Microbial communities comprise immense diversity, mediating numerous ecosystem functions such as nitrogen transformation in aquatic ecosystems. Yet, how microbial communities and ecosystem functions respond to changes in environmental conditions at different spatial scales in aquatic habitats is not fully understood. This thesis investigates the effect of changes in environmental conditions at different spatial scales on Baltic Sea microbial diversity and nitrogen transformation, specifically denitrification, with a focus on biodiversity-ecosystem function relationship. Two spatial scale settings were examined. A small spatial scale setting was examined with experimental mesocosms of benthic sediment of different salinity and dissolved oxygen levels and at different spatial arrangements, and an incubation experiment conducted with benthic sediments from two different aquatic habitats enriched with 15N nitrate isotope. Microbial diversity, denitrification gene transcript abundances, and potential denitrification rate were analyzed. At a large spatial scale of the Baltic Sea benthic and coastal seascape, a metadata-based modeling approach was employed to investigate how current environmental conditions and future climatic change scenarios affect the predicted spatial distribution of key taxa of denitrifiers and nitrogen-fixing filamentous cyanobacteria. Results showed that salinity and dissolved oxygen levels significantly influenced denitrification capacity, according to analyses of transcript abundances of nirS and nosZ genes, which was mainly driven by habitat environmental conditions rather than habitat spatial arrangement. Bacterial community composition in experimental mesocosms was significantly affected by salinity. Findings from the small scale mesocosms experiments indicate that salinity can be an important driver of microbial denitrification capacity and affect microbial diversity in studied mesocosms. At large coastal and seascape scales, multi-realm environmental variables from both marine and terrestrial domains had significant impacts on the predicted distribution of the studied microbial taxa. The predicted distribution under different global warming trajectories indicated that rising temperatures in year2050 and 2100 risk reducing the current bio-climatically suitable areas of the studied denitrifiers and filamentous cyanobacteria across the Baltic Sea. The studies with experimental mesocosms and modeling approaches presented in this thesis contribute knowledge and novel predictions on the spatial response of ecosystem function and microbial communities to ongoing and future environmental changes in the Baltic Sea.

Denitrifying microbial communities provide an important ecosystem function in aquatic systems. Yet, knowledge on predictive and modeling of these complex and changing communities is limited. The emergently challenging question of how the geographical distribution of denitrifiers responds to ongoing and future environmental change is not yet fully understood. In our study we use metadata-based correlative niche modeling to analyze the geographical distribution of selected putative denitrifiers in the genus Sphingomonas, Mycoplana, Shewanella, and Alteromonas at different predicted environmental conditions and future climatic scenarios across the Baltic Sea. Using the predictive power of an ensemble modeling approach and eight different machine-learning algorithms, habitat suitability and the distribution of the selected denitrifiers were evaluated using geophysical and bioclimatic variables, benthic conditions, and four Representative Concentration Pathway (RCP) trajectories of future global warming scenarios. All algorithms provided successful prediction capabilities both for variable importance, and for habitat suitability with Area Under the Curve (AUC) values between 0.89 and 1.00. Model findings revealed that salinity and nitrate concentrations significantly explained the variation in distribution of the selected denitrifiers. Rising temperatures of 0.8 to 1.8 °C at future RCP60–2050 trajectories are predicted to diminish or eliminate the bioclimatic suitable habitats for denitrifier distributions across the Baltic Sea. Multi-collated terrestrial and marine environmental variables contributed to the successful prediction of denitrifier distributions within the study area. The correlative niche modeling approach with high AUC values presented in the study allowed for accurate projections of the future distributions of the selected denitrifiers. The modeling approach can be used to improve our understanding of how ongoing and predicted future environmental changes may affect habitat suitability for organisms with denitrification capacity across the Baltic Sea.

Many factors affect the habitat selection for animal species, which in turn may greatly affect their distribution in different ecosystems. Understanding the processes that affect habitat selection is also critical for guiding and managing conservation initiatives. Our study aimed to assess the habitat selection by free-ranging Spiny-tailed lizard (Uromastyx aegyptia) by analyzing a geospatial data connecting its burrow parameters to different habitat characteristics within selected sites in Hail region, Saudi Arabia. We examined evidence and patterns of significant spatial clustering for (366) active burrows by linking their parameters (burrow entrance size, burrow entrance width and burrow entrance height), their reference geographical locations and, two habitat characteristics defined by soil type and vegetation cover. The objective of the analysis was to increase the understanding on the burrows aggregation process in the space and, to describe its possible relation to other spatial habitat configurations. Analysis of distances based on the Nearest Neighbor Index (NNI) and hotspots detection in Nearest neighbor hierarchical clustering (Nnh) suggested twelve (12) spatial clusters located within the study area. In addition, a spatial ordinary least square (OLS) and Poisson regression models revealed significant effects of soil type and vegetation cover on burrow parameters (OLS, p < 0.05; Poisson, p < 0.001), which indicate a strong association between burrows parameters and habitats characteristics. Findings from the study also suggest that other factors such as elevations, highways, and human settlement concentration spots could possibly play a major role in defining burrow spatial aggregation and furthermore have a significant impact on habitat selection.

Denitrification in sediments is a key microbial process that removes excess fixed nitrogen, while dissimilatory nitrate reduction to ammonium (DNRA) converts nitrate to ammonium. Although microorganisms are responsible for essential nitrogen (N) cycling, it is not yet fully understood how these microbially mediated processes respond to toxic hydrophobic organic compounds (HOCs) and metals. In this study, we sampled long-term polluted sediment from the outer harbor of Oskarshamn (Baltic Sea), measured denitrification and DNRA rates, and analyzed taxonomic structure and N-cycling genes of microbial communities using metagenomics. Results showed that denitrification and DNRA rates were within the range of a national reference site and other unpolluted sites in the Baltic Sea, indicating that long-term pollution did not significantly affect these processes. Furthermore, our results indicate an adaptation to metal pollution by the N-cycling microbial community. These findings suggest that denitrification and DNRA rates are affected more by eutrophication and organic enrichment than by historic pollution of metals and organic contaminants.

This study was conducted to assess the phytodiversity distribution in relation to altitudinal gradient in Salma Mountains, a naturally protected habitat in Ha'il region in the north of Saudi Arabia. Seventeen (17) sampling sites covering three altitudinal zones (i.e. valley, foot and top mountain) were randomly selected within the study area. Within each site and altitudinal zone, the floristic composition and the potential plant species of economic values were assessed and evaluated. A total number of 163 plant species belong to 101 genera and 41 families were identified in this study. Their economic values were also classified into forage (32%), edible (8.7%), medicinal (21.3%), ornamental (30.7%), weeds (18%) and rare species (14%). In addition, results obtained from different altitudinal zones of Salma Mountains, showed that 24 species were spread in the adjacent valley to the mountains, where 32 species spreading at the foot of the mountains and 40 found occupying the top of the mountains. The rest of species have been found to occupy larger geographical distribution in all altitudinal parts. Across the altitudinal gradients, species richness has been found to be consistent with a monotonically increasing pattern with a high richness at high altitudes. The current study suggests that such naturally protected ecosystem can provide a refuge for native plant species and serve as seed bank for a future restoration program in Ha'il region and other similar habitats in Saudi Arabia.