DOI: 10.5061/dryad.vhhmgqp3r
title: Burrowing crab effects across a tidal marsh successional chronosequence located along Mississippi sound

description: Burrowing crab engineers can affect the biological structure and sediment
 conditions of their environments. However, it is challenging to predict
 when and where burrow effects will manifest, as they are often site- and
 habitat-specific. We used a tidal marsh restoration chronosequence to
 explore crab burrow effects on plant communities (e.g., percent cover,
 biomass, stem heights) and sediment characteristics (e.g., bulk density,
 organic matter, carbon and nitrogen stocks) through early succession. In a
 field survey, we compared plants and sediments between plots with high and
 low crab burrow densities within three habitat zones: mud flat (i.e., 0-1
 years old), young marsh (i.e., ~1-6 years old), and old marsh (i.e., ~6+
 years old). In a manipulative experiment, we tested the physical effects
 of crab burrows on plants and sediments at the mud flat-young marsh
 ecotone using burrow mimics. In our field survey, crab burrow density did
 not influence plants or sediments. Rather, plant biomass and stem heights,
 as well as sediment bulk density, organic matter, carbon, and nitrogen,
 differed between habitat zones, following expected marsh successional
 trajectories of development over time. However, in our manipulative
 experiment, crab burrow mimics had a strong positive effect on plants at
 the ecotone, suggesting crab burrows can facilitate plant expansion into
 unvegetated mud flats. Thus, crab burrow effects appear to peak in early
 successional ecotones where burrowing mediates environmental stressors and
 promotes vegetative growth, with implications for the recovery of
 biological structure and sediment properties following restoration.
description: <strong><em>Site description
 </em></strong> To evaluate the relationship
 between burrowing crabs, plant communities, and sediment conditions along
 a successional gradient, we used a tidal salt marsh chronosqeunce at
 Greenwood Island, Pascagoula, Mississippi (hereafter, GWI;
 0°20'00.5"N 88°31'06.4" W). Specifically, GWI is a
 restored tidal marsh originally constructed through the beneficial use of
 dredged sediments in 2007 and was expanded with additional dredge-material
 applications from 2015 through 2021 (Mississippi Department of Marine
 Resources <em>personal communications</em>). This process
 produced a chronosequence of relative tidal marsh age since restoration,
 with older vegetated marsh habitat (i.e., ~6+ years old; hereafter, old
 marsh), younger vegetated marsh habitat (i.e., ~1-6 years old; hereafter,
 young marsh), and unvegetated mud flats (i.e., &lt;1 year old;
 hereafter, mud flat). The Mississippi Department of Marine Resources
 restored this tidal marsh via beneficial use of dredged sediments, which
 created a unique, natural experiment that allowed us to evaluate marsh
 development <em>in situ </em>using a space-for-time approach
 in a model tidal marsh ecosystem. The marsh habitats at GWI are dominated
 by <em>Spartina alterniflora </em>Loisel<em>
 </em>(hereafter, <em>Spartina)</em>, with some patches
 of <em>Distichlis spicata </em>Greene<em>
 </em>(hereafter, <em>Distichlis)</em> in older marsh
 habitats. Fiddler crab burrows (i.e., <em>Minuca
 longisignalis</em> [Salmon and Atsaides 1968)], <em>Minuca
 minax</em> [Le Conte 1855)], <em>Leptuca panacea
 </em>[Novak and Salmon 1974], and <em>Leptuca
 spinicarpa</em> [Rathbun 1900]) are common in marsh and mud flat
 habitats at GWI, with densities ranging from ~0-50 burrows m<sup>-2
 </sup>(Rinehart, Sharbaugh, and Dybiec <em>personal
 observation). </em>Purple marsh crabs, <em>Sesarma reticulatum
 </em>(Say, 1817), are also found in low densities in the oldest and
 highest elevation habitats at GWI, which were not included in this study
 due to the high potential for disturbance from ongoing management
 activities at the site. Overall, the community composition and relative
 densities of burrowing crabs observed at GWI are comparable to those
