Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • BCO-DMO Processing Description
• Data Files
• Related Publications
• Related Datasets
• Parameters
• Instruments
• Project Information
• Funding

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. 

- Imported "Data_percent_cover.xlsx"
- Replaced spaces in parameter names with underscores
- Removed existing computed "Sum" value, due to REF error in Excel file
- Created new "Sum" field combine all the identification values
- Added "pct" fields for all the identification values, where values were divided by the sum, in coordination with the submitter
- Exported file as "985410_v1_algal_percent_cover.csv"

NSF Award Abstract:
A well-known pattern in coastal marine systems is a positive association between the biomass of primary producers and the occurrence or intensity of upwelling. This is assumed to be caused by the increase in nutrient concentration associated with upwelling, enabling higher primary production and thus greater standing algal biomass. However, upwelling also causes large, rapid declines in water temperature. Because the metabolism of fish and invertebrate herbivores is temperature-dependent, cooler upwelled water could reduce consumer metabolism and grazing intensity. This could in turn lead to increased standing algal biomass. Thus upwelling could influence both bottom-up and top-down control of populations and communities of primary producers. The purpose of this study is to test the hypothesis that grazing intensity and algal biomass are, in part, regulated by temperature via the temperature-dependence of metabolic rates. Broader impacts include the training and retention of minority students through UNC's Course Based Undergraduate Research program, support of undergraduate research, teacher training, and various outreach activities.

The investigators will take advantage of the uniquely strong spatiotemporal variance in water temperature in the Galápagos Islands to compare grazing intensity and primary production across a natural temperature gradient. They will combine field monitoring, statistical modeling, grazing assays, populations-specific metabolic measurements, and in situ herbivore exclusion and nutrient addition to measure the effects of temperature on pattern and process in shallow subtidal communities. The researchers will also test the hypothesis that grazer populations at warmer sites and/or during warmer seasons are less thermally sensitive, potentially due to acclimatization or adaptation. Finally, the investigators will perform a series of mesocosm experiments to measure the effect of near-future temperatures on herbivores, algae, and herbivory. This work could change the way we view upwelling systems, particularly how primary production is regulated and the temperature-dependence of energy transfer across trophic levels.