Salinization and sedimentation drive contrasting assembly mechanisms of planktonic and sediment-bound bacterial communities in agricultural streams

Authors: DEVILBISS, STEPHEN - US Department Of Agriculture (USDA); Taylor, Jason; HICKS, MATTHEW - Us Geological Survey (USGS)

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2023
Publication Date: 8/7/2023
Citation: Devilbiss, S., Taylor, J.M., Hicks, M. 2023. Salinization and sedimentation drive contrasting assembly mechanisms of planktonic and sediment-bound bacterial communities in agricultural streams. Global Change Biology. 29:5615-5633. https://doi.org/10.1111/gcb.16905.
DOI: https://doi.org/10.1111/gcb.16905

Interpretive Summary: Intensification of agriculture has resulted in losses of valuable ecosystem services within rivers, lakes, and streams. Bacterial communities in streams can potentially mitigate many of these negative effects, like excessive nutrient concentrations. It is critical to understand how agricultural practices affect bacterial diversity in agricultural streams in order to improve resource management. Bacterial diversity actually increases with increasing agricultural land use and is strongly impacted by increasing salinity and sediment deposition, two widespread, agriculturally-associated water quality stressors. Because of their increased diversity in agricultural watersheds, bacterial communities may be useful for establishing water quality goals and monitoring improvements from best management practices in agroecosystems.

Technical Abstract: ture is the most dominant land use globally and is projected to increase in the future to support a growing human population but also threatens ecosystem structure and services. Bacteria mediate numerous biogeochemical pathways within ecosystems. Therefore, identifying linkages between stressors associated with agricultural land use and responses of bacterial diversity is an important step in understanding and improving resource management. Here, we use the Mississippi Alluvial Plain (MAP) ecoregion, a highly modified agroecosystem, as a case study to better understand agriculturally-associated drivers of stream bacterial diversity and assembly mechanisms. In the Map, we found that planktonic bacterial communities were strongly influenced by salinity. Tolerant taxa increased with increasing ion concentrations, likely driving homogenous selection which accounted for ~90% of assembly processes. Sediment bacterial phylogenetic diversity increased with increasing agricultural land use and was influenced by sediment particle size, with assembly mechanisms shifting from homogenous to variable selection as differences in median particle size increased. Within individual streams, sediment heterogeneity was correlated with bacterial diversity and a subsidy-stress relationship along the particle size gradient was observed. Planktonic and sediment communities within the same stream also diverged as sediment particle size decreased. Nutrients including carbon, nitrogen, and phosphorus, which tend to be elevated in agroecosystems, were also associated with detectable shifts in bacterial community structure. Collectively, our results establish that two understudied variables, salinity and sediment texture, are the primary drivers of bacterial diversity within the studied agroecosystem, while nutrients are secondary drivers. While numerous macrobiological assemblages respond negatively, we observed increasing bacterial diversity in response to agricultural stressors including salinization and sedimentation. Elevated taxonomic and functional bacterial diversity likely increases the probability of detecting community responses to stressors. Thus, bacteria assemblage responses may be more useful for establishing water quality goals within highly modified agroecosystems that have experienced shifting baselines.