typical “upcall” at 50 Hertz is likely to propagate farther in a warmer future ocean, the researchers found.

“We chose to talk about one megafauna species, but many trophic levels in the ocean are affected by the soundscape or use sound,” Affatati said. “All these hotspots are locations of great biodiversity.”

Future work will combine the global soundscape with other maps of anthropogenic impacts in the oceans to pinpoint areas of combined stressors, or direct needed observational research.

“With complicated problems like climate change, to combine different approaches is the way to go,” said author Chiara Scaini, an environmental engineer at the National Institute of Oceanography and Applied Geophysics.

More information: Affatati, A., Scaini, C., & Salon, S. (2022). Ocean sound propagation in a changing climate: Global sound speed changes and identification of acoustic hotspots. Earth’s Future, 10, e2021EF002099. https://doi.org/10.1029/2021EF002099

When variations in Earth’s orbit drive Phytoplankton evolution

Coccolithophores are microscopic algae that form tiny limestone plates, called coccoliths, around their single cells. The shape and size of coccoliths varies according to the species. After their death, coccolithophores sink to the bottom of the ocean and their coccoliths accumulate in sediments, which faithfully record the detailed evolution of these organisms over geological time.

A team of scientists led by CNRS researchers show, in an article published in Nature on December 1, 2021, that certain variations in Earth’s orbit have influenced the evolution of coccolithophores. To achieve this, no less that 9 million coccoliths, spanning an interval of 2.8 million years and several locations in the tropical ocean, were measured and classified using automated microscope techniques and artificial intelligence.

Coccolithophores, an important constituent of the plankton, evolved following the rhythm of Earth’s orbital eccentricity. Credit: Luc Beaufort / CNRS / CEREGE

The researchers observed that coccoliths underwent cycles of higher and lower diversity in size and shape, with rhythms of 100 and 400 thousand years. They also propose a cause: the more or less circular shape of Earth’s orbit around the Sun, which varies at the same rhythms. Thus, when Earth’s orbit is more circular, as is the case today (this is known as low eccentricity), the equatorial regions show little seasonal variation and species that are not very specialized dominate all the oceans. Conversely, as eccentricity increases and more pronounced seasons appear near the equator, coccolithophores diversify into many specialized species, but collectively produce less limestone.

The diversity of coccolithophores and their collective limestome production evolved under the influence of Earth’s orbital eccentricity, which determines the intensity of seasonal variations near the equator. On the other hand, no link to global ice volume or temperature was found. It was therefore not global climate change that dictated micro-algae evolution but perhaps the opposite during certain periods. Credit: Luc BEAUFORT / CNRS / CEREGE

Crucially, due to their abundance and global distribution, these organisms are responsible for half of the limestone (calcium carbonate, partly composed of carbon) produced in the oceans and therefore play a major role in the carbon cycle and in determining ocean chemistry. It is therefore likely that the cyclic abundance patterns of these limestone producers played a key role in ancient climates, and may explain hitherto mysterious climate variations in past warm periods.

In other words, in the absence of ice, the biological evolution of micro-algae could have set the tempo of climates. This hypothesis remains to be confirmed.

More information: Luc Beaufort et al., Cyclic evolution of phytoplankton forced by changes in tropical seasonality, Nature (2021). DOI: 10.1038/s41586-021-04195-7www.nature.com/articles/s41586-021-04195-7

Humans Guilty of Breaking an Fundamental Oceanic Law of Nature

A new international study carried out by the Institute of Environmental Science and Technology of the Universitat Autònoma de Barcelona (ICTA-UAB) has examined the distribution of biomass across all life in the oceans, from bacteria to whales. Their quantification of human impact reveals a fundamental alteration to one of life’s largest scale patterns.

As policymakers assemble in Glasgow for the UN Climate Change Conference, there is growing recognition that human impacts on the environment are going global and growing urgent. However, gaining a quantitative perspective on these impacts has remained elusive.

