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Microbial communities are important elements in the marine environment, contributing to nutrient cycling and biogeochemical processes. Estuaries comprise environments exhibiting characteristics from freshwater to marine, leading to distinct microbial communities across this environmental gradient. Here, we examine the spatial dynamics of microbial communities in Macquarie Harbour, an estuarine system on the West coast of Tasmania, Australia. Water was sampled along the estuary to explore the structure and composition of the microbial communities using 16S/18S rRNA gene amplicon sequencing. Multivariate analyses showed environmental variables and community compositions varying along a longitudinal (river to adjacent ocean) gradient at the surface. In the Harbour, differences in the microbial community were observed between surface (0–1 m) and intermediate depths (4.5–11 m depth). The results of differential abundance, network and Partial Least Square analyses suggest that Macquarie Harbour is a mixing zone, where the distributions of archaeal, bacterial and eukaryotic communities are influenced by oceanic and riverine inputs. These results provide critical insights into the Macquarie Harbour environment and the importance of understanding the role of microbial communities for similar systems elsewhere. • Network and machine learning revealed the microbial community structure in a highly stratified aquatic environment. • Environmental variables and community compositions varied along a longitudinal (river to adjacent ocean) gradient. • Oceanic and riverine inputs contribute to differences in the composition of microbial communities. • Pelagic heterotrophic microorganisms may play important roles in the nutrient cycle in the Macquarie Harbour. [ABSTRACT FROM AUTHOR]

Fish and macrocrustacean assemblage structure was analyzed along an estuarine gradient at Mad Island Marsh (MIM), Matagorda Bay, TX, during March 1998–August 1999. Eight estuarine-dependent fish species accounted for 94% of the individual fishes collected, and three species accounted for 96% of macrocrustacean abundance. Consistent with evidence from other Gulf of Mexico estuarine studies, species richness and abundance were highest during late spring and summer, and lowest during winter and early spring. Sites near the bay supported the most individuals and species. Associations between fish abundance and environmental variables were examined with canonical correspondence analysis. The dominant gradient was associated with water depth and distance from the bay. The secondary gradient reflected seasonal variation and was associated with temperature, salinity, dissolved oxygen, and vegetation cover. At the scales examined, estuarine biota responded to seasonal variation more than spatial variation. Estuarine-dependent species dominated the fauna and were common throughout the open waters of the shallow lake during winter–early spring when water temperature and salinity were low and dissolved oxygen high. During summer–early fall, sub-optimal environmental conditions (high temperature, low DO) in upper reaches accounted for strong spatial variation in assemblage composition. Small estuarine-resident fishes and the blue crab (Callinectes sapidus) were common in warm, shallow, vegetated inland sites during summer–fall. Estuarine-dependent species were common at deeper, more saline locations near the bay during this period. During summer, freshwater species, such as gizzard shad (Dorosoma cepedianum) and gars (Lepisosteus spp.), were positively associated with water depth and proximity to the bay. The distribution and abundance of fishes in MIM appear to result from the combined effects of endogenous, seasonal patterns of reproduction and migration operating on large spatial scales, and species-specific response to local environmental variation. [Copyright &y& Elsevier]