DOI: 10.5061/dryad.f7m0cfxtg
title: Sedimentation and overfishing drive changes in early succession and coral recruitment (Palau, Micronesia)

description: Sedimentation and overfishing are important local stressors on coral reefs
 that can independently result in declines in coral recruitment and shifts
 to algal dominated states. However, the role of herbivory in driving
 recovery across environmental gradients is often unclear. Here we
 investigate early successional benthic communities and coral recruitment
 across a sediment gradient in Palau, Micronesia over a 12-month period.
 Total sedimentation rates measured by ‘TurfPods’ varied from 0.03 ± 0.1 SE
 mg cm-2 day-1 at offshore sites to 1.32 ± 0.2 mg cm-2 day-1 at inshore
 sites. To assess benthic succession, three-dimensional settlement tiles
 were deployed at sites with experimental cages used to exclude tile access
 to larger herbivorous fish. Benthic assemblages exhibited rapid
 transitions across the sediment gradient within 3 months of deployment. At
 low levels of sedimentation (&lt; 0.6 mg cm-2 day-1), herbivory
 resulted in communities dominated by coral recruitment inducers (short
 turf algae and crustose coralline algae), whereas exclusion of herbivores
 resulted in the overgrowth of coral inhibitors (encrusting and upright
 foliose macroalgae). An “inducer threshold” was found under increasing
 levels of sedimentation (&gt; 0.6 mg cm-2 day-1), with coral inducers
 having limited to no presence in communities, and herbivore access to
 tiles resulted in sediment-laden turf algal assemblages, while exclusion
 of herbivores resulted in invertebrates (sponges, ascidians) and
 terrestrial sediment accumulation. A “coral recruitment threshold” was
 found at 0.8 mg cm-2 day-1, below which net coral recruitment was reduced
 by 50% in the absence of herbivores, while recruitment was minimal above
 the threshold. Our results highlight non-linear trajectories of benthic
 succession across sediment gradients and identify strong interactions
 between sediment and herbivory that have cascading effects on coral
 recruitment. Local management strategies that aim to reduce sedimentation
 and turbidity and manage herbivore fisheries can have measurable effects
 on benthic community succession and coral recruitment, enhancing reef
 resilience and driving coral recovery.
description: The study was conducted on inshore reefs of Palau archipelago
 (Micronesia). Study sites were located adjacent to three locations on the
 west coast of the Babeldaob: Ngardmau (“North”), Ngeremlengui (“Central”)
 and Ngermeduu (“South”). Within each location, five sites were chosen with
 increasing distance from river mouths, where site 1 was the closest to the
 river mouth, and site 5 was located in clear lagoonal waters. Within
 locations, sites were located within a total environmental gradient of
 &lt; 2.8 km, and each location was separated by &lt; 10 km
 distance. <b>Sedimentation rate
 2016-2017 (cylinder traps)</b> At each of the
 sites, three PVC tube cylinder traps (5cm diameter, 60 cm tall) were
 deployed to quantify rates of sedimentation (45 sediment traps in total)
 at each of the five sites within each location (<i>n</i> = 3
 per site). Cylinder traps were deployed for 3 month periods. Time
 indicates initial of deployment period, where 1 = May 2016, 2= August
 2016, 3 = December 2016, and 4 = March 2017. Following collection of traps
 and return to the lab, sediments were rinsed with freshwater, dried at
 55°C, ground using a mortar and pestle and dried at 105°C for 24 h to
 obtain total sedimentation rate (mg cm<sup>-2</sup>
 day<sup>-1</sup>). Samples were combusted at 550°C for 4 h and
 reweighed to determine organic material content. Carbonate content was
 determined following 32% hydrochloric acid digestion of carbonate
 material. All values are reported in mg cm<sup>-2</sup>
 day<sup>-1</sup>. <b>Cylinder trap
 and turf pod comparisons</b> To allow for
 comparisons among studies using different methods to quantify
 sedimentation rates, we conducted an additional experiment in May 2019
 were we simultaneously deployed “TurfPods” and cylinder traps
 (<i>n</i> = 3 each per site) across all study sites
 (<i>n</i> = 15) for a 5 day period (Figure S2). TurfPods were
 constructed from PVC tube 81mm wide, and covered with 3mm artificial turf
 (AstroTurf) to trap sediments. Following collection of traps and return to
 the lab, sediments were rinsed with freshwater, dried at 55°C, ground
 using a mortar and pestle and dried at 105°C for 24 h to obtain total
 sedimentation rate (mg cm<sup>-2</sup>
 day<sup>-1</sup>).  <b>Benthic
 community structure</b> To track benthic
 succession over time, we used “crevice tiles” to simulate patterns of
 algal colonization and succession in relation to microhabitats and
 herbivory.  Each tile measured 10 × 10 cm and had 24 equally spaced crowns
 and crevices, where each crown measured 1.2 cm<i>
 length</i> × 1.2 cm<i> width</i> × 1.0 cm
 <i>depth</i>. Within each site, tiles were assigned one of
 three treatments: caged, partially caged and open. Caged tiles were
 enclosed in PVC coated wire cages (20 cm <i>width</i> x 20 cm
 <i>length</i> x 30 cm <i>height</i>, galvanized
 2.5 cm wire mesh openings) to mimic the impact of overfishing by excluding
 large herbivorous fish from accessing the tile. Partial cages were
 constructed identical to cages but had had two opposing vertical sides and
 the lid removed to allow access to grazing fish. Open tiles had no cage
 structure to allow for unimpeded herbivore access to the tile.  Five
 replicate tiles from each treatment (caged, partial, open) were deployed
 onto sites 1, 3 and 5 of each location, resulting in 15 tiles per site, 45
 tiles per location, 135 tiles across all sites/locations. All tiles were
 placed at a depth of approximately 5-6 m within each site and separated by
 &gt;2m distance. Tiles were deployed in May 2016 and photographed at
 three, six, nine, and twelve-month intervals (August 2016, December 2016,
 March 2017 and June 2017). Tile images were analysed
 using Coral Point Count with Excel extensions (CPCe) program. Fifty-points
 were assigned per tile, and organisms under each point was categorised
 into functional groups: bare tile, terrestrial sediment, carbonate
 sediment, epilithic algal matrix and crustose coralline algae (merged into
 “EAM”), turf, upright foliose macroalgae (UFM), encrusting macroalgae
 (EFM), non-coral invertebrates (such as ascidians and sponges, merged into
 “invertebrates”), cyanobacteria and corals. We differentiated terrigenous
 and carbonate sediments into “finer-grained and darker-colored sediment”
 and “coarser, lighter-colored sediment” using visual assessments of
 particle size, colour and apparent texture, and differentiated sparse
 filamentous turf algae (&lt;5 mm) as EAM, while dense and/or tall
 (&gt;5 mm) mats of filamentous algae as “turf” due to their different
 ecological functions. From a grazing perspective, EAM is functionally
 analogous to short productive algal turfs (SPATs), while “turf” are
 analogous to long sediment-laden algal turfs (LSATs).
 <b>Coral recruitment</b> Coral
 recruitment on the crevice tiles was quantified at the last time point,
 after twelve months of succession. Tiles were observed under a dissecting
 microscope and any coral recruits were recorded along with maximum width,
 and identified to the lowest taxonomic resolution possible and
 subsequently grouped into <em>Acropora</em> spp.,
 <em>Pocillopora</em> spp., or “other growth form” (comprising
 of Poritidae, Merulinidae and ‎Lobophylliidae and other unidentifiable
 taxa) due to difficulties in taxonomic identification of coral
 recruits.
description: The data is organised into sheets in xlsx format as outlined in
 the methods