To test the impact of macroalgal species richness on coral performance, we manipulated macroalgal assemblages on the upper surface of 25 cm x 21 cm cement plots that each contained six Pocillopora verrucosa coral outplants (6 corals per plot, 72 plots, 432 corals total). Each plot was affixed to the substrate and elevated ~30 cm to prevent scour by sand and rubble. The upper surface of each plot contained a 4 x 3 grid space, with upturned soda bottle caps embedded within each space. To create a standardized coral population, 6–8 cm length branches of P. verrucosa were fragmented from colonies in situ and individually epoxied (Z-Spar A-788 Splash Zone Epoxy) into the cut-off necks of inverted plastic bottles (following methods of Clements and Hay 2015). Prior to outplanting, all corals and their epoxy/bottle-top base were wet-weighed in the field using an electronic scale (OHAUS Scout Pro) enclosed within a plastic container mounted to a tripod holding it above the water surface. Before weighing, each coral was gently shaken 30 times to remove excess water, and then weighed, immediately placed back into the water, and attached to their designated plot. Six corals that each originated from different P. verrucosa colonies were attached to plots by screwing the corals into bottle caps embedded within every other grid space within each plot.

We varied the macroalgal community on the upper surface of each plot by outplanting monocultures holding similar masses of: (1) Sargassum pacificum, (2) Turbinaria ornata, or (3) Amansia rhodantha, as well as (4) polycultures containing all three species (12 plots per treatment; Figure 1 of Clements et al., 2024). There were also two additional treatments: (5) plots with plastic algal mimics to control for effects of shading or abrasion unrelated to the biotic properties of live seaweeds and 6) control plots where corals, but not living algae or physical mimics, were present (n = 12 plots per treatment). 

To create six standardized units of macroalgae or algal mimics within every other space of the 4 x 3 grid of each plot, cable ties were first individually epoxied into the cut-off necks of inverted plastic bottles that were screwed into bottle caps embedded within the plots. An additional cable tie was then used to attach either Sargassum (two ~6-8 cm thalli), Turbinaria (two ~6-8 cm thalli), Amansia (two ~6-8 cm thalli), or algal mimics (two ~6 cm plastic aquarium plants) to the cable tie embedded in each soda bottle neck. Standardized units used in control plots only had a cable tie attached to the embedded cable tie. In polycultures, units of Sargassum, Turbinaria, and Amansia (2 units/species/plot) were distributed to ensure that conspecifics were not located adjacent to one another within the grid space of the plot and so that the mass of each species was approximately the same (Figure 1 of Clements et al., 2024). Any macroalgae or algal mimics displaced from their cable tie (e.g., because of wave action or herbivory) were replaced every 1-2 days, and all mimics were replaced weekly to minimize the influence of fouling organisms (e.g., diatoms, cyanobacteria) that might produce chemical or biological interactions confounding the physical effects these biologically inert "controls" are meant to produce. The different treatments were interspersed randomly to prevent confounding spatial effects with treatment effects.

The percentage growth and tissue mortality of individual corals in each plot were assessed at 9 weeks (experiment initiated on 30 May with treatment effects assessed on 7 August 2021). Each coral was visually examined from all sides and the percentage tissue mortality was estimated and assigned in 10% classes (0, 10, 20%, etc., up to 100%). Twenty-four to 48 hours before the second weighing session, each coral’s epoxy/bottle-top base was brushed clean of fouling organisms. To assess growth, corals were detached from the substratum, reweighed in the field as previously described to determine the percentage change in mass, and immediately placed back into the water and reattached to their respective bottle cap.

Taxonomic name, LifeSciences Identifier (LSID): 
Pocillopora verrucosa, urn:lsid:marinespecies.org:taxname:206954
Sargassum pacificum, urn:lsid:marinespecies.org:taxname:495052
Turbinaria ornata, urn:lsid:marinespecies.org:taxname:221490
Amansia rhodantha, urn:lsid:marinespecies.org:taxname:213667