Effects of a 70-year old artificial offshore structure on oxygen concentration and macrobenthos in the Gulf of Gdańsk (Baltic Sea)

Abstract Artificial structures with dense benthic assemblages on their surfaces are likely to affect the adjacent soft bottom and the communities that live there. In this study, we investigated environmental and benthic parameters in the vicinity of a 70-year old offshore concrete structure in the Gulf of Gdansk, in the southern part of the Baltic Sea. Samples were collected at three distances from the structure in autumn of 2015, 2016 and 2017. Each year, temperature and dissolved oxygen loggers were deployed 4–5 months prior to the collection of benthic samples. The obtained results revealed a significant impact of the structure on several parameters. The average oxygen concentration at 1 m distance was 0.6 mg L−1 lower than at 7 m and 50 m distance from the structure. We registered hypoxic events that lasted up to 4 days, with the oxygen concentration dropping to 0.57 mg L−1, and this is the first report on hypoxia in the coastal zone (depth