Increased standing macroalgal biomass in upwelling zones is generally assumed to result from higher nutrient flux associated with upwelled waters. However, other factors can also strongly impact macroalgal communities. For example, herbivory and temperature—through their effects on primary producers and the metabolic demands of consumers—can influence macroalgal biomass and productivity, respectively. Although there are numerous studies examining the interactive effects of herbivores and nutrients in both temperate regions, few have addressed these dynamics in tropical or subtropical upwelling systems. The purpose of this study was to assess the effects of herbivores, temperature, and nutrient availability on standing macroalgal biomass and total algal cover. Total algal cover is presented in the related dataset. We manipulated nutrient availability and herbivory in eight field experiments conducted under contrasting productivity and thermal regimes (cool upwelling vs. warm, non-upwelling seasons) on a subtidal nearshore rocky reef. Here, we present the raw macroalgal biomass data (dry weight in grams) collected from the shallow subtidal zone during September 2021 to April 2023 field experiments.

Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • 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 (~9-10 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 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, biomass data was 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 September 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.

Note:
In the data file, 0 indicates there was no growth of macroalgae for that specific replicate. NA indicates that the macroalgae sample was not recovered at the end of the trial (e.g. collection mesh bag got lost).

BCO-DMO processing replaces NA with blank values.

Version 1
- Imported original file "AFDW.xlsx" into the BCO-DMO system.
- Concatenated the data from separate Excel sheets (one per trial) into one data file.
- Renamed fields to comply with BCO-DMO naming conventions.
- Saved the final file as "algal_ash-free_dry_weight_cerro_mundo.csv".

Version 2
- Imported "Biomass data Sept-22.xlsx" into BCO-DMO system
- Imported original published dataset file "algal_ash-free_dry_weight_cerro_mundo.csv" into BCO-DMO system
- Concatenated the files, mapping "Dry_Preburn_weight" to "Preburn_weight"
- Renamed "Preburn_weight" to "Dry_weight"
- Exported file as 894169_v2_algal_ash-free_dry_weight.csv

**Added dry weight, or preburn weight, as biomass data for 2023 trials. Adjusted extents. Adjusted the language of the dataset to focus on biomass, rather than AFDW, which was not measured in the later trials. Adjusted abstract, methods, and parameter language as submitted. Added scale to list of instruments. Changed title to include Apr 2023.

Website: http://github.com/johnfbruno/Galapagos_NSF.git

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.