the baltic sea is choking, and so are many of the creatures that depend on it. 

oxygen levels of the baltic are so depleted that fish and plant species can’t survive in its waters, making it one of the most polluted seas in the world.

manure and fertilizers from its coastal countries account for large amounts of nitrogen and phosphorus in baltic waters. this induces over-enriched seawater to grow excessive amounts of algae, forming “algal blooms.” as the algae dies and sinks to the seafloor, bacteria use up oxygen as they decompose the material, causing “dead zones” – oxygen-depleted areas, in which fish and plant populations are decimated.

the baltic’s waters are among many gasping for oxygen as a result of this process called “eutrophication.” over the atlantic, u.s. scientists are grappling with similarly-polluted waterways in the chesapeake bay, gulf of mexico and the great lakes. earlier this month, marylanders reported “cloudy, streaky-looking water,” alerting algal blooms in six maryland rivers.

“it’s very deadly, it’s very dangerous,” dr. tara scully, biology professor and head of an oyster research lab at george washington university, said. “it causes fish kills, fish just start washing up on shore because this process has occurred.”

as with many environmental issues, there’s no single culprit, but european countries surrounding the sea are acknowledging responsibility by experimenting with natural water filters and trying to cut down on harmful sea inputs. 

the baltic blue growth (bbg) project, which ended in 2019, connected a network of research facilities in estonia, poland, sweden, latvia and denmark who have been trialing blue mussel farms to fight algal blooms.

blue mussels are filter feeders, meaning they take up nutrients in their environment and naturally clean the waters. once harvested, the nutrients they contain are recycled for food production or fish and poultry feed, which in turn benefits farmers.

jonne kotta, director of estonia marine institute and bbg partner, said that although countries surrounding the baltic sea have lowered their nutrient runoff by 15 to 30 percent since 1995, it is considered that nearly the entire baltic sea was still being affected by eutrophication as late as 2007-2011. 

“the amount of nutrients stored in the baltic sea ecosystem are so huge that even if all land based activities which cause eutrophication in the baltic stopped tomorrow, it would still take generations for the baltic to recover to an acceptable state,” kotta explained.

environmental policy is starting to show that reducing nutrient inputs hasn’t changed the already nutrient-excessive waters. this is an especially salient concern for a body of water with such limited water exchange – water exchange with the north sea takes about 30 years.

“while land-based pollution control measures can and will continue to make an important contribution to solving the baltic sea eutrophication problem, it is a mistake to think they are enough to solve the problem,” kotta said.

dr. scully studies another bivalve – the eastern oyster – as a solution to the chesapeake’s nutrient pollution and algal blooms. the chesapeake oyster alliance, a coalition of organizations aiming to restore the health of the bay, even launched an initiative to add 10 billion oysters to the bay by 2025 to combat algal blooms eutrophication.

the bivalves’ kryptonite 

but nils hedberg, marine ecotoxicologist at stockholm university, believes bivalves are not necessarily the answer for baltic-like environments. 

although he said the blue mussel clean-up method is attractive because it’s pitched as a low-cost solution with potential to stimulate new farming markets, hedberg co-authored a report highlighting the limitations of relying heavily on bivalves to clean the baltic.

it’s unclear if the baltic’s waters can support the large-scale farms that would be required to make a significant difference to the baltic’s nutrient content. examples from the united states, new zealand, sweden, and the netherlands have shown that bivalve cultivation alters the ecosystem in a negative way and can even increase nutrient levels in affected waters.

“the risks will increase with the scale and intensity of the farm, and it is very likely that we need quite large farms to compensate for the slow growth and the high production costs,” he explained. “small farms will not cause any significant problems but will probably not solve any either.”

there is still very little research about the environmental impact of large mussel farms. in theory, hedberg explained mussels will eat most of the plankton in large farming areas, which makes it harder for fish to thrive there. 

he is instead a proponent of external methods which aim to decrease nutrients by changing land conditions so less nutrients reach the sea. 

external methods include agricultural nutrient recycling – farming methods that reduce the amount of nutrient-rich fertilizers. current practices, like intensive animal farming and overusing fertilizers, cause extra nutrients to leach into the soil and, through groundwater and streams, find their way into the baltic.

meanwhile, the baltic’s brackish waters – water with salinity levels between sea water and fresh water – limit bbg’s circular economy plan. blue mussels are native to high-salinity waters: this is true of western baltic waters, but the eastern waters are either brackish or have a lower salinity, which causes the mussels to grow more slowly and be smaller overall.

smaller mussels are harder to sell on the seafood market, making it a challenge for farmers in the east to commercially benefit from a push for mussel cultivation.

kotta said bbg still found ways to cultivate larger volumes of mussels in more areas of the batlic than originally thought possible, through developing farming technologies.for example, kotta said one of the new pieces of equipment was the “fuzzy rope,” which allows more surface area for mussels to grow.

next steps

kotta is now focusing on kicking off a mussel farm off estonia’s coastline, one of the most distant coastlines from the open sea.

hedberg explained one of the biggest issues is the knowledge gap in understanding the baltic’s seascape, like ecosystem habitats and currents. with the little research there is now, it’s hard to predict at the rate negative aspects of mussel cultivation will occur.

it is still unclear if the baltic’s waters can support the large-scale farms that the project requires.

“i think it is safe to say that we researchers never agree on all the details, and in this complex case, with a lot of knowledge gaps and large scale processes, we have a lot to debate,” said hedberg.

“perhaps it is possible to find a perfect balance.”