DOI: 10.5061/dryad.95x69p8qh
title: Sediment texture and nutrient data from: Bacterial assembly in agricultural streams

description: Agriculture 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. Although numerous
 macrobiological communities respond negatively, we observed increasing
 bacterial diversity in response to agricultural stressors including
 salinization and sedimentation. Elevated taxonomic and phylogenetic
 bacterial diversity likely increases the probability of detecting
 community responses to stressors. Thus, bacteria community responses may
 be more reliable for establishing water quality goals within highly
 modified agroecosystems that have experienced shifting baselines.
description: Prior to analyses, sediments were dried, ground, and pre-sieved
 through a 2 mm sieve to remove any coarse material. We measured sediment P
 (Sed P) concentrations using a Melich-3 extraction method. Briefly, 1 g of
 dried sediment was digested using an extract composed of 0.2 M acetic
 acid, 0.25 M ammonium nitrate, 0.015 M ammonium fluoride, 0.013 M nitric
 acid, and 0.001 M ethylene diamine tetraacetic acid. Phosphorus content of
 extracted sediment samples was then determined using the standard
 ascorbic-ammonium molybdate method using a MULTISKAN SkyHigh microplate
 reader (Thermo Scientific). We determined percent carbon (C) and N of 1 g
 of dried sediment using a CN analyzer (Elementar). We determined sediment
 texture and particle size distribution using a digital hydrometer and the
 integral suspension pressure method (ISP+) (Pario, METER AG, Munich)
 following manufacture protocols (see Durner and Iden 2021 for theory).
 Briefly, dried sediments were shaken overnight in 5% Sodium
 Hexametaphosphate to disperse aggregates. The dispersed sample was
 transferred to a 1-L graduated cylinder with a valve for draining
 effluent. We homogenized the sample for one minute prior to inserting the
 digital hydrometer into the sample tube. The density of the sediment
 solution was continuously measured for 2.5 hours. After 2.5 hours, we
 drained the top portion of the effluent containing the clay fraction
 through the valve into a pre-weighed, 250 mL beaker to determine the mass
 of the clay fraction in the sample. We wet-sieved the sand fraction
 through 500, 250, 125, and 63 µm sieves to determine the mass of different
 sand fractions. Based on the mass of the clay fraction in the effluent and
 the mass of different sand fractions, an inverse model of suspension
 pressure was fit using the Pario software to determine the continuous size
 distribution and percent sand, silt, and clay.