Reduced sea ice – big consequences

Reduced sea ice – big consequences

Top image: Ice algae thrives under the sea ice. This algae is fundamental for the Arctic marine ecosystem. Photo: Janne Søreide/UNIS.

New research reveals that reduced sea ice coverage in the Arctic will have large consequences for the algae bloom. As key marine species rely on this blooming, any changes in the timing of this event can have huge consequences for the whole marine ecosystem.

16 July 2010
Text: Eva Therese Jenssen

There is no doubt that the Arctic sea ice coverage has been reduced quite dramatically over the past decade. However, until recently no investigation into the corresponding consequences for the most important marine biological processes have been done.

Ice algae and phytoplankton are two distinct primary producers which are fundamental for the Arctic marine ecosystem. The ice algae grow within and on the underside of the sea ice, while phytoplankton grow in open waters. These algae are the basis for all life in the Arctic, as they produce exclusively omega-3 fatty acids, which humans also need.

New research shows that key marine species have fine-tuned their reproduction and growth to these two algal blooms. Changes in the underwater light climate due to reduced ice coverage will change the current algal bloom regime with unknown consequences for the vulnerable Arctic marine ecosystem.

Scientists are now studying how changes in the extent of the sea ice affect the timing, quantity and quality of primary- and secondary producers (zooplankton), such as marine copepods Calanus. Together the algae and copepods form the basis of the Arctic marine food web, from fish and all the way up to the polar bear.

It’s all about the fat
The omega-3 production of the ice algae and phytoplankton has “peaks” which grazing zooplankton are effectively utilizing. The marine copepods Calanus glacialis, Calanus finmarchicus and Calanus hyperboreus are abundant in the northern oceans.

In the spring and summer, when 24 h daylight takes place and the biomass production in the Arctic Ocean is at its peak, these small copepods feed effectively on the algae.

The energy from the algal food is stored as liquid fat in a lipid sack, which can fill almost the whole body cavity of the animal when full.

Up to 80 % of the body weight of the Calanus glacialis and Calanus hyperboreus can consist of fat, and the largest of these three copepods, Calanus hyperboreus, is able to survive 2 years on this fat deposit without feeding.

The medium-sized Calanus glacialis accounts for up to 80 % of the zooplankton biomass in Arctic ice covered shelf seas, and is one of most important food item for, among other, the bowhead whale, little auk and the polar cod.

Calanus glacialis

Calanus glacialis. The fat sack is clearly visible. Photo: Janne Søreide/UNIS.

Explosive food production
In the spring, when the sea ice cover withdraws and the sunlight penetrates into the ocean the phytoplankton literally “explodes” and provide the ocean with large amounts of high quality food rich in omega-3 fatty acids.

However, this productive period in the High Arctic is usually too short for a full development of important grazers in just one summer season. This is due to a combination of low temperatures that prolong the development time, and a rapid decrease in phytoplankton growth, since strong stratification of the water masses – caused by ice melting – prevents new nutrient supplies to the upper layers.

C. glacialis , however, extend its grazing season by also grazing on the ice algae. The ice algae can start blooming up to two months earlier than phytoplankton, since they are specially adapted to grow under dismal light conditions.

Finely tuned
Scientists in the Norwegian IPY project “CLEOPATRA” followed the seasonal development of ice algae, phytoplankton and population development of C. glacialis in a high-Arctic fjord in 2007. The scientists, from UNIS, Norwegian Polar Institute and the Alfred Wegener Institute (Germany) aimed at uncover the intimate coupling between the solar cycle, food quality peaks, and the onset and duration of primary and secondary production.

The fieldwork was done in Rijpfjorden, which is the northern part of Nordaustlandet on Svalbard. This fjord is directly linked to the polar ocean. Data collection was done by using both a stationary ocean observatory and regular in-situ fieldwork throughout the winter, spring and summer of 2007.

– The first algae bloom, of ice algae, occurred in late April, while Rijpfjorden still was covered by thick sea ice. The second bloom, of phytoplankton, started in early July, after the fjord ice had melted, says Janne Søreide at UNIS.

– The life cycle of C. glacialis was perfectly tuned to these two algal bloom events. Females efficiently utilized the first ice algae bloom for early maturation and high egg production, thereby enabling the first feeding stages of the new generation to take full advantage of the later occurring phytoplankton bloom, Søreide explains.

Reduced sea ice – huge consequences
Because these biological processes at the bottom of the food web are so nicely tuned, may a reduced sea ice coverage, both in extent and thickness, lead to a mismatch between primary producers and key Arctic grazers with negative consequences for the whole Arctic marine ecosystem. Less sea ice will result in earlier melting and an earlier phytoplankton bloom, and a shorter blooming period for the ice algae.

Fat-rich Arctic grazers such as C. glacialis might be replaced by less “fatty” Atlantic organisms, which can have direct negative impact on the rest of the fat-based High Arctic marine ecosystem, according to Søreide.

Future research will be focusing on getting longer time-series of these important biological processes as studied in Rijpfjorden.

– In addition to study the algae and grazers, we want to include one or two key predators, as well as include studies of how changes in timing, quantity and quality of the algal blooms affects the rich benthic communities in the Arctic shelf seas, Søreide says.

Rijpfjorden will thus become an important research area in the future, along with Billefjorden (an ice covered threshold fjord in the inner Isfjorden system), which is more accessible than Rijpfjorden. The scientists also want to study the biological processes during the Polar night in Billefjorden – which until now has been fairly unknown.

Janne Søreide, Eva Leu, Jørgen Berge, Martin Graeve, Stig Falk-Pettersen: ”Timing of blooms, algal food quality and Calanus glacialis reproduction and growth in a changing Arctic. Global Change Biology (2010), doi: 10.1111/j.1365-2486.2010.02175.x

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