Scientists from the ICTA-UAB in Spain, the Max Planck Institute for Mathematics in the Sciences in Germany, Queensland University of Technology in Australia, Weizmann Institute of Science in Israel, and McGill University in Canada have used advances in ocean observation and large meta-analyses to show that human impacts have already had major consequences for the larger oceanic species, and have dramatically changed one of life’s largest scale patterns – a pattern encompassing the entire ocean’s biodiversity, from bacteria to whales.

Early samples of marine plankton biomass from 50 years ago led researchers to hypothesize that roughly equal amounts of biomass occur at all sizes. For example, although bacteria are 23 orders of magnitude smaller than a blue whale, they are also 23 orders of magnitude more abundant. This size-spectrum hypothesis has since remained unchallenged, even though it was never verified globally from bacteria to whales. The authors of the study, published in the journal Science Advances, sought to test this hypothesis on a global scale for the first time. They used historical reconstructions and marine ecosystem models to estimate biomass before industrial scale fishing got underway (pre-1850) and compared this data to the present-day.

Figure about the research. Credit: Ian Hatton, Eric Galbraith et al.

“One of the biggest challenges to comparing organisms spanning bacteria to whales is the enormous differences in scale,” recalls ICTA researcher and lead author Dr. Ian Hatton, currently based at the Max Planck Institute for Mathematics in the Sciences. “The ratio of their masses is equivalent to that between a human being and the entire Earth. We estimated organisms at the small end of the scale from more than 200,000 water samples collected globally, but larger marine life required completely different methods.”

Their approach focused on 12 major groups of aquatic life over roughly 33,000 grid points of the ocean. Evaluating the pre-industrial ocean conditions (pre-1850) largely confirmed the original hypothesis: there is a remarkably constant biomass across size classes.

“We were amazed to see that each order of magnitude size class contains approximately 1 gigaton of biomass globally,” remarks co-author Dr. Eric Galbraith of the ICTA-UAB and a current professor at McGill University. However, he was quick to point out exceptions at either extreme. While bacteria are over-represented in the cold, dark regions of the ocean, the largest whales are relatively rare, thus highlighting exceptions from the original hypothesis.

In contrast with an even biomass spectrum in the pre-1850 ocean, an investigation of the spectrum at present revealed human impacts on ocean biomass through a new lens. While fishing and whaling only account for less than 3 percent of human food consumption, their effect on the biomass spectrum is devastating: large fish and marine mammals such as dolphins have experienced a biomass loss of 2 Gt (60% reduction), with the largest whales suffering an unsettling almost 90% decimation. The authors estimate that these losses already outpace potential biomass losses even under extreme climate change scenarios.

“Humans have impacted the ocean in a more dramatic fashion than merely capturing fish. It seems that we have broken the size spectrum – one of the largest power law distributions known in nature,

reflects ICTA researcher and co-author Dr. Ryan Heneghan. These results provide a new quantitative perspective on the extent to which anthropogenic activities have altered life at the global scale. 

More information: The global ocean size spectrum from bacteria to whales (2021). Hatton, Heneghan, Bar-On and Galbraith, 2021, Science Advances.
DOI: 10.1126/sciadv.abh3732

Marine Plankton helps produce clouds, but existing clouds keep new ones at bay

Marine plankton breathe more than 20 million tons of sulfur into the air every year, mostly in the form of dimethyl sulfide (DMS). In the air, this chemical can transform into sulfuric acid, which helps produce clouds by giving a site for water droplets to form. Over the scale of the world’s oceans, this process affects the entire climate.

But new research from the University of Wisconsin–Madison, the National Oceanic and Atmospheric Administration and others reveals that more than one-third of the DMS emitted from the sea can never help new clouds form because it is lost to the clouds themselves. The new findings significantly alter the prevailing understanding of how marine life influences clouds and may change the way scientists predict how cloud formation responds to changes in the oceans.

NOAA

 By reflecting sunlight back into space and controlling rainfall, clouds play significant roles in the global climate. Accurately predicting them is essential to understanding the effects of climate change.

More information: Novak et al PNAS October 19, 2021 118 (42) e2110472118; https://doi.org/10.1073/pnas.2110472118

Australian wildfires triggered massive Phytoplankton blooms in the Southern Ocean

Clouds of smoke and ash from wildfires that ravaged Australia in 2019 and 2020 triggered widespread phytoplankton blooms in the Southern Ocean thousands of miles downwind to the east, a new Duke University-led study by an international team of scientists finds.

