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

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.