 observed throughout tidal marshes in the northern Gulf of
 Mexico. <strong><em>Crab burrow density
 survey (file name-- survey)</em></strong>
             In November 2022, we conducted an observational
 survey in each habitat zone (i.e., mud flat, young marsh, old marsh) to
 evaluate the relationships between crab density, plant population traits,
 and sediment conditions. Specifically, in each habitat zone, we
 established five, 0.25 m<sup>2</sup> (0.5m x 0.5 m, length x
 width) plots containing high crab burrow densities (i.e., ≥16 burrows
 m<sup>−2</sup>; hereafter, high crab plots) and five, 0.25
 m<sup>2</sup> plots with low crab burrow densities (i.e., ≤12
 burrows m<sup>−2</sup>; hereafter, low crab plots; see Figure
 S1). All plots were at least 0.5 m apart, but they were generally
 clustered to reduce the natural variability in marsh structure. At the
 time of the survey, we documented the total number of crab burrows in each
 plot, and the diameter of up to five representative crab
 burrows.  <em>Plant populations:</em> In
 each plot, we estimated the total percent cover of all species (i.e.,
 <em>Spartina</em> and <em>Distichlis</em>) and the
 mean stem height of <em>Spartina</em>. We estimated the mean
 <em>Spartina </em>stem height by measuring five, randomly
 selected <em>Spartina </em>stems in each plot. We harvested
 the aboveground biomass in each plot by clipping stems at the air-sediment
 interface. Harvested plant material was then dried at 60˚ C to a constant
 weight. Additionally, to assess belowground biomass, we collected one
 sediment core (Ø = 7.9 cm, depth = 10 cm) from the center of each clipped
 plot. Belowground biomass cores were sub-sectioned in the field at 5 cm
 intervals and bagged. In the lab, belowground biomass subsections were
 rinsed and sorted by tissue type (i.e., roots, shoots, rhizomes) before
 being dried at 60°C to a constant mass.
 <em>Sediment properties</em>: In each plot, we
 collected five, 24.5 cm<sup>3</sup> soil cores to a depth of
 10 cm using a Russian Peat Corer. We standardized the locations of each
 core within every plot to minimize bias. Specifically, we took one core in
 each corner and one in the center of each plot. Each core was then
 sub-sectioned at 2.5 cm depth intervals in the field and bagged.
 Sub-sections were dried at 60 °C to a constant weight to obtain bulk
 density. For analysis of bulk density, we calculated the mean bulk density
 of replicate cores collected at 1) 0-2.5 cm and 2.5-5 cm depths and 2)
 5-7.5 cm and 7.5-10 cm depths to obtain plot-level mean bulk density at
 0-5 cm and 5-10 cm, respectively. Once dried, samples were ground with a
 mortar and pestle and then ashed in a muffle furnace (six hours at 550˚ C)
 to estimate sediment organic matter (SOM) via loss-on-ignition. We pooled
 sediment from replicate cores collected at 1) 0-2.5 cm and 2.5-5 cm depths
 and 2) 5-7.5 cm and 7.5-10 cm depths to obtain a single plot-level sample
 representing 0-5 cm and 5-10 cm, respectively. These pooled samples were
 then sent to the Alabama Stable Isotope Laboratory for analysis of %C and
 %N using a Costech 4010 ECS interfaced with a Thermo Delta V isotope ratio
 mass spectrometer via a Thermo Conflo IV.
 <strong><em>Crab burrow mimics at marsh ecotones
 experiment (file name-- experiment)</em></strong>
 In May 2023, we established 20, 1 m<sup>2</sup> plots
 along the boundary between the mud flat and young marsh habitat.
 Specifically, we placed one edge of the plot parallel with the vegetation
 line (hereafter, upper edge), so the entire plot extended into the
 unvegetated mud flat habitat and contained minimal vegetation at the
 beginning of the study (i.e., &lt; 3% <em>S.
 alterniflora</em> cover). Plots were at least 1m apart and were
 placed in a line along the young marsh-mud flat ecotone (hereafter,
 ecotone).  In half of the plots (every other plot; n = 10; hereafter, crab
 burrow plots), we created crab burrow mimics by inserting a hollow, 1.5 cm
 diameter PVC pole vertically into the marsh sediment to a depth of 20 cm.
