Microbenthos under Pressure : Impacts of human activities on bacteria and meiofauna communities in Baltic soft sediments

Abstract: The marine benthic zone is one of the largest habitats on the planet, harbouring a large diversity of life that underpin many important ecological processes. However, these habitats have been under growing stress from human activities, profoundly altering their living communities. How microbial communities respond to various anthropogenic disturbances has been a central topic in ecology, but the response of meiofauna (benthic microeukaryotes, 40 µm ≥ 1 mm in size) and their use as potential bioindicators in a whole-community approach, has received less attention. The Baltic Sea has been of particular interest in studying how ecological processes are affected due to its history and continuous exposure to various anthropogenic stresses. In this thesis, I explored the effects of chemical contamination (Study I), recreational boating activity (Study II), bottom trawling fishing (Study III), and hypoxia (Study IV) on the Baltic microbenthos in terms of structure and diversity using DNA-inferred community analysis in the context of controlled experiments (Studies I and IV) and field studies (Studies II and III). Study I showed a clear increase in contaminated sediments in PAH degradation, relative abundance of Pseudomonas amongst bacteria, and nematodes amongst meiofauna. Study IV showed that high meiofauna abundance, and its bioturbation, altered sediment oxygen penetration and sulphide oxidation in hypoxic conditions with clear effects on bacterial community structure. Studies II and III showed that select bacteria and meiofauna taxa were favoured in areas with high levels of disturbance. In Study II, the ostracod family Cypridoidea was found in higher abundance in bays with high recreational boating activity (marinas) whilst Cytheruridae was significantly lower. Amongst bacteria, Burkholderiaceae showed to be favoured in active marinas and served as an indicator of chemical contamination. In Study III, highly trawled areas appeared to favour Rhabdocoels flatworms and were accompanied by a decrease in Rhizobiales amongst bacteria, suggesting improved feeding opportunities for motile flatworms, and a negative pressure on substrate dependent bacteria. However, the exposure to the open sea was a major factor in structuring both meiofauna and bacterial communities in shallow bays (Study II) and the impact of trawling (Study III) was more pronounced between areas that were geographically closer. This highlights the importance of abiotic and location specific co-factors that drive microbial and meiofauna assemblages and stress the impact of survey design and sampling locations. Additionally, a common trend throughout my work emphasised the potential of select microbial taxa as bioindicators. For example, the bacteria families Pseudomonadaceae and Burkholderiaceae, and the nematode genera Sabatieria and Leptolaimus could be used as bacterial and meiofauna bioindicators of chemical contamination. In brief, chemical stress had a significant effect on bacterial community structure, whereas physical disturbances had a more pronounced effect on meiofauna, both in terms of structural changes as well as alpha diversity. As such, combining the study of meiofauna and microbial assemblages could be a useful approach in assessing the effects of a wide range of disturbances in benthic habitats.