The spatiotemporal dynamics of bacterial and fungal abundance in Great Plains non-perennial streams

Abstract

Non-perennial streams comprise approximately 50% of global stream length and 59% across the continental U.S. Non-perennial streams are also predicted to become more common, since increased length and time of drying in rivers and streams is a widespread symptom of climate change. These substantial hydrologic fluctuations have the potential to impact the stream surface water and benthic microbiomes. Aquatic microorganisms are important drivers of carbon and nitrogen retention and immobilization, thus controlling both downstream water quality and integrated ecosystem metabolism. Surface flow connectivity to the local microbial habitat provides organic energy and dissolved nutrients that support microbial growth, and in turn, nutrient immobilization. However, the sensitivity of microbial populations to stream drying and rewetting is not well understood. Thus, learning more about the sensitivity of aquatic microbiomes to drying and rewetting cycles will improve our ability to assess future impacts to water quality. We predicted that greater hydrological connectivity would support higher bacterial and fungal populations, to a plateau level where the growth response becomes saturated. Across fifty sampling locations in South Fork Kings Creek, Kansas, USA, microbial populations in surface waters, benthic (stream bed) sediments, epilithic biofilms (rock surfaces), and leaf surfaces were not related to the duration of flowing conditions before sampling, as expected, but did vary based on distance from the outlet of the watershed and differed between wet and dry sampling sites. During sequential hydroperiods (low flow, wet-up, dry-down, and disconnected) at three streams in Kansas and Oklahoma, USA, microbial abundance in the same four microhabitats changed during the wet-up and dry-down periods with patterns qualitatively consistent with spatial wet/dry differences. Specifically, epilithic biofilms tended to have higher microbial populations in wet conditions, whereas microbial populations on leaves and in benthic sediments, unexpectedly, tended to be lower in wet than dry conditions. These results suggest that as they grow, epilithic biofilm bacterial and fungal populations are more likely nutrient sinks during wet conditions. In contrast, larger sediment microbial populations can support more biogeochemical cycling, and provide potential refugia for stream microbiota, during dry conditions. Network continuum patterns also show greater surface water bacterial and fungal loads higher upstream, suggesting decreasing terrestrial dispersal pressure downstream in the stream network. This research establishes baselines on how microbial populations change in response to drying and rewetting over space and time, which will be used to inform water quality changes in non-perennial streams across the country. Future work should consider how bacterial, fungal, and protistan abundance, diversity, and activity can teach us more about the integrated functions of non-perennial stream ecosystems.