 We then removed the pole, creating a 1.5 cm diameter, 20 cm deep burrow
 mimic. We based the diameter of crab burrow mimics on ambient diameters
 observed along the ecotone at GWI (1.2 - 1.6 cm; 1.4 ± 0.04 cm; mean ±
 1SE). The depth of crab burrow mimics was based on the 1) lower limits of
 Fiddler crab burrows observed in southeastern US tidal marshes (i.e.,
 16-75 cm depth; Aspey 1978) and 2) the general rooting depths of
 <em>Spartina </em>(i.e., 0-20 cm; Darby and Turner 2008). Crab
 burrow plots each received 20 crab burrow mimics, consistent with the
 density of crab burrows observed along the ecotone at GWI (range: 20-24
 burrows m<sup>-2</sup>; 22.4 ± 0.98 burrows
 m<sup>-2</sup>; mean ± 1SE). We maintained crab burrow mimics
 monthly until the study concluded in November 2023. The other plots (n
 =10) did not receive crab burrow mimics and served as controls (hereafter,
 no burrow plots). <em>Plant populations:
 </em>At the conclusion of the ecotone study, we estimated the total
 plant cover<em> </em>in each plot. We divided each plot into
 four quadrats (each 0.5 x 0.5 m; length x width) and estimated the
 total<em> </em>cover in each quadrant to understand the
 spatial variation in plant cover between the crab burrow and no burrow
 plots. To further understand how crab burrows influence plant growth at
 the ecotone, we quantified the distance that <em>Spartina
 </em>stems and rhizomes migrated into the unvegetated mud flat from
 the original vegetation edge. We quantified the distance that
 <em>Spartina </em>stems migrated by measuring the distance
 between the upper edge of the plot (i.e., the initial location of the
 plant ecotone) and the farthest <em>Spartina </em>stem at
 five, equidistant points (every 0.2 m) in each plot. We estimated the
 distance migrated by plant belowground biomass by taking three sediment
 cores, one at the center of the upper edge of the plot (0-10 cm from
 initial ecotone border, one at the middle of the plot (50-60 cm from
 initial ecotone border, and one at the center of the lower edge of the
 plot (90-100 cm from the initial ecotone border). We used a t-corer (i.d.
 = 10 cm) to take 5 cm deep cores at each of these points, and then
 searched through each core and noted the presence/absence of plant
 belowground biomass. If no belowground biomass was observed in any core, a
 zero was recorded for belowground expansion. We also estimated the mean
 height of <em>Spartina </em>in each plot by measuring the
 heights of five randomly selected stems.
 <em>Sediment properties</em>: In November 2023, we
 collected two, 49.1 cm<sup>3</sup> soil cores to a depth of 5
 cm using a Russian Peat Corer. We standardized the locations of each core
 within every plot to minimize bias. Specifically, we took both cores ~0.25
 m from the upper edge of each plot.  We took cores close to the original
 ecotone, as these areas were most likely to be colonized by
 <em>Spartina </em>over the 6-month study, meaning that these
 sediments should be more likely to show signs of organic matter and carbon
 stock development. All cores were dried at 60 °C to a constant weight to
 obtain bulk density and SOM, as described above. 
description: # Burrowing crab effects across a tidal marsh successional chronosequence
 located along Mississippi sound.
