What does the seafloor sound like ? First exploration of a softbottom underwater soundscape in the Baltic sea along a hypoxia gradient.

Benthic animals, such as mussels, crabs and worms, have major effects on the ecology, function and services of marine ecosystems. With their overwhelming biodiversity, they create diverse living habitats for other marine organisms, occupy a key link in marine foodwebs and participate in biogeochemical flows. However, one of the great scientific challenges is studying the benthic organisms and their functioning in the ecosystem, i.e. what the animals do on the seafloor. In the Baltic sea, benthic organisms and also aquatic insects are the food of fish species with high economic stakes. The Baltic Sea is now seeing large areas die of hypoxia and little is known about how this phenomenon works and its impact on the inhabitants of the mud in terms of behavior. Knowledge is needed on these ecosystems. Traditional methods (cores, dredges, photography and video) enable identification of for example species and number of individuals but struggles with providing information on the behavior of organisms in a natural setting and in response to human activities. Additionally, they are not cost-effective across large areas and long periods. An innovative and effective tool that have proven to overcome these drawbacks is ecoacoustics or soundscape ecology. By using underwater hydrophones and data loggers, it is possible to passively and non-intrusively record and monitor marine organisms in their natural surroundings.

Through a field work (14 measurement station of 12 to 36h each) exploring muddy habitats in the Baltic Sea (Finnish Aland Island), we demonstrate that the soundscape of these low(/no)-light habitats contains benthic transient sounds but also fish sounds, insects sounds and geophonic sounds (rain, waves, bubbles). The abundance and diversity of these sounds vary with the distance to the coast, the depth of the deployment and the hypoxic state of the measurement site (3 to 6mg/L).

We report on the abilities of passive acoustic monitoring :

• to enhance information on target fish species’ reproduction behavior living in muddy habitats
• to quantify the benthic and fish (and insect) biodiversity through acoustic indicators in the light of parameters such as hypoxia