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  • Halonen, Viivi (2021)
    During the last century, a decline in the canopy-forming foundation species Fucus vesiculosus has been observed in the Baltic Sea. The widely studied typical form of F. vesiculosus, that lives anchored to hard substrata, is at risk of further declines in the following century due to eutrophication and changes in water temperature and salinity. Fucus vesiculosus also exists in the Baltic Sea as a less common free-living form, which lives deposited in sheltered and shallow bays. This free-living form has been left understudied and little is known about their role in the ecosystem or the potential consequences of its disappearance. However, their occurrence may be equally or more under threat in the event of the aforementioned environmental changes. Additionally, it is currently unknown if mats of F. vesiculosus cause anoxia in the sediment below. This thesis will investigate the macroepifaunal and macroinfaunal communities associated to the presence of free-living F. vesiculosus across different sites in both Finland and Sweden. We will also estimate if F. vesiculosus causes anoxia. For this study, replicate frames of F. vesiculosus, including all vegetation and epifaunal community, were collected using mesh bags. Infaunal samples were randomly collected using benthic cores, both under the mat of F. vesiculosus and the adjacent bare soft bottom. All macroinvertebrates were identified to the lowest possible taxa, counted and weighed. Morphological measurements of F. vesiculosus thalli, such as length of thallus and wet weight, were recorded for every frame. Our results showed that the presence of free-living F. vesiculosus has a consistent effect across the two study locations. We found that increasing wet weight of F. vesiculosus significantly increased the abundance and biomass of the macroepifauna. The highest infaunal animal abundance and biomass were found in the bare sediment with high occurrence of opportunistic taxa. However, we found potential evidence to suggest that the presence of F. vesiculosus mats does not cause anoxia in the sediment. This study provides a much-needed first look into the macrofaunal communities associated to the free-living Fucus vesiculosus. Our study demonstrated that free-living F. vesiculosus is a potential foundation species in shallow, sheltered bays of the Baltic Sea by increasing the number of present taxa compared to adjacent bare sediment. Higher F. vesiculosus biomass directly increased the abundance and biomass of the macroepifaunal community, and the presence of free-living Fucus vesiculosus was not found to have significant negative effects on the associated macroinfaunal community.