New major study: Baltic Sea food webs remain out of balance despite decades of measures

Responsible for the study is a large international research team from the Baltic region, who have mapped how food webs – the network of who eats whom – have changed. The study encompasses seven Baltic sub-basins with time-series between 26-46 years.

– This is the first time food web status has been assessed across all sub-basins in the Baltic Sea using the same method, and the results are worrying, says Carolyn Faithfull, researcher at the Swedish University of Agricultural Sciences (SLU) and lead author of the study published in the ICES Journal of Marine Science.

In all seven areas studied, the balance between different groups of organisms has been negatively affected by human activity. Under EU environmental legislation (Marine strategy framework directive), this indicates poor food web status.

Fishing, eutrophication and a warmer climate

The two biggest drivers of change in the food web are overfishing and nutrient pollution from land. Excess nutrients fuel algal growth, which sinks and degrades on the seabed, consuming oxygen and creating low-oxygen zones where few species can survive. 

At the same time industrial fishing has had a large effect on fish populations. After the fishing industry depleted predatory fish, such as cod, fishing pressure shifted to herring and sprat. Now large declines in herring are becoming apparent. When a historically abundant fish species like herring massively declines, this sets off chain reactions throughout the food web.

– Climate change is making these effects worse. Rising water temperatures are amplifying the effects of both overfishing and eutrophication, as warmer water holds less oxygen and fish reproduction is affected by warmer winters and changes in prey availability, says Carolyn Faithfull. 

Reducing pressures is not always enough for recovery

Perhaps most troubling is that even where pressures have been reduced the food web has not always recovered. Since the 1980s phosphorus inputs have fallen by around 50%. Despite this, oxygen depleted areas of the seabed continue to increase in size and concentrations of available phosphorus are continuing to increase in some sub-basins.

Cod shows a similar pattern. Fishing mortality has decreased dramatically in both the western and central Baltic Sea, but cod stocks have not recovered. The food web is still shaped by the lack of predatory deep-water fish such as cod, with a greater dominance of flatfish and plankton-eating fish such as herring and sprat. 

– Even though some pressures have been reduced, the effects on the food web have remained. The ecosystem appears to have shifted to a new state that it can't easily leave, says Carolyn Faithfull. 

Dealing with climate change demands global measures. But the study also describes overfishing and high nutrient concentrations as important pressures in the Baltic – both which are possible to remediate through regional management. 

– There are many steps we can take to improve the situation here and now. But it requires strong, coordinated action across the Baltic region. We need to deal simultaneously with multiple pressures that are affecting the ecosystem, says Carolyn Faithfull

Read the scientific article “Systemic fooa web reorganization due to multiple pressures in seven Baltic Sea sub-basins” in ICES Journal of Marine Science