DOI: 10.25338/b82h1n
title: Stratified vertical sediment profiles increase burrowing crab effects on salt marsh edaphic conditions

description: Burrowing animals can profoundly affect the biological structure and
 ecosystem functions of their environments. For instance, burrowing crabs
 in soft-sediment coastal ecosystems, like salt marshes, can increase
 sediment deposition and facilitate sediment homogenization and turnover,
 with potential impacts to sediment biogeochemistry. However, the relative
 importance and overall impacts of burrowing crabs on sediment dynamics can
 vary considerably between, and within, salt marsh habitats. Past studies
 have suggested that sediment properties can influence how burrowing crabs
 will affect edaphic conditions in salt marshes, but these studies often
 assume homogenous sediment conditions and fail to consider how marsh
 sediment properties change with depth. Here, we conducted a series of
 field surveys in three tidal salt marshes with variable sediment
 properties to understand if salt marsh vertical sediment profiles can help
 predict the nature of burrowing crab-sediment relationships. We found that
 burrowing crabs homogenize sediments in all marshes, but their effects on
 sediment homogenization and edaphic conditions were greater in marshes
 with highly stratified vertical sediment profiles. Our study suggests that
 understanding the vertical sediment profile of a salt marsh may provide
 critical insights into how crab burrowing may influence the edaphic
 conditions and physical characteristics of marsh surface sediments —
 especially in restored, created, and managed salt marshes.
description: <em><strong>Site Descriptions
 </strong></em>             To evaluate if
 marsh vertical sediment profile stratification informs burrowing crab
 effects on sediment edaphic conditions, we used a complex of two,
 34-year-old constructed marshes and one natural marsh (~2,140 ± 15 - 2,290
 ± 20 years old; Smyth 2020) along the West Fowl River in southern Alabama,
 USA (CON-1: 30°22’02.3” N, 88°09’06.8” W; CON-2: 30°22’03.5” N,
 88°09’02.6” W; NAT: 30°22’02.5” N, 88°09’37.2” W). The natural marsh
 (hereafter, NAT) is approximately 1km from the two constructed sites
 (hereafter, CON-1 and CON-2), which are hydrologically connected to NAT by
 a dredged tidal channel. All three sites experience diurnal microtides
 with an amplitude of approximately 0.26 m (Smyth 2020) and range in
 elevation from 0.26-0.36 NAVD88 (Ledford and others 2021).
             The plant community at each marsh is dominated by
 <em>Juncus roemerianus</em> (hereafter, Juncus), with smaller
 patches of <em>Spartina</em> <em>alterniflora</em>
 and <em>Distichlis</em> <em>spicata</em> along
 creek banks and in the high marsh, respectively. The aboveground biomass
 at all three sites is comparable; however, CON-1 and CON-2 have lower
 belowground biomass, porewater and extractable nutrient concentrations,
 and organic matter content than NAT (Ledford and others 2020, Tatariw and
 others 2021). These differences likely result from the mitigation methods
 employed to create CON-1 and CON-2, which were converted to tidal marshes
 by harvesting pine savanna habitat and excavating topsoil down to a clay
 layer that was parallel with the water table (Vittor and others  1987).
 Today, this clay layer is still a prominent feature of CON-1 and CON-2
 that can be found ~10-15 cm below the surface at both sites, under a layer
 of organic material, and results in a distinctly stratified vertical
 sediment profile (Figs 1a-b, Griffin Wood 2020).
 Burrowing crabs are present at all three sites in similar
 densities [CON-1: 14 ± 3 burrows m<sup>-2</sup> (mean ± 1SE);
 CON-2: 16.5 ± 4 burrows m<sup>-2</sup>; NAT: 16 ± 3 burrows
 m<sup>-2</sup>), with their communities being primarily
 comprised of fiddler crabs (<em>Minuca</em>
 <em>longisignalis</em>, <em>M</em>.
 <em>minax</em>, <em>Uca</em>
 <em>panacea</em>, and <em>U</em>.
 <em>spinicarpa</em>). However, we have observed five
 <em>Sesaema</em> <em>reticulatum</em> individuals
 and several large burrows (2–3 cm diameter) — indicative of <em>S.
 reticulatum</em> activity— in the <em>Juncus</em> and
 <em>D. spicata</em> zones at NAT (Rinehart and Dybiec personal
 observations). <em><strong>Characteristics
 of excavated sediment.</strong></em>
             We collected excavated sediments from areas within 2
 cm of crab burrow openings at CON-1 (n = 4), CON-2 (n = 9) and NAT (n =
 8). These sediments were returned to the laboratory where we dried them in
 an oven at 60˚ C to a constant weight to obtain bulk density. We ground
 the dried sediments with a mortar and pestle then ashed them in a muffle
 furnace (six hours at 550˚ C) to quantify the sediment organic matter
 (SOM) content via loss-on-ignition. Following our
 initial evaluation of excavated sediment characteristics, we wanted to
 test if burrowing crabs were excavating clays from the dominant clay layer
 (~10–15 cm deep) at CON-1 and CON-2 (Griffin Wood 2020).  To do this, we
 collected additional excavated sediments from areas adjacent to crab
 burrow openings at all three marshes (n = 3
 marsh<sup>-1</sup>). The excavated sediments were then
 incubated for 12 hours in 10 ml of deionized (DI) water at ambient
 temperature. We also incubated three samples of only 10 ml of DI water (no
 sediment added) at ambient temperature to serve as a control. Following
 incubation, we used a Thermo Orion Star pH meter (model A211) to measure
 the pH of the sediment-water slurries and our control DI water samples. We
 then calculated the difference between the pH of our sediment-water
 slurries and the pH of our DI water controls. We implemented this approach
 because suspended clay particles can acidify water. Thus, slurries made
 from clay-rich sediments should acidify the DI water more than other
 sediment types (Keller and Matlack 1990).
