Heatwaves in the Ocean: A Risk to Ecosystems?

An unusually long-lasting warm water bubble -- nicknamed 'The Blob' -- spread across the surface of the Northeast Pacific from winter 2013/2014 to the end of 2015. The warm water bubble at times measured up to 1,600 kilometres in diameter and had water temperatures of more than 3 degrees Celsius above the long-term average. Because warm surface water has a lower density than the cold deep water, the exchange of nutrient-rich deep water with warm surface water was reduced, especially along the west coast of North America. This had far-reaching consequences for marine organisms and ecosystems: the growth of phytoplankton decreased due to the reduced supply of nutrients, and some zooplankton and fish species migrated from the warm and nutrient-poor water to cooler regions. By contrast, researchers found pygmy killer whales in the North Pacific for much longer than usual: this tropical whale species is usually observed 2,500 kilometres further south.

A stronger but shorter heatwave hit Australia's west coast at the turn of the year 2010/2011, with sea temperatures of up to 6 degrees Celsius above normal levels for that time of year. The seabed along the coast of Western Australia is known for its high concentration of brown algae. These marine 'kelp forests' have similar functions as terrestrial forests: they provide habitat and food resource to numerous species; in particular a large number of fish. Australian researchers demonstrated that most of the kelp forest stocks rapidly disappeared during this heatwave. In total, an area of 1,000 square kilometres of kelp forest was lost -- this corresponds to twice the size of Lake Constance. Today, algae stocks haven't recovered yet. Instead, a new ecosystem with tropical fish and seaweeds has developed.

As the world's oceans continue to warm, marine heatwaves are likely to become more frequent and intense. Observations and model simulations also demonstrate that other factors such as ocean acidification and deoxygenation are putting additional stress on marine organisms and ecosystems.

Until recently, climate models were unable to accurately represent the relevant physical and biogeochemical processes to simulate extreme events in the ocean and predict future changes. The uncertainties in future projections, particularly at the regional scale, were simply too large. New model simulations linking the global carbon and oxygen cycle with high-resolution physical processes now enable us to make quantitative predictions about the frequency, strength and spatial distribution of future extreme events in the ocean for the first time. And this is precisely what my scientific research focuses on. But in order to better understand the impact of these extreme events on individual organisms or entire ecosystems and their socioeconomic services, interdisciplinary collaborations are urgently needed. Research on understanding such events is only just beginning.

https://www.sciencedaily.com/releases/2016/09/160919131958.htm