Climate change is having a greater impact on oceans, lakes and rivers across the globe than previously thought. A recent study shows that global temperature rise is contributing to an expansion of dead zones around the world. Elke Bunge reports.
Dead zones are areas in waters where there is too little oxygen for fish or other marine animals to survive. And they are growing steadily. The main culprit is excess runoff of agricultural nutrients, such as nitrogen and phosphorous, which end up in the sea and cause algae to grow. The massive algal blooms sink when they die, where they are then decomposed by bacteria. The bacteria consume oxygen during the degradation process, which results in low-oxygen areas on the seafloor.
Fertilisers and climate change
Scientists have long speculated that fertiliser use isn’t the only reason behind the expansion of dead zones, but that climate change could be exacerbating the problem.
In a recent study from the Smithsonian Tropical Research Institute (STRI) in Panama, the Smithsonian Environmental Research Center in Maryland and the University of Maryland, research team Andrew Altieri and Keryn Gedan investigated the influence of climate change on biological, physical and chemical factors.
They found that a temperature rise of two degrees Celsius will lead to stronger melting of the poles, rising sea levels, but also more wind, storms and precipitation. As river levels rise, a variety of agricultural chemicals will be flushed into the sea. As Altieri and Gedan demonstrate, 94 per cent of the dead zones that they surveyed are located in areas that will be affected by this.
Dead zones double every ten years
Since the early 2000s, scientists have known that dead zones have been doubling in coastal regions since the 1960s, wrote the researchers in their study. “These dead zones have significant consequences for marine biological diversity and the functioning of ecosystems, but also on the way people use them, such as fisheries,” says Altieri, a biologist whose research focuses on how marine organisms interact with the environment.
Co-author Gedan adds: “About 40 per cent of the world’s population live in coastal regions. We live off marine resources. No one wants to see dead fish or rising algal blooms on their doorstep.”
In their study the researchers noted that a temperature rise of 2.4 degrees Celsius would lead to a dramatic increase in dead zones. In 2008 there were already 400 dead zones along the eastern seaboard of the United States and in the Gulf of Mexico, with the Black Sea and Baltic Sea also among the affected regions. Studies show that one-sixth (70,000 km2) of the approximately 412,500 km2 Baltic Sea is affected.
Acidification leads to less oxygen
Their research also shows a correlation between ocean acidification and a loss of oxygen. Together with global warming, ocean acidification is one of the most serious changes to the environment caused by humankind. Their study shows that the survival rate of crustaceans and fish, feeding activity and spawning decrease as pH values fall in acidic oceans.
The immune system of marine animals is also damaged by less oxygen. Mussels, which actually have a ‘control function’ over algal growth, can lose this role in acidified zones. According to Altieri and Gedan, mussel populations will be destroyed and entire ecosystems imbalanced. The study is unambiguous in this regard: Wherever an increase is ocean acidification is observed, the area is at risk of developing a dead zone.