Dramatic temperature rises in the Arctic hits vital sea ice algae
Top image: Temperatures in the Arctic are rising more than twice as fast as on Earth – this has consequences for algae and wildlife. Former UNIS PhD candidate Ane Cecilie Kvernvik doing fieldwork in Van Mijenfjorden. Photo: Eva Leu
Original text (in Norwegian): Eva Leu, senior researcher, Akvaplan-niva
Unicellular, microscopically small algae that are invisible without a microscope form the basis of the entire marine ecosystem. The reason for this is that they are the only organisms that can use solar energy to form biomass through photosynthesis.
This also applies in the Arctic Ocean. In icy waters, we find mainly two groups of these algae:
1) Sea ice algae are living inside the sea ice itself in tiny meltwater channels or attached to the lowermost layer of the ice where it meets the water.
2) Phytoplankton are unicellular algae that float around in the water masses in all types of water.
Both are completely dependent on the environment to have good growth conditions, and this environment is now rapidly changing.
Dramatic temperature increases in the Arctic
As many may know, temperatures in the Arctic are rising more than twice as fast as elsewhere on the planet. New estimates suggest thrice as fast. This naturally leads to many dramatic changes in the environment, of which sea ice melt is perhaps the best known.
When sea ice melts, becomes thinner and more transparent, it is not just the “home” of the ice algae that dissolves – it also contributes to increased light supply to the sea and more stratified surface water. The melting of ice also causes a layer of fresh water to form on the surface. The reason for this is that sea ice contains much less salt than ordinary seawater, and when it melts, the meltwater is lighter than the seawater, so it settles on top. This counteracts the water masses being mixed by the wind, which would normally have led to an increased supply of nutrients from deeper water layers.
Another change is that the increased emissions of CO2 into the atmosphere result in ocean acidification. This means that the water’s pH becomes lower, and that the ocean’s ability to absorb new CO2 from the air gradually decreases. It has been shown that several organisms struggle when the sea becomes more acidic. This is especially true for species that form structures of lime, as lime simply dissolves when the water becomes too acidic.
All life in the sea depends on algae
Ultimately, all marine life depends on the biomass formed by tiny algae. Therefore, it is impossible to predict what the future marine ecosystems in the Arctic will look like without understanding how climate change and ocean acidification will affect these key organisms.
We have studied this in a 5-year research project funded by the Research Council of Norway with the title: “FAABulous: Future Arctic Algae Blooms – and their role in the context of climate change”. A key question in the project has been whether ice algae and phytoplankton will react differently to the changes we are now seeing.
Smaller algae provide less food for all marine life
The reason why this is an important question is that these two algae usually bloom at different times, and that there are different types of animals that utilize them as food.
Changes in the relative contribution of ice algae or phytoplankton to the total production at the lowest level of the Arctic food network will potentially lead to major changes: Both with respect to in how the ecosystem functions, but also with respect to which groups of animals in this system will be able to cope with the ongoing changes in the best way.
To find the answer to this important question, we chose two approaches:
1) We compared natural communities of ice algae and phytoplankton that we collected in and under the sea ice in the Van Mijenfjord on Svalbard with regard to their reaction to environmental change.
2) To study the underlying mechanisms more closely, we exposed two key species (one ice algae, and one phytoplankton species) to a stress experiment in the laboratory where we pre-cultured them under identical conditions prior to the experiment. The stress we inflicted on them during the experiment (strong increase in light, with and without ocean acidification) was meant to mimic conditions that we assume they will experience in the Arctic of the future.
Sea ice algae were very stressed
Both studies gave us a very clear answer: while phytoplankton seemed to adapt quickly to new conditions and were very good at taking advantage of increased access to light – the reaction of sea ice algae was completely opposite.
The ice algae were very stressed by high light intensities and used a lot of energy to get rid of excess light energy. As a result, ice algae could not benefit from the increasing amount of light, even though light serves as energy source for photosynthesis.
Specialists in challenging environments
Sea ice algae did also tolerate ocean acidification much less than phytoplankton. When they were exposed to high light in addition, the negative impact was extra strong. A possible explanation for the strong differences in the algae’s responses towards future environmental conditions probably lies in their different ways of living they have specialized in over long time: ice algae have adapted to the rather challenging environment they usually grow in with very low temperature, low light, and sometimes extreme conditions with respect to salt content and concentration of nutrients and CO2.
Phytoplankton cells, on the other hand, experience continuous fluctuations surroundings because they “float in the water” and get mixed to different depths on very short time scales. Thus, they experience changes in light and nutrient levels in a very short time. Perhaps it goes without saying that those who survive and are productive in such an environment must be able to cope with rapid change.
But back to why it is important to understand these contexts. Scientists often use models to try to predict how climate change will affect marine ecosystems. These models include a lot of background knowledge about how environmental changes will affect the production rate of algae, as well as knowledge about how altered algae production will affect species further up the food chain.
Our research results show that it is crucial that such models consider that sea ice algae and phytoplankton react differently to the same changes. If one does not take this into account, then the results of different types of model calculations will be completely wrong. Since most studies focus on either ice algae or phytoplankton, the results from our FAABulous project are quite unique.
Our hope is that through the project we have created better knowledge that helps us predicting the impact of ongoing climate change on marine Arctic ecosystems.