Warmer waters dumb down food webs

Blue-green algae proliferation caused by climate change could drive marine food webs to simple, less productive ecosystems, threatening fisheries worldwide, new research has found.

A study in the journal PLOS demonstrates that warming temperatures trigger marine food web collapse in a laboratory environment.    

Researchers Hadayet Ullah, Ivan Nagelkerken and Damien Fordham of the University of Adelaide, in Australia, show that increased temperatures reduce the vital flow of energy from the primary food producers at the bottom of a food web, such as algae, to intermediate consumers such as herbivores, and on up to predators. 

The researchers created model ecosystems in several 16,000 litre tanks, each containing a number of species from different trophic levels, including algae, sponges, shrimp, and fish. Each tank was then subjected to warming and acidification treatments over a six month period.

Ullah and the team discovered that the collapse in species biomass of the entire food web was driven by the replacement of beneficial turf algae, favoured by herbivores, with blue green algae, known as cyanobacteria, which is not exploited as a nutrient source. The reduced food availability then had negative affects for the upper trophic levels, or predators. 

 “Whilst climate change increased the productivity of plants, this was mainly due to an expansion of cyanobacteria,” says Ullah. 

“This increased primary productivity does not support food webs, however, because these cyanobacteria are largely unpalatable and they are not consumed by herbivores.”

The global climate stressors of warming and acidification are often researched in short term or single-species studies. 

“If we are to adequately forecast the impacts of climate change on ocean food webs and fisheries productivity, we need more complex and realistic approaches, that provide more reliable data for sophisticated food web models,” adds Nagelkerken.