Isabel Silva 1,2, Margarita Brandt2,3, Esteban Agudo-Adriani1,2, Nicole Chico-Ortiz, John Bruno1,2
1University of North Carolina, USA - 2Galapagos Science Center, Universidad San Francisco de Quito USFQ & University of North Carolina, Ecuador - 3Universidad San Francisco de Quito USFQ, Ecuador
It is generally assumed that oceanographic processes are the main drivers of primary production on coastal nearshore habitats. In upwelling zones, higher nutrient flux is associated with increased macroalgal biomass, exerting a bottom-up control of communities. This increase in primary producers can also result from cooler temperatures lessening the metabolic demand of ectothermic consumers (i.e., weakening grazing). Thus, upwelling influences primary producers due to a simultaneous enhancement of nutrients and a relaxation of top-down control. Conversely, increased ocean temperature can accelerate the metabolism of ectotherms promoting top-down forcing from consumers and boosting the photosynthetic activity of primary producers. These ideas have been explored broadly at higher latitudes and temperate regions, yet what makes the highest contribution to benthic primary production remains untested in tropical upwelling systems. The purpose of this study was to quantify the relative and interactive effects that herbivores, temperature, and nutrients have on macroalgal biomass and cover in the Galapagos rocky subtidal. We conducted field trials from July 2021 to March 2023 and manipulated the presence of herbivores and nutrient availability across seasons and upwelling periods. We found that herbivore treatment and upwelling significantly influenced macroalgal biomass. Across trials, biomass accumulation, measured in grams per day, was highest in the herbivore exclusion treatment, followed by open plots. The lowest values were observed in the urchin inclusion treatment. Overall, biomass was greater during upwelling periods when compared to non-upwelling periods. At the subtidal level, the importance of grazing by macroherbivores surpasses and is not affected by nutrient flux, contrary to the traditional assumption that benthic primary production is largely controlled and limited by nutrients. Algal composition differed between treatments with Ulva sp. and CCA being the most abundant algal groups that developed in our experimental units. Our results suggest that, even on the highly productive Galapagos upwelling system, consumption by herbivores seems to be the most important driver on seaweed assemblages, exerting a strong top-down control and modulating ocean productivity.
Biography
Isabel is a graduate student at Dr. John Bruno’s lab and her focus lies in marine community ecology. Her research focuses on how abiotic environmental factors, such as upwelling and temperature, impact species interactions, biodiversity, and food webs — particularly in the context of herbivores as consumers and seaweed as primary producers. She conducts fieldwork in the Galapagos archipelago and uses the shallow rocky subtidal as a study system.