The study study, published in Nature, is the first to conclusively link a large-scale response in marine life to fertilization by pyrogenic—or fire-made—iron aerosols from a wildfire.

It shows that tiny aerosol particles of iron in the windborne smoke and ash fertilized the water as they fell into it, providing nutrients to fuel blooms at a scale unprecedented in that region.

Phytoplankton require iron for photosynthesis

The study’s co-author Prof Peter Strutton, of the University of Tasmania’s Institute for Marine and Antarctic Studies, likened the phytoplankton bloom to “the entire Sahara desert turning into a moderately productive grassland for a couple of months”. “The entire Southern Ocean is basically low in iron because it’s a long way from dust sources, so any small amount of iron that gets deposited there can cause a strong response,” Strutton said.

The discovery raises intriguing new questions about the role wildfires may play in spurring the growth of microscopic marine phytoplankton, which absorb large quantities of climate-warming carbon dioxide from Earth’s atmosphere through photosynthesis and are the foundation of the oceanic food web.

A satellite image shows smoke from the 2020-21 Australian wildfires covering parts of the Southern Ocean. Credit: Japan’s National Institute of Information and Communication Technology.

Our results provide strong evidence that pyrogenic iron from wildfires can fertilize the oceans, potentially leading to a significant increase in carbon uptake by phytoplankton,” said Professor Nicolas Cassar, of Duke’s Nicholas School of the Environment.

The phytoplankton blooms triggered by the Australian wildfires were so intense and extensive that the subsequent increase in photosynthesis may have temporarily offset a substantial fraction of the fires’ CO2 emissions, he said. But it’s still unclear how much of the carbon absorbed by that event, or by blooms triggered by other wildfires, remains safely stored away in the ocean and how much is released back into the atmosphere. Determining that is the next challenge, Cassar said.

Carbon uptake

The researchers estimate the amount of carbon taken up by phytoplankton cells as a result of the bloom was equivalent to around 95% of the emissions generated by the 2019-20 bushfires.

For that carbon to be permanently removed from the atmosphere, however, the phytoplankton cells would have to sink into the deep ocean and be stored there, Strutton said.

There’s a lot of recycling of energy and biomass that happens in the surface waters. It’s likely that a lot of that carbon that was initially taken up might have been re-released to the atmosphere when those phytoplankton cells started to break down or were eaten.

Large wildfires, like the record-breaking blazes that devastated parts of Australia between 2019 and 2020 and the fires now raging in the western U.S., Siberia, the Amazon, the Mediterranean and elsewhere, are projected to occur more and more frequently with climate change, noted Weiyi Tang, a postdoctoral fellow in geosciences at Princeton University, who co-led the study as a doctoral candidate in Cassar’s lab at Duke.

These fires represent an unexpected and previously under-documented impact of climate change on the marine environment, with potential feedbacks on our global climate.

Pyrogenic aerosols are produced when trees, brush and other forms of biomass are burned. Aerosol particles are light enough to be carried in a fire’s windborne smoke and ash for months, often over long distances.

More information: Widespread phytoplankton blooms triggered by 2019–2020 Australian wildfires, Nature (2021). DOI: 10.1038/s41586-021-03805-8 , www.nature.com/articles/s41586-021-03805-8

Ocean microbes team up and cooperate to gather food when it’s scarce

What do phytoplankton do when the pickings are slim? They must continue to capture nutrients – nitrogen, phosphorus, or iron – to survive, even when these nutrients have become depleted in the sun light layers of the ocean.

One ingenious solution to this challenge is reported in Proceedings of the National Academy of Sciences. In low-nutrient environments, marine microbes can clump together and hook up with even tinier cells that have vibrating, hairlike appendages (cilia) on their surface. The beating cilia create microcurrents that can pull up to 10 times more nutrients within the microbes’ reach – thereby serving up a meal through cooperative work.

