The Baltic Sea is one of the busiest seas in the world. A new study has now revealed a previously underestimated consequence of this shipping traffic. In it, researchers examined how the turbulence caused by ship’s propellers and wake affects the water column and seabed of the Baltic Sea. This revealed: In the shallow areas of the Baltic Sea, the vortices of the ships reach to the bottom and cause sand displacement and erosion. In addition, the turbulence generated by large ships destroys the stratification of seawater and thus changes the exchange of substances in the water column. Both effects suggest that shipping traffic has a greater impact on the ecosystems and material cycles of the Baltic Sea than previously assumed.
The Baltic Sea is one of the most intensively used marine areas in the world. Around 85 million people live in its catchment area and a significant part of the freight and passenger traffic between the neighboring countries is carried out by sea. At the same time, the Baltic Sea is almost completely surrounded by land and is particularly shallow: “Around 26 percent of the Baltic Sea has a water depth of less than 20 meters. In the southwest of the Baltic Sea, such shallow water areas even make up 56 percent,” report Jacob Geersen from the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) and his colleagues. “As a result, many of the human-caused pressures affect the entire water column, from the boundary of the atmosphere to the ocean floor and below.” The effects of heavy shipping traffic on the marine environment have so far been examined primarily with regard to ship emissions, noise and the risk of accidents.
Erosion on the seabed
But another factor was missing so far: “With our study we wanted to investigate the question of whether and how ships also act as a mechanical disruptive factor that extends to the seabed,” says Geersen. Larger ships in particular stir up the water due to their ship’s propellers and the wake waves generated during the journey. To find out what effects this has, Geersen and his team focused their study on the Bay of Kiel. “It is consistently shallower than 20 meters and is characterized by intensive commercial shipping traffic,” said the team. On average, 90 freighters and passenger ships pass through this sea area every day. For their study, the researchers measured the seabed from a research vessel down to centimeter accuracy, recording even the smallest depressions and elevations. They compared the resulting map with previous surveys of the Bay of Kiel by the Federal Maritime and Hydrographic Agency in 2014.
The mapping revealed thousands of depressions in the seabed of the Bay of Kiel, around ten meters in size and up to one meter deep. These elliptical depressions were particularly noticeable around larger stones and occurred predominantly along the main shipping routes. There they were aligned in a strikingly uniform manner, as the team reports. Between the depressions there were also sand dune-like sediment accumulations and ridges on the sea floor. These structures were primarily created by the lateral displacement of fine-grained sediment, as Geersen and his colleagues report. Their comparison of the current mapping with that of 2014 showed that these structures re-form within a few years to decades and sometimes disappear again. “This substantial erosion of the seabed over the course of ten years is clearly due to ship traffic,” the researchers state. Accordingly, the shear stresses generated by the ship’s propellers are often sufficient to set the sediment on the Baltic Sea floor in motion. In some cases, the fine-grained sand is carried to the side by up to 60 meters.
…and disturbed stratification of the water column
But even above the seabed, ship propellers and wakes leave stronger traces than previously assumed. This was shown by temperature and salinity measurements in the water column and acoustic studies in the area around shipping routes and ships. To do this, the researchers crossed the wake behind three freighters and two ferries with their ship and an echo sounder lowered into the water. These analyzes showed that the turbulence of the sea water behind a large ship reaches deeper than expected. Geersen and his colleagues discovered that they cause turbulence and air bubbles at a depth of twelve to 16 meters. In the majority of documented ship passages, these turbulences reached very close to the bottom of the Baltic Sea. “The waves can break through the summer stratification of seawater and mix previously separated water masses,” report the researchers. This influences the exchange of oxygen, nutrients and dissolved trace elements between surface and bottom water in the Baltic Sea.
According to the research team, the observed impacts of shipping traffic could have far-reaching consequences for the marine ecosystems of the Baltic Sea. “Although the environmental consequences of this anthropogenic pollution have not yet been quantified, our results leave little doubt that they influence marine ecosystems and material budgets across the Baltic Sea,” write Geersen and his colleagues. This repeated disturbance could have significant effects, especially in shallow sea areas. “The results make it clear that shipping traffic must be viewed as an active designer of marine habitats,” emphasizes Geersen. “Extrapolated to the entire Baltic Sea, we estimate that around 7.5 percent of the sea area could be affected by ship-induced sediment changes.” But even if only part of the shallow and heavily trafficked areas of the Baltic Sea were affected, this would make a relevant contribution to large-scale sediment and material flows.
The scientists therefore raise the question of whether adjustments to shipping traffic might make sense and be necessary.
“Adjustments to fairways, speed controls or alternative routes could, in the long term, help to relieve pressure on particularly sensitive seabed areas,” says the team. “Whether such measures are effective should be accompanied by further research.”
Source: Jacob Geersen (Leibniz Institute for Baltic Sea Research Warnemünde) et al., Nature Communications, doi: 10.1038/s41467-026-68875-6