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Coupled changes in pH, temperature and dissolved oxygen impact the physiology and ecology of herbivorous kelp forest grazers

Citation

Donham, Emily; Strope, Lauren; Hamilton, Scott; Kroeker, Kristy (2022), Coupled changes in pH, temperature and dissolved oxygen impact the physiology and ecology of herbivorous kelp forest grazers, Dryad, Dataset, https://doi.org/10.5061/dryad.8sf7m0cq7

Abstract

Understanding species’ responses to upwelling may be especially important in light of ongoing environmental change. Upwelling frequency and intensity are expected to increase in the future, while ocean acidification and deoxygenation are expected to decrease the pH and dissolved oxygen of upwelled waters. However, the acute effects of a single upwelling event and the integrated effects of multiple upwelling events on marine organisms are poorly understood. Here, we use in situ measurements of pH, temperature, and dissolved oxygen to characterize the covariance of environmental conditions within upwelling-dominated kelp forest ecosystems. We then test the effects of acute (0-3 days) and chronic (1-3 month) upwelling on the performance of two species of kelp forest grazers, the echinoderm, Mesocentrotus franciscanus, and the gastropod, Promartynia pulligo. We exposed organisms to static conditions in a regression design to determine the shape of the relationship between upwelling and performance and provide insights into the potential effects in a variable environment. We found that respiration, grazing, growth, and net calcification decline linearly with increasing upwelling intensity for M. francicanus over both acute and chronic timescales. Promartynia pulligo exhibited decreased respiration, grazing, and net calcification with increased upwelling intensity after chronic exposure, but we did not detect an effect over acute timescales or on growth after chronic exposure. Given the highly correlated nature of pH, temperature, and dissolved oxygen in the California Current, our results suggest the relationship between upwelling intensity and growth in the 3-month trial could potentially be used to estimate growth integrated over long-term dynamic oceanographic conditions for M. franciscanus. Together, these results indicate current exposure to upwelling may reduce species performance and predicted future increases in upwelling frequency and intensity could affect ecosystem function by modifying the ecological roles of key species.

Methods

Biological data were collected during Spring 2019 (Experiment 1) and Summer 2020 (Experiment 2). During each experiment, red sea urchins, Mesocentrotus franciscanus, and brown turban snails, Tegula pulligo, were reared across a gradient of upwelling conditions in a laboratory mesocosm system. Within each replicate treatment aquarium, we placed sea urchins and gastropods (N = 4-8 and N = 9-10 respectively) in individual 0.5 L cages where they were fed ad libitum for the duration of the experiment (except during respirometry and grazing trials). For experiment 1, at the beginning and after one month, two months and three months in treatment conditions, we measured wet and buoyant weight. After one, two and three months we measured respiration rate and per capita grazing rate of all individuals. For experiment 2, we measured respiration rate and per capita grazing rate of all individuals immediately after being placed in experimental conditions and after 48 hours.

Chemistry data were collected during two mesocosm experiments testing the effects of (1: chronic, 2: acute) upwelling on kelp forest grazers. These data were used to characterize the carbonate chemistry system in experimental treatments. Discrete samples for carbonate chemistry analyses were collected approximately every two weeks for the duration of both experiments using best practices for ocean CO2 measurements. Associated salinity, temperature and dissolved oxygen measurements were made within an hour of discrete sample collection.