Coscinodiscus wailesii diatom (left image) and with attached Pseudovorticella coscinodisci ciliate epibionts (right image). Streak lines were derived from flows generated by the ciliate epibionts. Kanso et al, PNAS, 2021

Even if the ocean is wildly turbulent, microbes can piggyback into consortia for division of labor, says senior corresponding author John H. Costello of Providence College and the Marine Biological Laboratory (MBL), Woods Hole, where much of the research was conducted.

For all conditions but the most radically extreme mixing, these microbial cells live in fluid spaces that are smaller than the eddies caused by ocean mixing,” Costello says. “In their world, the surrounding fluid is always viscous and they do not experience turbulent eddies as humans feel them.”

The team used a technology called Particle Image Velocimetry (PIV) to measure the direction and magnitude of fluid flows around a photosynthetic marine diatom, Coscinodiscus wailesii, with and without an attached ciliate “partner,” Pseudovorticella coscinodisci. They found that fluid flows generated by ciliary beating can increase nutrient flux to the diatom’s cell surface 4-10 times greater than fluxes to the diatom alone.

This cooperative solution is one way microbes can cope in low-nutrient environments. Another previously known tactic for individual cells is sinking to greater depths, which creates relative motion between the cell and surrounding water and increases its exposure to higher nutrient concentrations.

“Sinking might work well in low-nutrient conditions where mixing will recirculate the cells back up from the depths to the sunlit layers,” Costello says. “That way, the risk to the diatom of sinking might be countered by the probability of being returned to high-light environments. But in low-mixing conditions, forming consortia with ciliates could be a more favorable solution to low nutrient availability.”

Diatoms are among the most important groups of single-celled photosynthesisers for removing carbon dioxide from the atmosphere. Thus, the study helps to illuminate ocean-atmospheric exchanges that have become increasingly important for understanding climate change.

More information and link to paper: Eva A. Kanso, Rubens M. Lopes, J. Rudi Strickler, John O. Dabiri, John H. CostelloProceedings of the National Academy of Sciences Jul 2021, 118 (29) e2018193118; DOI: 10.1073/pnas.2018193118

Predicting the future of cod stocks in the North Atlantic

New fisheries management planning tool developed with fewer stocks expected

The future of cod stocks in the North Sea and the Barents Sea may be much easier to predict than before. This is the result of an international research project led by the Helmholtz-Zentrum Hereon and its Institute of Coastal Systems – Analysis and Modeling. For the first time, the team has succeeded in predicting the development of stocks for ten years in advance, taking into account both changes due to climate and fishing. Traditionally, fisheries experts provide catch recommendations for about a year in advance, on the basis of which fishing quotas are negotiated and set internationally. This involves first estimating the size of current cod stocks and then calculating how much cod can be caught in the coming year without endangering the stocks as well as harvesting the stock optimally. The climatic change, long-term changes in water temperature, circulation and mixing, which have a decisive influence on how well cod reproduce, are not included in this prediction, so that the development of stocks can only be predicted in the short term.

Cod stocks will probably decrease in the future: Photo: David Young via Fotolia

Warm North Sea causes stress

As the experts around climate modeler Vimal Koul und Corinna Schrum of Hereon now write in the journal Nature Communications Earth and Environment, they have taken temperature into account in their calculations. For the North Sea, the climate forecast continues to predict temperatures at a high level, so that cod stocks are unlikely to recover or reach earlier levels. As a result, catches are expected to remain low. Things look better for the Barents Sea, where stocks can be managed sustainably.

For the researchers, the challenge was that climate models cannot calculate how much fish there will be in the oceans in the future. They only provide information about expected temperatures. “So we first had to develop a program that translates water temperature into fish quantities,” says Vimal Koul. Among other things, this took into account the ocean temperature in the North Atlantic. The researchers were then able to run their prediction model. The model starts with today’s conditions – the current temperature conditions and the current carbon dioxide content of the atmosphere, and can then calculate how the situation will change as carbon dioxide concentrations increase. The future temperatures are then translated into expected fish abundance and stock sizes.

To test how reliably the model works, it was first compared with real fish data from the 1960s to the present. As it turned out, it was able to correctly estimate fish stocks for the ten-year periods since the early 1960s. In this respect, the researchers led by Vimal Koul can assume that the current view of the coming ten years is also correct.

