July 19, 2011 The spring bloom of plant plankton in Disko Bay has been unusually long this year. While in some years, it may have a short burst of just two weeks, this year Disko Bay was filled with plankton alga for more than six weeks.
"Right up to the end of May we were observing high concentrations of alga, and it is only recently that the bloom has started to decrease. It is the first time that we have got measurements and data from such a long bloom. It will be interesting to examine the data in detail," says Professor Torkel Gissel Nielsen from the National Institute of Aquatic Resources (DTU Aqua) in Denmark.
The duration of algal blooms depends on ice conditions, wind, and how much the sun shines. When the ice breaks up quickly, and there is a lot of sun the bloom occurs very rapidly. However, when the ice remains solid, and the weather is cloudy like this year, the plant plankton grows slowly, and it takes longer for it to absorb the nutrients in the water.
Copepods come to feast A long algal bloom is good news for the small microscopic crustaceans, the copepods, which feed on the plant plankton, because it increases their chances of coming to the surface at a point when there is enough food. And what is good news for the copepods is also good news for the rest of the food chain; because without the copepods, there would be no fish, seabirds, seals or bowhead whales.
"Most copepods survive the long arctic winter by hibernating in the deep water. It is not until the spring that they come to the surface to feed and reproduce. Therefore, if the algal bloom is short, the copepods risk arriving too late for the feast, so it is more likely there is still enough food around when they come up to the surface when the bloom is long like this year," says Torkel Gissel Nielsen, DTU Aqua.
Avoiding the nasty-smelling alga
Biologists from DTU Aqua have therefore returned from this year's fieldwork with important new measurements about how the lower links in the food chain work when there is an apparent abundance of food. That is, if the copepods do in fact eat the alga that was so plentiful this year. This is also something this year's data will provide new knowledge about.
In order to find out what the copepods' favorite dishes are, scientists at the lab at the Arctic Station carried out so-called "grazing experiments," which are designed to examine what kind of plant plankton the different copepod species in the Bay prefer to eat.
"This year, there has been an awful lot of the nasty-smelling colony-forming alga, Phaoecystis, which forms large colonies, and gathers in what can best be described as small gel-like clusters. Preliminary results suggest, that copepods avoid these and do not eat them," says Torkel Gissel Nielsen from DTU Aqua.
Enter the thin southern copepods
In recent years, scientists have focused particularly on which copepod species inhabit Disko Bay. Preliminary estimates show that over the last few years, a thin southern species has become more and more prevalent, while the numbers of the fatter Arctic species have been declining.
"There has never been so few of the Arctic copepod, Calanus glacialis, as there is right now, while the southern species, Calanus finmarchicus, has become very dominant this year. This may affect the area's wildlife because the southern species are much less fatty than the Arctic species. This means, that an animal has to eat many more of them to obtain the fat needed to survive and multiply and get through the long Arctic winter," says DTU Aqua's Torkel Gissel Nielsen.
Making life hard for their enemies
As part of this year's fieldwork, researchers have also, for the first time in the Arctic, examined whether diatom alga blooms can make the water alkaline.
"Laboratory experiments have shown that small, single-celled zooplankton, whose diet includes diatoms, perform badly in conditions with high pH values. So you could say that by affecting the water's pH, diatoms may make life harder for their predators. If this is the case, it may have repercussions higher up the food chain as copepods and fish larvae may then not be getting enough to eat," says the professor.
Researchers have not yet, however, reached any definitive conclusion about it this year: "Because it's been very windy for much of the year, resulting in the water being mixed up quite a lot, the bloom was extended for longer than usual. As a consequence, the concentration of alga was never very high, and therefore the pH value was never very high. So this is something we're very much looking forward to exploring over the next few years," says Torkel Gissel Nielsen, DTU Aqua.
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