As drivers of biogeochemical cycles and nutrient recycling, such as carbon turnover, the microbial community is essential in sustaining functioning ecosystems. Together with the metazoan community, the microbial community constitute the majority of all life in the benthos. Environmental change in biotic and abiotic factors may influence the dynamics of these communities, for example through a sorting or driving effect on the community structure through assembly processes. Environmental change, e.g. change in dissolved oxygen concentration, salinity and temperature, can directly or indirectly affect community composition. How, in what way, and to what extent, benthic bacterial and meiofaunal community composition in the eutrophied, brackish benthic environments, in the Baltic Sea sub-basin the Baltic Proper, respond to environmental change is understudied, both at local and seascape scale. This thesis aimed to study and understand the effects of environmental variation on the diversity and biogeographic patterns of Baltic Sea sediment bacterial and meiofaunal communities. A further aim was to understand the links between the different community levels by studying the interaction between meiofaunal- and macrofaunal communities in relation to environmental variation. Community diversity was analysed along a latitudinal transect of national environmental monitoring stations in the Baltic Proper using a framework of metapopulation and metacommunity theory. The analyses were based on environmental genomics, with high-throughput sequencing, bioinformatics

and statistics. The total community genome was analysed using phylogenetic marker gene fragments as a proxy for taxonomic diversity, to investigate diversity, community structure and dynamics. Salinity and oxygen were found to be the main abiotic environmental drivers of benthic community composition and alpha- and beta-diversity patterns. Furthermore, macrofauna-meiofauna interactions were significantly more complex in higher salinity environments. Results also showed that both enhanced environmental gradients and dispersal following a major inflow of saline and oxygenated water from the Atlantic Ocean, influenced the composition of sediment bacterial communities at the seascape scale of the Baltic Sea, as shown by a reduced beta-diversity and increased alpha-diversity, and the development of a significant distance-decay of community similarity. This study also identified strong metapopulation dynamics of the benthic sediment bacterial communities with many satellite and a few core taxa. The outcomes from this study contribute to the understanding of how environmental variation and environmental change relate to changes in Baltic Sea benthic community diversity and composition, and important factors and processes governing community dynamics.

The Baltic Sea is heavily affected by eutrophication caused by nutrient overload, climate, and infrequency in major inflow events, resulting in widespread areas of oxygen depleted waters and sediments. A Major Baltic Inflow event (MBI) brings saline and oxygenated Atlantic Ocean water into the Baltic Sea, as occurred in 2014. Using a theoretical framework based on metapopulation and metacommunity theory we predicted a transition of the sediment bacterial community after the MBI, from a more heterogeneous community pattern driven by local colonisation-extinction dynamics towards a more pronounced environmental gradient but with reduced beta-diversities. Community diversity patterns before and after the MBI were investigated by 16S rRNA gene sequencing of samples from 42 Baltic Sea environmental monitoring stations. Results showed strong metapopulation dynamics with many satellite and few core taxa. NMDS-ordination showed distinct geographical clustering. After the MBI, alpha-diversity increased, beta-diversity decreased and a significant distance-decay relationship developed. Changes in community composition correlated significantly with changes in oxygen and salinity from 2010 to 2015. Our results indicate strong metapopulation and metacommunity structuring of sediment bacterial diversity and composition in the Baltic Sea and how movements of large-scale water bodies affect bacterial communities through changes in large-scale environmental gradients and dispersal patterns.

Coastal benthic biodiversity is under increased pressure from climate change, eutrophication, hypoxia, and changes in salinity due to increase in river runoff. The Baltic Sea is a large brackish system characterized by steep environmental gradients that experiences all of the mentioned stressors. As such it provides an ideal model system for studying the impact of on-going and future climate change on biodiversity and function of benthic ecosystems. Meiofauna (animals < 1 mm) are abundant in sediment and are still largely unexplored even though they are known to regulate organic matter degradation and nutrient cycling. In this study, benthic meiofaunal community structure was analysed along a salinity gradient in the Baltic Sea proper using high-throughput sequencing. Our results demonstrate that areas with higher salinity have a higher biodiversity, and salinity is likely the main driver influencing meiofauna diversity and community composition. Furthermore, in the more diverse and saline environments a larger amount of nematode genera classified as predators prevailed, and meiofauna-macrofauna associations were more prominent. These findings show that in the Baltic Sea, a decrease in salinity resulting from accelerated climate change will likely lead to decreased benthic biodiversity, and cause profound changes in benthic communities, with potential consequences for ecosystem stability, functions and services.

The social environment of animals, particularly in the early stages of life, can have great impact on species-specific and sex-specific behaviours. These changes can be irreversible and continue during the entire life. In the present study we asked the question whether the social environment of male Endler's guppies, Poecilia wingei, housed in an all-male community could affect their preference response to female or male odour cues in a flow through Y-maze. After 30 days in an all-male group males were tested for their preference-avoidance responses to conspecific odours. The males were attracted to male-scented water but not to water scented by females. In simultaneous choice between male and female odours they demonstrated no significant preference. The males were attracted to male-scented water after they were kept for 48 h or 12 days with females. After the Y-maze tests the males' were placed with two females and their courting behaviour were recorded. The males showed low frequencies of reproductive behaviours. In the all-male group the males had been courting each other. The results show that the social environment influence sexual odour preference and courting behaviour in guppy males.