Fishing intensity taken into account

Another interesting aspect of the study is that the team of climate modelers, fisheries biologists and oceanographers took four different fishing scenarios into account. This allowed them to determine how cod stocks would fare if they were fished at different levels – from intensive to sustainable. In this respect, the results of the current study are very practical. “The 10-year estimates will help the fishing industry better plan catches in the future – so that cod stocks are fished sustainably and gently despite changes in climate,” says Vimal Koul. The new 10-year calculation model could also help fishing companies in their strategic planning – by providing a secure basis for investments in new vessels or processing facilities.

More information: Koul, V., Sguotti, C., Årthun, M. et al. Skilful prediction of cod stocks in the North and Barents Sea a decade in advance. Commun Earth Environ 2, 140 (2021). https://doi.org/10.1038/s43247-021-00207-6

Oceans’ microscopic plants known as diatoms capture carbon dioxide via biophysical pathways

Diatoms are tiny unicellular plants — no bigger than half a millimeter — which inhabit the surface water of the world’s oceans where sunlight penetration is plenty. Despite their modest size, they are one of the world’s most powerful resources for removing carbon dioxide (CO2) from the atmosphere. They currently remove, or “fix,” 10-20 billion metric tons of CO2 every year by the process of photosynthesis. But not much is known about which biological mechanisms diatoms use, and whether these processes might become less effective with rising ocean acidity, temperatures, and, in particular, CO2 concentrations. A new study shows that diatoms predominantly use one pathway to concentrate CO2 at the vicinity of carbon fixing enzyme and that this continues to operate even at higher CO2 concentrations.

“We show that marine diatoms are super smart in fixing atmospheric CO2 even at the present-day level of CO2 — and the variability in surface seawater CO2 levels did not impact the gene expression and abundance of the five key enzymes used in carbon fixation,” says the group leader of the study, Dr Haimanti Biswas from the National Institute of Oceanography-CSIR (Council of Scientific and Industrial Research), India. “This answers a key question about how marine diatoms may respond to the future increase in atmospheric CO2 levels.”

Centric diatoms

The plant kingdom has evolved a wide range of mechanisms for concentrating CO2 from the air, or water, and transforming it into organic carbon. In this way, plants convert CO2 into glucose and other carbohydrates, which they use as building blocks and energy storage. But these different mechanisms have varied strengths and weaknesses. Somewhat ironically, the only carbon-fixing enzyme, RuBisCO, is notoriously inefficient at fixing CO2 and hence plants need to keep CO2 levels high In the vicinity of this enzyme.

To better understand which mechanism diatoms use to concentrate CO2, Biswas and her collaborators, Drs Chris Bowler and Juan Jose Pierella Karluich from the Institut de Biologie de I’Ecole Normale Supérieure, Paris, France, mined a data set from the Tara Oceans research expedition. The international Tara expedition collected marine plankton samples from around the world over several years (2009 to 2013). These included more than 200 metagenomes (which show the abundance of the genes responsible for the five key enzymes) and over 220 metatranscriptomes (showing expression of the genes for the five key enzymes) from diatoms of different size classes.

Biswas and her collaborators were particularly interested in how often the genes of five key carbon-fixing enzymes are present, and whether there were any differences in their abundance and expression levels depending on location and conditions. Across all of the samples measured, one enzyme was roughly ten times more abundant than any of the other enzymes. This enzyme — called carbonic anhydrase — is especially informative because it also confirms that diatoms are actively pumping in dissolved CO2 inside the cell, as opposed to biochemically transforming CO2 first.

The team also observed complex different patterns of the key enzymes’ gene expression, which varied depending on latitude and temperature. The researchers hope to learn more using new datasets from more widely-traveled future expeditions.

“So far, our study indicates that despite variability in CO2 levels, these tiny autotrophs are highly efficient in concentrating CO2 inside the cell,” says Biswas. “That’s the probable reason for their ability to fix nearly one-fifth of the global carbon fixation on earth.”

