Dataset: Algal cover from herbivore exclusion and nutrient enrichment experiments conducted in the Florida Keys National Marine Sanctuary during 2009-2012

Natural history of the study site:
This experiment was conducted in the area of Pickles Reef (24.99430, -80.40650), located east of Key Largo, Florida in the United States. The Florida Keys reef tract consists of a large bank reef system located approximately 8 km offshore of the Florida Keys, USA, and paralleling the island chain. Our study reef is a 5-6 m deep spur and groove reef system within this reef tract. The reefs of the Florida Keys have robust herbivorous fish populations and are relatively oligotrophic. Coral cover on most reefs in the Florida Keys, including our site, is 5-10%, while macroalgal cover averages ~15%, but ranges from 0-70% depending on location and season. Parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) are the dominant herbivores on these reefs as fishing for them was banned in 1981. The other important herbivore on Caribbean reefs, the urchin Diadema antillarum, remains at low densities across the Florida Keys following the mass mortality event in 1982-3

Exclosure and nutrient enrichment experimental design details
In order to simulate the effects of overfishing, nutrient loading, or the combination of these stressors, we conducted a three-year field experiment. Four pairs of 9m² plots were established. One member of each of these pairs was enriched with nitrogen and phosphorous, while the other remained at ambient nutrient levels. These plots were >10 m from each other in all cases. Each 9 m² plot was delineated into nine 1 m² subplots with metal nails driven into the reef at the corners and center of each plot. The locations of the plots were selected such that initial variation in rugosity and algal cover within each subplot was minimal. Within each plot, two randomly-selected subplots were enclosed with herbivore exclosures, while two other random subplots were selected as exclosure controls. Exclosure controls were fitted with open-topped exclosures. These controls allowed access by herbivorous fishes but acted as controls for other potential artifacts of the cages.

All exclosures were made of plastic-coated wire mesh with 2.5 cm diameter holes. This diameter mesh generally excludes most fishes >10 cm total length. Smaller or juvenile herbivorous fishes are able to enter the exclosures, but these smaller herbivores generally contribute little to overall grazing rates on reefs and have minimal impacts on the algal communities. Additionally, access by smaller herbivores reflects patterns seen under intensive fishing, in which larger fish species are preferentially harvested while leaving smaller size classes of fish. We scrubbed both exclosures and exclosure controls every 4-6 weeks to remove fouling organisms.

Nutrient pollution was simulated using slow-release fertilizer diffusers applied to each nutrient enrichment plot. Each diffuser was a 15 cm diameter PVC tube, perforated with six 1.5 cm holes. The open ends of the PVC tube were wrapped in fine plastic mesh to keep fertilizer pellets inside, but allow diffusion of soluble nutrients. 175 g of Osmocote^® (19-6-12, N-P-K) slow-release fertilizer was loaded into each diffuser. PVC enrichment tubes were attached to each metal nail within the 9m² enrichment plots for a total of 25 enrichment tubes per enrichment plot. Nutrients were replaced every 30-40 days to ensure continued delivery of N and P. Previous studies have shown Osmocote delivery using similar methods to be an effective way of enriching water column nutrients in benthic systems.

Quantification of Benthic Cover
At least once every season (e.g. Spring, Summer, Fall, Winter at 12-14 week intervals), we visually quantified benthic cover within four, 50 cm X 50 cm quadrats in each of the 1 m² treatment areas. These quadrats were divided into 49 points, and benthic organisms under each point were identified to species or genus. Algae that are challenging to identify taxonomically under field conditions (e.g. crustose coralline algae, filamentous algae) were classified into algal functional groups (e.g. 52). Filamentous algae were classified into short algal turf (< 0.5 cm in height) or algal turf (> 0.5 cm in height) given that taller, thicker algal turf can often be deleterious to coral health and growth.

Benthic cover was quantified in June 2009 one week before treatments were initiated to provide a baseline from which to assess changes in algal abundance and community structure. No significant differences among treatments in algal abundance could be detected at the beginning of the experiment, as expected given random assignment of subplots to treatment conditions. Further, during the summer of each year (2009-2012) when algal cover was often at its highest, we also surveyed open areas of reef (areas that did not have three-sided exclosure controls) with the 9 m² plots to assess whether the exclosure controls had any effect on algal abundance or community composition. We did not detect any differences in algal abundance or community composition between the open unmanipulated areas and exclosure controls.