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Global WarmingOceansScience

Arctic Plankton Blooms Arriving Earlier, Fish May Be Imperiled

Over the past decade, the Arctic’s annual “bloom” of phytoplankton has been arriving earlier each year. The trend in earlier blooms of this crucial,  primary producer of the Arctic’s food web is occurring largely along coastal and ice edge areas within the Arctic circle, with the exception of large patches in the northern Pacific Ocean.

A recent survey by a team of oceanographers from the Scripps Institution of Oceanography (UC San Diego), Mexico and Portugal, found evidence of earlier algal blooms in roughly 11 percent of the area ringing the Arctic Ocean, and closest to the North Pole. Delayed blooms were found more commonly in southerly regions around the Arctic (note: algal blooms can occur in both warmer and colder waters, but the composition of the plankton community will vary).

Using satellite imaging data depicting ocean color and phytoplankton production, the scientists found that in some areas around the Arctic, peak blooms have been arriving up to 50 days earlier over this ten year period.

One of thousands of kinds of phytoplankton life forms

The problem, of course, is that large numbers of fish depend, directly or indirectly, on phytoplankton. The greatest increase in the fish population is tied to the peak plankton numbers of these one to two week bloom periods. If, however, many of these plankton blooms are trending earlier each year, then the seasonal return/growth of the fish population in these areas is gradually becoming “out of sync” with the primary producers in this region. This may mean insufficient food supply to maintain robust fish populations.

Phytoplankton are tiny, mostly microscopic, plants that occupy the base of the region’s food web, providing food for zooplankton (microscopic animals, the primary consumers) which in turn get eaten by larger fish, and so on. Large marine mammals such as some species of baleen (mysticene) whales survive entirely on a diet of plankton.

It is hypothesized that warming temperatures and melting ice are responsible for this early bloom trend (these earlier blooms tend to occur where the ice breaks up, leaving patches of open (and warmer) ocean for the blooms to spread).

Arctic, plankton blooms, trend map, oceanography
Significant trends toward earlier phytoplankton blooms (blue) were detected in about 11 percent of the area of the Arctic Ocean closest to the North Pole, delayed blooms (red) were evident to the south. Credit: Scripps Institution of Oceanography, UC San Diego

If this trend is real (and the data supports this analysis), then the concern is that fish populations will not be able to match these earlier blooms with the timing of their own egg-laying and larval development. This could have major impact on the near-term survival of many fish species. Many scientists speculate that this mismatch may be responsible for the well-documented, annual variability in Arctic fish stocks.

An additional concern is that this trend could expand into more and more areas in the region, which will have potentially dramatic consequences for the entire food chain.

The ecological pyramid

And it’s not just the food supply that may be impacted; the regions carbon cycle could be impacted as well. Algal organisms “lock up” a massive amount of carbon as they absorb CO2 from the atmosphere and convert it to food and organic carbon forms (via photosynthesis). As these plants die, or the marine animals that eat them die, they sink into the depths, sequestering all that carbon along with them.

A disruption in the food web can produce a disruption in the carbon cycle.

There is also concern over another planktonic trend in the Arctic ocean — the trend towards smaller-sized phytoplankton species (known as picophytoplankton; see Li et al, Science, 23 Oct, 2009) believed to be driven by freshening (less salty) sea water. This too is impacting both the food supply in this region and the amount of carbon locked up via photosynthesis (smaller “community” size structure is strongly correlated to ecosystem carbon flux). Such smaller forms of plankton cannot support large exports of biogenic carbon (thus less extraction from the atmosphere and less sequestration in the ocean).

The Arctic region is a great, dynamic ecosystem, and even small changes (or changes in the small) can have huge and long-term impacts on this system’s capacity to sustain life (in numbers that we are accustomed to, and depend on for food) and too regulate the climate through sequestering carbon.

As always, more study is needed, but the trend, if it continues, does not bode well for the region’s fish, food chain, and climate response.

The team used satellite  from 1997-2010 to create their bloom maps. The results were published in the journal Global Change Biology (March 9, 2011)

Top photo:  Ice edge blooms often follow retreating ice, as shown here on July 5, 2007, south of Wrangel Island in the eastern Chukchi Sea. Satellite data captured by the NASA MODIS-Aqua sensor, processed by Mati Kahru. Credit: Scripps Institution of Oceanography, UC San Diego




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