Dramatic changes in the Arctic ocean floor

Dramatic changes in the Arctic ocean floor

Top image: Kongsfjorden. Photo: Eva Therese Jenssen/UNIS.

A new study based on a 30-year period show that the ocean floor communities of both Kongsfjorden and Smeerenburgfjorden have changed dramatically.  Existing organisms are replaced by warmth- and light-loving species. – A biological tipping point, according to the authors.

23 August 2012
Text: Eva Therese Jenssen / UNIS

In a new study, performed by researchers from the University in Tromsø, UNIS, Akvaplan-Niva and University of Cambridge, the aim has been to investigate changes in the rocky-bottom community structure over 30 years of two Svalbard fjords during a period of rapid warming.

The researchers have been utilizing photographic surveys of certain bottom areas in Kongsfjorden and Smeerenburgfjorden during this three decade period with considerable sea ice decrease and temperature increase in the Arctic.

The findings have been published in the prestigious Proceedings of the National Academy of Sciences (PNAS). The article’s lead author is Susanne Kortsch, and much of the analysis was done at UNIS while Kortsch was a master student. Kortsch is now a PhD student at the University of Tromsø.

Rapid warming and regime shift
Over the past 30 years, the global climate has become warmer, with an increase in global surface temperatures of about 0.2 °C per decade. In the Arctic, the temperatures have increased twice the global average. In addition, the sea ice cover at the end of the summer has decreased by 30%.

Until now there have been few long-term studies looking at the marine ecosystem responses to these climatic changes.

In the period 1980 to 2010 the length of the ice-free season for Western Svalbard increased by 3.3 days per year, and the annual average sea-surface temperature increased by 0.5 °C. For the first 12–14 years of the three decade long time series the two benthic communities in Kongsfjorden and Smeerenburgfjorden remained relatively stable, but thereafter changed quite rapidly.

A major shift in the rocky-bottom communities was the abrupt and persistent in macroalgal (brown algae) cover in both fjords. In Kongsfjorden macroalgal cover was scant, on average 8%, until 1995. But the year after it increased to 80%! After this, the brown algae cover fluctuated around 40%.

In Smeerenburgfjorden, which is further north, the shift in brown alagae coverage happened in 2000, five years after the change in Kongsfjorden. Here, the macroalgal (both brown and red algae) increase from average 3–26%. The researchers say this is indicative of an abrupt ecological regime shift in both fjords.

In Kongsfjorden, initially characterized by calcareous algae, sea urchins and sea anemones, the brown algae established itself quite rapidly and led to an 80% loss in the originally dominant sea anemones. The macroalal expansion led to an increased diversity in the ocean floor community.

Ocean bottom floor in Kongfjorden and Smeerenburgfjorden

Photographs of the rocky bottom communities (15 m of depth) from two Arctic fjords: Kongsfjorden (78°58.60′N, 11°30.10′E) and Smeerenburgfjorden (79°41.33′N, 11°04.00′E). The photographs are representative for the communities before (1984) and after (2006) the macroalgal regime shift. The macroalgal coverage is encircled by a white line. Image: Kortsch et. al. 2012.

Tipping point
The researchers suggest that these Arctic benthic communities have crossed a critical threshold, a so-called tipping point, from the original state in 1980 with little macroalgae to a new state with relatively high density of macroalgae.

This shift has resulted in a decline of the original benthic community species and opened up for the introduction of species that are reliant of warmer temperatures and more light, which is consistent with warmer sea-surface temperatures and longer ice-free seasons.

The structural change will lead the primary production sources to shift from encrusting calcareous algae to erect, habitat-forming macroalgae, thus facilitating species dependent on physical support or refuge, and will promote increased biodiversity, according to the authors.

Reference
Susanne Kortsch, Raul Primicerio, Frank Beuchel, Paul E. Renaud, João Rodrigues, Ole Jørgen Lønne and Bjørn Gulliksen: Climate-driven regime shifts in Arctic marine benthos. PNAS 2012 : 1207509109v1-201207509. Link to the article.

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