Seasonal and Spatial Dynamics of Microbial Communities in Streams

Introduction to Temporal Dynamics in Stream Microbes. Seasonal changes in microbial diversity are a fundamental characteristic of stream ecosystems. As environmental conditions shift within the stream and its watershed, the sources of microorganisms and the resources they depend on undergo significant transformations. Key drivers include fluctuations in temperature, discharge rates, and the concentrations and quality of nutrients and organic matter. The interplay between resource availability, temperature, and hydrology collectively dictates these temporal patterns. While seasonality is a dominant force, the scale of temporal changes varies among studies, indicating that other factors are also at work in shaping microbial community dynamics over time.

Methodological Influences and General Trends. Differences in methodological approaches inevitably influence the generalizations that can be drawn from ecological studies. However, as highlighted by Zeglin [4], overarching trends do emerge from the literature. This review concludes that seasonally predictable variables—specifically water temperature, hydrology, organic matter, and nutrient loads—are consistently correlated with observable shifts in microbial diversity. These factors provide a predictable framework for understanding how microbial communities assemble and change throughout the year, despite the complexities introduced by different research techniques.

Biofilm Succession as a Window into Temporal Change. A key component of understanding temporal change involves studying the succession of biofilm communities on new substrates introduced into the stream environment. Researchers employ various materials for this purpose, including leaves, ceramic tiles, and nutrient-diffusing artificial substrates. These studies operate on a different temporal scale, typically tracking changes over weeks to months, and sometimes compare these successional patterns across different seasons. In one illustrative study, Santmire and Leff [16] used glass beads as a substrate and concluded that substratum size influenced overall bacterial abundance, while different taxonomic groups exhibited similar response patterns to the changing environment.

Analyzing Environmental Drivers Through Field Studies. The understanding of environmental drivers is largely built upon multivariate analyses of data from field studies, which often involve repeated sampling of the same locations. For instance, Ghosh et al. [17] attributed seasonal changes in microbial communities to dramatic shifts in nutrient concentrations, which subsequently altered the pool of dissolved organic matter. Similarly, Olapade and Leff [18] concluded that the organic matter pool directly impacted bacterial communities, and that these responses themselves changed seasonally. These findings underscore the complex feedback loops between chemistry and biology in stream ecosystems.

Contrasting Findings and Habitat Specificity. In contrast to clear seasonal patterns, some studies report that temporal changes are driven by intermittent physicochemical events rather than predictable seasonal cycles [19]. Such discrepancies are likely partially attributable to differences in the habitat studied. For example, Olapade and Leff [18] focused on biofilm communities, whereas Portillo et al. [19] examined planktonic bacteria in the water column. Furthermore, interannual differences can be significant; Anderson-Glenna et al. [20] found substantial variations in bacterial community composition from year to year, which they attributed to differences in temperature and other overarching conditions.

The Role of Spatial Scale in Microbial Distribution. Spatial differences in microbial communities are inherently a matter of scale. Variation in diversity among different streams on a landscape level is often greater than the differences observed along an upstream-to-downstream gradient within a single stream. A study by Fierer et al. [21] in the Hubbard Brook watershed demonstrated that pH was the strongest environmental variable correlated with bacterial community composition. This highlights how regional geology and chemistry can override local longitudinal patterns in shaping the microbial inhabitants.

The Interconnection of Space and Time. Spatial and temporal dynamics are frequently interconnected in stream ecology. For example, Febria et al. [22] discovered that when sedimentary and hyporheic zone habitats became hydrologically isolated, microbial richness declined. This suggests that stream intermittency can influence bacterial communities over time scales ranging from days to years. On an even larger spatial scale, geological differences between streams in Finland were directly linked to variations in fungal community composition, demonstrating that broad-scale landscape features create lasting legacies on the microbial life within flowing waters.