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Sedimentation and overfishing drive changes in early succession and coral recruitment (Palau, Micronesia)

Citation

Roff, George; Wakwella, Ama; Mumby, Peter (2020), Sedimentation and overfishing drive changes in early succession and coral recruitment (Palau, Micronesia), Dryad, Dataset, https://doi.org/10.5061/dryad.f7m0cfxtg

Abstract

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 (< 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 (> 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.

Methods

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 < 2.8 km, and each location was separated by < 10 km distance.

Sedimentation rate 2016-2017 (cylinder traps)

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 (n = 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-2 day-1). 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-2 day-1.

Cylinder trap and turf pod comparisons

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 (n = 3 each per site) across all study sites (n = 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-2 day-1). 

Benthic community structure

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 length × 1.2 cm width × 1.0 cm depth. 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 width x 20 cm length x 30 cm height, 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 >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 (<5 mm) as EAM, while dense and/or tall (>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).

Coral recruitment

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 Acropora spp., Pocillopora spp., or “other growth form” (comprising of Poritidae, Merulinidae and ‎Lobophylliidae and other unidentifiable taxa) due to difficulties in taxonomic identification of coral recruits.

Usage Notes

The data is organised into sheets in xlsx format as outlined in the methods