Massive algae blooms change composition of sea floor food chain
By Summit Voice
SUMMIT COUNTY — Arctic Ocean ecosystems are sure to change in as-yet unexpected ways as sea ice continues to shrink. This summer, German Polar researchers and microbiologists documented one of those changes, observing an unprecedented bloom of ice-loving algae on patches of thin summer ice.
The researchers hypothesized three years ago that ice algae could grow faster under the thinning sea ice of the Central Arctic. This past summer’s observations support the hypothesis: The ice algae were responsible for almost half of the primary production in the Central Arctic Basin. The paper is published in the journal Science.
“We were able to demonstrate for the first time that the warming and the associated physical changes in the Central Arctic cause fast reactions in the entire ecosystem down to the deep sea,” said Lead researcher Dr. Antje Boetius, of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research.
When the ice melts, the algae sink rapidly to the bottom of the sea at a depth of several thousands of meters. Deep sea animals such as sea cucumbers and brittle stars feed on the algae, and bacteria metabolise what’s left, consuming the oxygen in the sea bed.
The findings illustrate a short-term reaction of the deep sea ecosystem to changes in sea ice cover, according to the researchers from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research and from the Max Planck Institute for Marine Microbiology in Bremen.
Boetius said the Arctic ecosystems are changing faster than expected.
“The seabed at a depth of more than 400 meters was littered with clumps of ice algae which had attracted lots of sea cucumbers and brittle stars,” Boetius said. “It has been known for some time that diatoms of the type Melosira arctica can form long chains under the ice. However, such a massive occurrence has so far only been described for coastal regions and old, thick sea ice,” she said.
The research was done late in the summer of 2012 during the sea ice minimum from aboard the research icebreaker Polarstern. The scientists gathered samples and made visual observations with a variety of instruments and remote-operated vehicles.
Large algal deposits covered up to ten percent of the seabed, and the scientists were able to measure the bacterial and faunal oxygen consumption directly in the deep sea using micro-sensors.
Life was thriving under the algae cover: Bacteria had started to decompose the algae as evident from a greatly reduced oxygen content in the sediment. By contrast, the sea bed in the adjacent algae-free areas was aerated down to a depth of 80 centimeters and had virtually no algal residues.
Normally, the small phytoplankton cells sink only very slowly through the water column and are largely consumed near the ocean surface layer. By contrast, the long chains of algae formed by Melosira arctica are heavy and can quickly sink to the bottom of the sea. In this way, they exported more than 85 per cent of the carbon fixed by primary production from the water surface to the deep sea.
The researchers suppose that the algae had grown recently because they found only one-year old ice in the Central Arctic, and because the algae extracted from the guts of sea cucumbers were still able to photosynthesise upon return to the ship’s laboratory. The good nutritional state of the sea cucumbers was also evidence of the massive food supply.
The animals were larger than normal and with highly developed reproductive organs – an indication that they had been eating abundantly for some two months, according to zoologist Dr. Antonina Rogacheva of the P.P. Shirshov Institute of Oceanology.
The sea ice physicists on board investigated why ice algae are able to thrive beneath the thinning Arctic sea ice, and how they may also lose their habitat quickly due to the increasing ice melt. They determined the ice thickness with an electromagnetic probe dragged by a helicopter and by drilling the ice.
“At the end of the summer we still found a lot of ice algae remains, and could quantify them by using an under-ice ROV. The increasing cover by melt ponds permits more light to permeate the ice, and makes the algae grow faster,” said Dr. Marcel Nicolaus, of the Alfred Wegener Institute. Since the ice has become so much thinner in recent years, and the Arctic so much warmer, the ice algae will melt out more quickly from the ice and sink, he explained.
The deep sea has so far been seen as a relatively inert system affected by global warming only with a considerable temporal delay. The fact that microbial decomposition processes fueled by the algal deposits can generate anoxic spots in the deep sea floor within one season alarmed the researcher.
“We do not know yet whether we have observed a one-time phenomenon or whether this high algal export will continue in the coming years,” Boetius said. “We still understand far too little about the function of the Arctic ecosystem and its biodiversity and productivity, to be able to estimate the consequences of the rapid sea-ice decline.“