Ecology and biogeochemistry of fluvial microphytobenthic communities on soft sediments

Freshwater ecosystems are the link between the terrestrial and the marine biomes. Terrestrially-derived nutrients travel through, and are transformed within these ecosystems, outflowing eventually to the sea. A major player in small rivers are microphytobenthic (MPB) communities - groups of algae, bacteria, fungi and other protists existing within a polysaccharide matrix. These biofilm communities exist on a transitional zone (the hyporheic zone) between surface water and underground water. This thesis explores the ecology and biogeochemistry of microphytobenthic communities over multiple spatial scales within the Hampshire Avon river catchment, south England. Across three contrasting sub-catchment geology types - clay, greensand and chalk - algal abundance and composition, algal extracellular organic carbon, bacterial abundance and physical characteristics including organic carbon and nutrient concentrations were examined. Results from this study showed that the contrasting habitats indeed carried communities under different physical and ecological pressures. Variability between samples highlighted the heterogeneity of MPB communities and confirmed through comparisons of multiple rivers within the same sub-catchment. Diatom community composition showed a distinct gradient, where the greensand rivers were found to form a transitional zone between the two extremes of clay and chalk rivers. Interactions found between algal extracellular carbon and pore- and surface water chemistry hinted at a change in interactions between autotrophs and heterotrophs, and indicated the first known example of shifting facilitation/competition found in MPB communities on natural sediment. Potential nutrient flux was also shown to vary between geology and light levels, with differences being found between habitats within the same river. An attempt to remove variability in these systems using simulated natural sediment was not fully successful, although supported previous findings regarding the impact of light on biogeochemical cycles through oxygen flux. This work shows that further investigation is needed on natural MPB communities, with greater sampling effort required to overcome heterogeneity.