Dataset: Macroalgal biomass data collected in the nearshore shallow subtidal during eight field experiments conducted at Cerro Mundo Bay in the Galapagos Islands between Jul 2021 and Mar 2023

We conducted a fully factorial 4x2 caging experiment (n = 8) from September 2021 to April 2023 in a nearshore rocky subtidal habitat (~10 meters deep) 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 each cage. 

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, biomass data were collected at the end of each trial. 

To obtain biomass data, macroalgae accumulated on each experimental unit were scraped and vacuumed into independent mesh bags (0.12 m × 0.18 m, 400 µm mesh size). Samples were transported to the Marine Ecology laboratory at the Galapagos Science Center, where they were dried at 60 °C for 24 hours and subsequently weighed to obtain dry weight as a proxy for biomass.

For trials between July 2021 and May 2022, ash-free dry weight (AFDW) was also determined. This was calculated as the difference between preburn dry weight and postburn weight, the latter obtained by combusting samples in a muffle furnace for 4 hours at 500 °C after the initial 24-hour drying step at 60 °C.