India and Jonna both did their work within the large research project Thriving bays and investigated whether boat traffic has any clear influence on the water-clarity in the shallow bays of the Baltic Sea.

India heard about the project through a professor at her own university. She contacted the researchers and became increasingly interested.

– It sounded really exciting to discover Sweden and explore the Baltic Sea, says India. And I was not disappointed. The Stockholm archipelago is beautiful, I had never seen anything like that before. I loved spending so much time outdoors and living in a remote field station. It was also really fun to get to know Jonna and our supervisors Jocke and Sofia, and to learn about their Swedish culture.

A piece of the puzzle for healthy bays

The ‘Thriving bays’ project aims to investigate management measures to reverse the negative development seen in many shallow bays in the Baltic Sea. In many areas, clear, fish-rich waters with meadows of underwater plants and numerous small animals and fish fry have turned into a murky algae soup without predatory fish.

– There are usually several reasons underlying this sad transformation, says India, but the role of boat traffic has never been properly investigated in the area. My study was about finding out to what extent boats can affect the health of shallow bays, and which criteria make the bays more or less sensitive to boat traffic.

Master's student India Findji on the way to field work. Photo: Private

The large Swedish archipelago provides excellent conditions for boating, and Sweden is one of the countries with the highest number of boats per inhabitant. In wave-protected bays along the coast, boats can moor safely at jetties and in natural harbors. Many bays are also used extensively by anglers.

– Unfortunately, we only have limited knowledge on how boating affects the environment in shallow bays, especially the magnitude in effect in relation to other pressures, says India's supervisor Joakim Hansen, a researcher at Stockholm University Baltic Sea Centre. Therefore, we do not know if boating needs to be restricted – and if so how – in order to improve the environmental conditions in the bays.

An exciting project

During the spring and summer, India has therefore carefully studied a number of different bays in the Stockholm archipelago trying to understand how much turbidity is caused by boats and for how long the resuspended sediment stays in the water.

– It was exciting to be involved from the beginning in a project like this, says India. I especially liked to work on a current and local problem, and using a relatively simple approach to suggest solutions.
Bays in the Stockholm archipelago are typically dominated by soft sediment bottoms and often harbor a high diversity of aquatic vegetation of both freshwater and marine origin.

– In my study, we performed a field-experiment and collected environmental data in fifteen different shallow bays to assess levels of sediment resuspension induced by leisure boat activity. The bays we selected ranged from very isolated with small exposure to wind and waves, to relatively open bays with higher exposure. The idea behind the experiment was to link factors such as the bays’ morphometry, sediment characteristics and vegetation cover to changes in sensitivity to boat-induced sediment resuspension and elevated turbidity levels.

Field experiments in fifteen bays

The experiment designed for this study consisted of generating a standardized boat activity as a disturbance, and assessing variations in water turbidity in samples taken prior to and following the disturbance.

– The driving behavior we used as disturbance for the experiments was a rapid acceleration from neutral to about 3 000 rpm, with a five meters long leisure boat powered by a 50 horse-power engine, says India. It simulated a start from stationary position against a moderate headwind. We only did this in favorable weather conditions, to avoid wind-driven resuspension to interfere with our results. We did this experiment at two different depths, 1 meter and 1.5 meter, in all the bays.

First, India snorkelled out to a buoy and took a water sample to measure turbidity in the water before the start of the experiment. A boat would then accelerate at the buoy, generating the disturbance. Immediately after the boat’s passage, another water sample was taken at the centre of the resuspended sediment ‘cloud’ formed due to the boat acceleration. The decrease in turbidity formed by the resuspended particles was then measured over a longer period of time. In some of the water samples, the concentration of nitrogen and phosphorus was also measured to investigate whether nutrients were released with the sediment resuspension process.

– To investigate the role of the vegetation in the boat-induced sediment resuspension process, the experiment was performed both in early spring, when vegetation biomass is relatively low, and in late summer, when the vegetation biomass peaks, India tells. There was also a gradient in vegetation cover and composition among the fifteen bays, allowing for analysis of the potential effect of vegetation on boat-induced turbidity within season too.

Heavily turbid water sample collected by India in snorkelling gear. Photo: Joakim Hansen

Useful results

Now, when India has compiled all the values and made a number of statistical calculations and tests, the results are ready.

– Our results show that the boat acceleration caused large increases in turbidity in almost all the sampled bays, both at 1 meter and 1.5 meters depth. The vegetation cover did not have an obvious protecting effect. Even smaller leisure boats, which are commonly spotted in the area, can lead to high levels of sediment resuspension in shallow waters. This result is especially relevant when considering that many boats moor near shores in bays, and therefore need to accelerate at shallow depths to return to deeper waters.

India is very satisfied with her master's project.

– With the knowledge I have helped to develop, recommendations can now be made on a scientific basis: Avoid driving in shallow water, especially in very sheltered bays where the bottom is muddy. Drive slowly and carefully if you have to drive where it is shallower than two meters. Above all, avoid rapid accelerations unless it is really deep.

– All in all, this feels really good, India proudly concludes.

Master's presentation

India Findji will present her master's project "Experimental assessment of boat-induced sediment resuspension in a morphometric gradient of Baltic Sea bays." at the University of Groningen this fall.

Text: Annika Tidlund