 <em><strong>Characteristics of deposited
 sediment.</strong></em>             We
 deployed 10 cm long, 2.5 cm internal diameter PVC crab burrow mimics along
 large (&gt; 2m wide) tidal creeks at CON-1 and NAT in September 2021.
 Crab burrow mimics were sealed at one end with PVC socket caps. At each
 marsh, we deployed seven burrow mimics at 25 m intervals starting at the
 mouth of the tidal creek and running 150 m along the edge of the creek
 into the marsh. The crab burrow mimics were inserted vertically into the
 marsh sediments (sealed end down) until the open end of the mimic was
 flush with the sediment surface. We placed all crab burrow mimics ~0.5 m
 in from the marsh-tidal creek edge (horizontal distance) in stands of
 <em>Juncus</em> and <em>S. alterniflora</em>. All
 inserted mimics were filled with deionized water to prevent sediment
 deposition from rapidly flushing water (Wang and others 2010).
 We collected the contents of each crab burrow mimic in October
 2021 after one month. The collected sediments were transported in plastic
 bottles to the University of Alabama where the deposited sediments were
 oven dried at 60˚ C to a constant weight to obtain the dry weight
 burrow<sup>-1</sup>. We then ashed the dried sediment in a
 muffle furnace (6 hours at 550˚ C) to estimate SOM via loss-on-ignition.
 We standardized total sediment deposition and SOM deposition by
 calculating the rate of total sediment deposition and SOM deposition crab
 burrow mimic<sup>-1</sup>
 day<sup>-1</sup>.
 <em><strong>Crab burrow density and habitat
 characteristic survey. </strong></em>
             In October 2021, we conducted an observational survey
 at all three marsh sites in the high <em>Juncus</em> zone to
 evaluate the relationships between crab burrow density, Juncus traits,
 porewater biogeochemistry, and sediment properties. Specifically, at each
 site we identified four, 0.25 m<sup>2</sup> plots with high
 crab burrow densities (i.e., ≥ 16 burrows m<sup>-2</sup>;
 hereafter High Crab plots) and four, 0.25 m<sup>2</sup> plots
 with low crab burrow densities (i.e., ≤ 12 burrows
 m<sup>-2</sup>; hereafter Low Crab plots). All plots were at
 least 0.5 m apart, but we did cluster all plots in the same habitat to
 minimize natural variability in marsh structure. We documented the total
 number of crab burrows and the diameter of every crab burrow in each plot
 at the time of sampling. Juncus
 <em>traits</em>. We estimated percent cover, stem density, and
 mean stem height of <em>Juncus</em> in every plot.
 <em>Juncus</em> stem density was estimated by counting the
 number of individual stems within a 0.1 m<sup>2</sup> quadrat
 that was placed in the center of each plot. We then extrapolated these
 counts to the entire 0.25 m<sup>2</sup> plot. We estimated the
 mean <em>Juncus</em> stem height by measuring five,
 randomly-selected <em>Juncus</em> stems in each
 plot. <em>Biogeochemistry</em>. We
 collected pore water samples from the center of each plot. Pore water was
 collected by inserting a 30 cm-long acrylic tube (4.7 mm diameter) with
 1.5 mm holes drilled along the bottom 3 cm vertically into the sediment to
 a depth of 5 cm (McKee and others 1988). From each plot, we pulled two
 separate samples that were filtered in the field (0.45 μm, VWR) into vials
 and stored on ice. The first sample was pulled to determine nutrient
 concentrations and was frozen in the laboratory until analyses. We used
 colorimetric analyses for phosphate
 (PO<sub>4</sub><sup>3-</sup>) and NOx
 (NO<sub>3</sub><sup>-</sup> +
 NO<sub>2</sub><sup>-</sup>) using a UV-vis
 spectrophotometer (Thermo Scientific GENESYS 50; Grasshof and others 1983,
 Schnetger and Lehners 2014). Additionally, we used fluorometric analysis
 for ammonium (NH<sub>4</sub><sup>+</sup>) using a
 fluorometer (Turner Designs 7200-02) equipped with a color-dissolved
 organic matter (CDOM)/ NH<sub>4</sub> UV module (Holmes and
 others 1999). The second porewater sample was transported to the
 laboratory where we used a Thermo Orion Star pH meter (model A211) and a
 YSI conductivity and salinity instrument (model 3100) to measure pH and
 salinity, respectively. <em>Sediment
 properties.</em> To understand potential relationships between crab
 burrows and sediment conditions, we collected three, 10 cm deep sediment
 cores using a 5 cm diameter Russian peat corer. We standardized the
 locations of each core within every plot to minimize bias. Specifically,
 cores were all separated by 10 cm and formed a triangle in the center of
 the plot. All collected cores were then sub-sectioned in the field at 2.5
 cm intervals and bagged. Sub-sections were weighed to obtain a wet mass
 (used in calculations of water content) and then oven dried at 60˚ C to a
 constant weight to obtain bulk density. Once dried, samples were ground
 with a mortar and pestle and then ashed in a muffle furnace (six hours at
 550˚ C) to estimate SOM via loss-on-ignition. Additionally, we pooled
 sediment from replicate cores within each plot (n = 3 cores plot-1) and
 sent material from the 0–2.5 cm and 2.5–5 cm core subsections for analysis
 of %C and %N at the Alabama Stable Isotope Laboratory (ASIL). We then used
 %C and %N to calculate the mean C and N stocks in the top 0–2.5 and 2.5–5
 cm of every plot.
description: All files are CSVs and should be compatable with excel and other
 basic data-base software.