The Baltic Sea’s biomass of bladder wrack (Fucus vesiculosus) may soon decline due to rising sea temperatures and the presence of increased nutrients in the ocean, say a team of marine scientists from Kiel and Rostock. Bladder wrack forests are integral to the ecosystem due to their role as a long-term nutrient store and as a nursery for fish species with economic importance. Their decline would have negative repercussions for our economy and society.
The bladder wrack is responsible for the species-rich ecosystems along the rocky shores of the Baltic Sea. It creates habitats for other species by colonising pebbles and rocks. The algae that grow on it are grazed upon by snails, isopods and amphipods. In addition, other species like crustaceans, mussels and predatory fish thrive amidst the bladder wrack. It is a main producer of organic matter in the Baltic Sea and plays an important role due to its biodiversity and biogeochemical cycles.
However, climate change may lead to the loss of some of these functions.
According to marine biologists from GEOMAR Helmholtz Centre for Ocean Research Kiel and Rostock University – within the German research network BIOACID (Biological Impacts of Ocean Acidification) – the amount of bladder wrack in the Baltic Sea may decline due to rising sea temperatures and entrophication. “It is important to consider the effects of a combination of global and local environmental factors on entire ecosystems,” said Dr Franziska Julie Werner, a post-doctoral researcher in marine geology from GEOMAR. She is the lead author of two studies that have been published in the journals Limnology and Oceanography and Oecologia.
“Our studies also illustrate how important it is to promote a successful eutrophication management in the Baltic Sea – a factor which, unlike rising global temperatures, could be achieved by national commitment,” she added.
The experiments involved twelve experimental tanks (called Kiel benthocosms) that held one and a half cubic metres of water each. Bladder wrack from the Kiel Fjord, smaller species of algae, and grazers like isopods, amphipods and periwinkles were placed inside them. A flow-through system continuously pumped fjord water into the benthocosms, to simulate the fjord environment.
In the first study, the temperature and carbon dioxide concentrations inside half the benthocosms were raised to levels projected for the region’s future as a result of climate change. It lasted a year and was divided into four seasonal phases. The second study focused on the impact of increased nutrient supply under greenhouse conditions of increased temperature and carbon dioxide concentrations.
The results of the first experiment showed that the increased carbon dioxide concentrations had no effect on the bladder wrack over the four seasons. However, the increased temperature brought about a negative effect on the bladder wrack, especially in summer. After six weeks the bladder wrack in the simulated benthocosms had been reduced by half, compared to those kept under present conditions.
The second experiment then confirmed the observations. The biomass of the bladder wrack dropped by 80 percent if nutrients were added into the water (simulating eutrophication). The negative consequences were visible even when the increase of nutrients was moderate. “Smaller species of algae that naturally grow on Fucus benefitted from the additional nutrient supply. But snails, isopods and amphipods were not able to graze these epiphytes because they suffered from the increased temperature. Fucus suffered from both, thermal stress and intensified overgrowth by epiphytes. That’s a double negative effect,” said Dr Werner.
The eutrophication of its coastal waters is one of the Baltic Sea’s oldest environmental problems. The scientists emphasise that European water management directives have not fully realised their vision for a good chemical and ecological status in water management. This is due in part to the slow water exchange in the Baltic Sea, which causes nutrients to circulate within the system for a long time. In addition, run-off from over-fertilised fields and farms still ends up in the ocean.
“Climate change does not only entail changes in temperature, but also changes in precipitation patterns. Increased run-offs from land due to stronger rainfall could carry additional nutrient loads from industrial agriculture into the coastal waters of the Baltic Sea,” said Dr Werner.
Further information: www.geomar.de
Links to the studies: