Dataset: Algal Percent Cover Data collected in the nearshore shallow subtidal during eight field experiments conducted at Cerro Mundo Bay in the Galapagos Islands between Sep 2021 and Apr 2023

We conducted a fully factorial 4x2 caging experiment (n = 8) from September 2021 to April 2023 in a nearshore rocky subtidal habitat at Cerro Mundo reef (0°52’06.0” S; 89°35’04.0” W), San Cristóbal Island, Galápagos. Sixty-four experimental cages were deployed at a depth of nine meters to simultaneously manipulate in situ grazer presence and nutrient availability across a natural temperature gradient. Cages were based on designs from Witman et al. (2017) and consisted of circular concrete platforms (diameter = 0.43 m, height = 0.06 m) with Aquamesh (plastic-coated, galvanized mesh, 0.05 m mesh size) incorporated as needed per cage design. Treatments provided varying degrees of herbivore exclusion: open plots with unrestricted grazer access; full exclusion cages preventing access by large herbivores (i.e., fish, urchins, turtles, and marine iguanas) but not mesograzers (e.g., amphipods, small gastropods); and grazer inclusion cages maintaining a constant density of a single urchin species.

Cage type was crossed with two nutrient levels: ambient and enriched. Nutrient enrichment was achieved by attaching two nutrient pillows (drawstring pouches, 0.001 m mesh size, 0.1 × 0.1 m) containing 50 g total of slow-release fertilizer (Osmocote, NPK 19-6-12, without micronutrients) to half of the cages.

Eight independent four-week trials were conducted during peak periods of the warm and cold seasons. To discern the effects of the treatments (cage type × nutrient level) on the productivity and dynamics of benthic macroalgae, percent cover data were collected at the end of each trial.

To determine percent cover, photographs of the substrate of the 64 cages were taken using a Nikon COOLPIX W300 camera. Percent macroalgal cover was quantified for each photograph in Adobe Illustrator (version 28.0) by superimposing a grid of 200 points and identifying the substrate beneath each point. The categories used included: Ulva spp., crustose coralline algae (CCA), brown foliose algae, haploid phase of macroalgae (HPM), red filamentous algae, Asparagopsis sp., Schizymenia sp., and "other" (including concrete, sand, fish, urchins, snails, and shells). The number of points classified as algae was summed and divided by two to obtain the total algal cover percentage.