 [https://doi.org/10.5061/dryad.vhhmgqp3r](https://doi.org/10.5061/dryad.vhhmgqp3r) ## Description of the data and file structure ### Files and variables #### File: Preliminary\_crab\_burrow\_desnisty\_across\_seasons.csv **Description:** Data collected from the preliminary assessment of crab burrow densities across the three habitat types (i.e., mud flat, young marsh, old marsh) between March and November.  ##### Variables * Zone: The habitat zone (i.e., mud flat, young marsh, old marsh) where the data was collected. * Month: The month in which the data was collected from the permanent sampling plots.  * Burrow density_1m2: The density of crab burrows scales up from 0.25m2 to 1m2.   #### File: Experiment.csv **Description:** Data collected from a manipulative field study looking at the physical effects of crab burrowing on the expansion and growth of *Spartina alterniflora* along a mud flat-young marsh ecotone. Physical crab burrow effects were generated using burrow mimics (i.e., fake burrows).  ##### Variables * Plot Code: Individual codes assigned to each plot in the study. * Crab: Indicates treatments assignment (FB = Fake crab burrow; NB = No crab burrow).  * PC-Q1: Percent Spartina cover in Q1. * PC-Q2: Percent Spartina cover in Q2. * PC-Q3: Percent Spartina cover in Q3.  * PC-Q4: Percent Spartina cover in Q4.  * PC-Total: The total percent cover, calculated using PC-Q1 through PC-Q4. * AVG HGT: Average Spartina stem height.  * AVG ECO: Average distance Spartina stems expanded (grew) into the mud flat (mean of five individual measurements).  * ECO RHIZ: Maximum distance Spartina belowground biomass expanded into the mud flat.  * AVG-BD: Mean sediment bulk density (mean of two sediment cores, 0-5 cm depth). * AVG-SOM: Mean sediment organic matter content (mean of two sediment cores, 0-5 cm depth).  #### File: Survey.csv **Description:** Data collected during an observational survey assessing the relationships between relative crab burrow density (high vs. low), foundation plant growth, and sediment development across three habitats representing a successional chronosequence (i.e., mud flat = 0 yrs old; young marsh = 1-6 yrs old; old marsh = 6+ yrs old).  ##### Variables * Plot: Individual codes assigned to each plot in the study.  * Zone: Indicates the successional stage of the habitat along the tidal marsh chronosequence (mud flat = 0 yr old; young marsh = 1-6 yrs old; old marsh = 6+ yrs old). * Crab: Indicates treatments assignment (Low = low crab burrow density; High = high crab burrow density).  * Burrows_1m2: Mean crab burrow density in plots scaled to 1m2.  * SPAL_cover_%: The percent cover of *Sartina alterniflora.* * DISP_cover_%: The percent cover of *Distichlis spicata.* * TOT_cover_%: The total plant percent cover.  * Mean_SPAL_height_cm: The mean height of five randomly selected *Spartina alterniflora* stems.  * TOT_AGB_g/m2: The total plant aboveground biomass scaled to grams per m2.  * TOT_BGB_0-5_g/cm3: The total plant belowground biomass in the top 0-5 cm of sediment scaled to grams per cm3.  * TOT_BGB_5-10_g/cm3: The total plant belowground biomass in the top 5-10 cm of sediment scaled to grams per cm3.  * BD_0-5_g/cm3: The mean sediment bulk density of the top 0-5 cm presented in grams per cm3.  * BD_5-10_g/cm3: The mean sediment bulk density of the top 5-10 cm presented in grams per cm3.  * BD_0-2.5_g/cm3: The mean sediment bulk density of the top 0-2.5 cm presented in grams per cm3.  * BD_2.5-5_g/cm3: The mean sediment bulk density of the top 2.5-5 cm presented in grams per cm3.  * BD_5-7.5_g/cm3: The mean sediment bulk density of the top 5-7.5 cm presented in grams per cm3.  * BD_7.5-10_g/cm3: The mean sediment bulk density of the top 7.5-10 cm presented in grams per cm3.  * SOM_0-5_%: The mean sediment organic matter in the top 0-5 cm presented as a percent.  * SOM_5-10_%: The mean sediment organic matter in the top 5-10 cm presented as a percent.  * SOM_0-2.5_%: The mean sediment organic matter in the top 0-2.5 cm presented as a percent.  * SOM_2.5-5_%: The mean sediment organic matter in the top 2.5-5 cm presented as a percent.  * SOM_5-7.5_%: The mean sediment organic matter in the top 5-7.5 cm presented as a percent.  * SOM_7.5-10_%: The mean sediment organic matter in the top 7.5-10 cm presented as a percent.  * TotalN_0-5_%: The total nitrogen content in the top 0-5 cm presented as a percent.  * TotalC_0-5_%: The total carbon content in the top 0-5 cm presented as a percent.  * CN_0-5: The ratio of carbon-to-nitrogen in the top 0-5 cm.  * TotalN_5-10_%: The total nitrogen content in the top 5-10 cm presented as a percent.  * TotalC_5-10_%: The total carbon content in the top 5-10 cm presented as a percent.  * CN_5-10: The ratio of carbon-to-nitrogen in the top 5-10 cm.  ## Code/software No special software required.  ## Access information Other publicly accessible locations of the data: * NA Data was derived from the following sources: * NA