Organic matter and metal loadings influence the spatial gradient of the benthic bacterial community in a temperate estuary

Omics-based monitoring using bacterial marker genes can provide valuable mechanistic insights into the functioning of ecosystems. Here, we present a 2.5-year dataset with monthly sampling of sediment genomic bacterial DNA (n = 160) in a temperate, urbanised estuary in Tasmania, Australia. Molecular data were collected with physical and biochemical bottom water data, sediment organic matter and metal concentrations. Our study supports evidence that sediment-specific variables (organic matter composition) have a larger influence over the sediment bacterial community than do large-scale environmental conditions (seasonal water changes). The observed spatial and temporal differences are interesting, given the significant seasonal variation in bottom water data (e.g. temperature differences of up to 10°C and 3-fold increases for NOx concentrations in the bottom water between summer and winter months). Whereas bottom water parameters changed seasonally, metal concentrations in the sediments did not show seasonal variations. Metal concentrations explained a larger variance in the bacterial community among sites but not on an estuary-wide scale. The disconnect between environmental bottom water conditions and the sediment bacterial communities has important ramifications, because it indicates that seasonal changes have little effect on the compositional dynamics of sediment microbes and may, therefore, be difficult to trace with marker-gene surveys.