More information: https://www.frontiersin.org/articles/10.3389/fpls.2021.657821/full

Potentially toxic plankton algae may play a crucial role in the future Arctic

As the sea ice shrinks in the Arctic, the plankton community that produces food for the entire marine food chain is changing. New research shows that a potentially toxic species of plankton algae that lives both by doing photosynthesis and absorbing food may become an important player in the Arctic Ocean as the future sea ice becomes thinner and thinner.

Microscopic plankton algae, invisible to the naked eye, are the foundation of the marine food web, feeding all the ocean´s living creatures from small crustaceans to large whales. Plankton algae need light and nutrients to produce food by photosynthesis.

A thick layer of sea ice – sometimes covered with snow – can reduce how much sunlight penetrates into the water and stop the algae getting enough light. However, as the sea ice is becoming thinner and less widespread in the Arctic, more and more light is penetrating into the sea. Does this mean more plankton algae and thus more food for more fish, whales and seabirds in the Arctic? The story is not so simple.

More light in the sea will only lead to a higher production of plankton algae if they also have enough nutrients – and this is often not the case. With the recent increase in freshwater melt from Arctic glaciers and the general freshening of the Arctic Ocean, more and more fresh and nutrient-depleted water is running out into the fjords and further out into the sea. The fresher water lies on top of the more salty ocean and stops nutrients from the deeper layers from mixing up towards the surface where there is light. And it is only here that plankton algae can be active.

Mixotrophic algae play on several strings

However, a new study published in the journal Nature – Scientific Reports shows that so-called mixotrophic plankton algae may play a crucial role in the production of food in the Arctic Sea.

When the spring sets in in the Arctic, the metre-thick sea ice begins to melt. Melt ponds on the surface of the sea ice bring so much sunlight into the underlying seawater that the mixotrophic plankton algae start to grow dramatically. During an approx. 9-day period, the plankton can produce up to half of the total annual pelagic production in the high-Arctic fjord, Young Sound, in northeast Greenland. Several mixotrophic algae species are toxic. Photo credit: Lars Chresten Lund Hansen and Dorte H. Søgaard

Mixotrophic algae are small, single-celled plankton algae that can perform photosynthesis but also obtain energy by eating other algae and bacteria. This allows them to stay alive and grow even when their photosynthesis does not have enough light and nutrients in the water.

In northeast Greenland, a team of researchers measured the production of plankton algae under the sea ice in the high-Arctic fjord Young Sound, located near Daneborg.

“We showed that the plankton algae under the sea ice actually produced up to half of the total annual plankton production in the fjord,” says Dorte H. Søgaard from the Greenland Climate Research Centre, Greenland Institute of Natural Resources and the Arctic Research Centre, Aarhus University, who headed the study.

“Mixotrophic plankton algae have the advantage that they can sustain themselves by eating other algae and bacteria as a supplement to photosynthesis when there isn’t enough light. This means that they are ready to perform photosynthesis even when very little light penetrates into the sea. In addition, many mixotrophic algae can live in relatively fresh water and at very low concentrations of nutrients – conditions that often prevail in the water layers under the sea ice in the spring when the ice melts,” Dorte H. Søgaard explains.

Toxic algae kill fish

For nine days, the researchers measured an algal bloom driven by mixotrophic algae occurring under the thick but melting sea ice in Young Sound during the Arctic spring in July, as the sun gained more power and more melt ponds spread across the sea ice, gradually letting through more light.

The algae belong to a group called haptophytes. Many of these algae are toxic, and in this study they bloomed in quantities similar to those previously observed in the Skagerrak near southern Norway. Here, the toxic plankton algae killed large amounts of salmon in Norwegian fish farms.

“We know that haptophytes often appear in areas with low salinity – as seen in the Baltic Sea, for example. It is therefore very probable that these mixotrophic-driven algae blooms will appear more frequently in a more freshwater-influenced future Arctic Ocean and that this shift in dominant algae to a mixotrophic algae species might have a large ecological and socio-economic impact.” says Dorte H. Søgaard.

The researchers behind the project point out that it is the first time that a bloom of mixotrophic algae has been recorded under the sea ice in the Arctic.

More information: An under-ice bloom of mixotrophic haptophytes in low nutrient and freshwater-influenced Arctic waters.http://www.nature.com/articles/s41598-021-82413-y