Dataset: Benthic community composition from a field experiment manipulating consumer pressure and nutrient availability on a coral reef in Moorea, French Polynesia from 2018 to 2022

Experimental Setup

In June 2018, we established a factorial experiment manipulating consumer pressure and nutrient availability in areas that mimicked disturbance from a cyclone. At 12m depth on the north shore forereef of Moorea, French Polynesia (17.47° S, 149.82° W) we established 16 ~30m² plots. Within each plot, we then nested 4 different consumer exclosures (~1.25 m² each) with different size openings that allowed different size fishes (herbivores and corallivores, hereafter referred to as ‘consumers’) access to the benthos. The exclosure frames consisted of 0.5cm stainless steel all-thread drilled into the reef matrix and epoxied into place. These frames were then wrapped with plastic-coated, galvanized wire to create the following levels of consumer pressure: 1) Very Low (2.5cm × 2.5cm (1inx1in) openings); 2) Low (5cm × 5cm (2inx2in) openings); 3) Medium (7.5cm × 7.5cm (3inx3in) openings); 4) High (4 sides of 2.5cm × 2.5cm (1inx1in) openings but no top). Others have used a similar design to create a gradient of consumer pressure to mimic the effects of different levels of fishing (Holbrook et al. 2016, Schmitt et al. 2019). We included sides, but not tops, on the High consumer pressure treatment to control for potential artifacts on water flow, although we have shown these are minimal (Zaneveld et al. 2016). Exclosures were scrubbed every 12-16 weeks to remove fouling organisms.

Within the exclosures we effected a second treatment in order to simulate the effects of a cyclone. To do so, we removed all branching corals (mostly Acropora spp. and Pocillopora spp.) and transplanted them to other areas of the reef away from the experiment. The removal of branching corals simulated a pulse disturbance such as the cyclone that reduced coral cover to < 5% on the forereefs of Moorea in 2010. Remaining patches of encrusting corals (mostly Montipora spp.) and mounding corals (mostly Porites spp.) and macroalgae (which were rare) were scrubbed with wire brushes to mimic the scouring from sediments that occurs during a large cyclone. Thus, after coral and macroalgae removal, mean coral cover in the plots was ~7% and macroalgae was ~0.5%. These treatments were called 'Disturbed' as opposed to plots where we did not remove corals that are referred to as 'Intact'. 

We also included a third treatment of Ambient or Enriched nutrient conditions with each plot (which included four nested consumer exclosures) was then assigned to either Ambient or Enriched nutrient conditions. Thus, each combination of consumer pressure (Very Low, Low, Medium, High) and nutrients (Ambient or Enriched) had n=4 for replication. For the enrichment we placed 175 g of Osmocote^® (19-6-12, N-P-K) slow-release garden fertilizer into 5 cm diameter PVC tubes with 10, 1 cm holes drilled into them. These tubes were wrapped in fine plastic mesh to retain the fertilizer. This method is similar to our previous work (e.g., Zaneveld et al. 2016). PVC enrichment tubes were attached to the corners of each exclosure and onto a piece of stainless steel all-thread in the center of each plot (5 enrichment tubes per exclosure). We replaced enrichment tubes every 12-16 weeks except for two periods during the COVID-19 pandemic when travel to Moorea was not possible and enrichment tubes were deployed for longer than usual before replacement (deployed from 01-30-2020 to 08-31-2020, and from 08-31-2020 to 02-08-2021). We analyzed water samples from the experimental plots to evaluate the effect of nutrient enrichment.

The treatments were arranged as in this example:

• Plot A1 was Intact (coral remained) with Ambient nutrients and contained one replicate of each of the exclosure treatments (Very Low, Low, Medium, and High consumer pressure).
• Plot A2 was Intact (coral remained) with Enriched nutrients and contained one replicate of each of the exclosure treatments (Very Low, Low, Medium, and High consumer pressure).
• Plot A3 was Disturbed (coral removed) with Enriched nutrients and contained one replicate of each of the exclosure treatments (Very Low, Low, Medium, and High consumer pressure).
• Plot A4 was Disturbed (coral removed) with Ambient nutrients and contained one replicate of each of the exclosure treatments (Very Low, Low, Medium, and High consumer pressure).

n=4 for each disturbance-nutrient-consumer pressure combination e.g., - Intact/Enriched/High consumer pressure; Disturbed/Ambient/Very Low consumer pressure etc. 

Data Collection

At three time points each year (April, July/August, November - except for two time points missed due to the COVID-19 pandemic, April 2020 and November 2020), we quantified benthic cover in each exclosure via point contacts on orthorectified photomosaics. Using Olympus TG cameras, we took ~64 individual photographs of each exclosure and then generated photomosaics by stitching photographs together using Agisoft Metashape software (Agisoft Metashape Professional Version 17.0). These photomosaics generate high-resolution imagery that allows identification of benthic space holders. Percent cover of benthic space holders was estimated via 225 random points per m² exclosure using CoralNet software (Beijbom et al. 2015) with the space holder underlying each of the 225 points identified to the lowest taxonomic level possible (usually genus for most macroalgae, corals, and other invertebrates). It was not possible to reliably distinguish among filamentous turf algae, bare space, and very small encrusting algae (including small CCAs and other encrusting algae) from photographs, therefore these taxa were categorized as one group. We used these data to generate percent cover estimates for each exclosure at each timepoint.