Fluctuating pH simulating natural variability modulates larval growth for Strongylocentrotus droebachiensis

Abstract

Ocean acidification (OA) is the downward trend of ocean pH mainly resulting from the absorption of carbon dioxide (CO2) emissions from anthropogenic sources. pH of the open ocean is expected to drop to 7.7 by the end of the century. Previous experiments investigating biological response to OA often use this open ocean prediction to assess response and neglect the potential modulating effects of a dynamic, fluctuating coastal ecosystem. pH in the Gullmar Fjord on the west coast of Sweden experiences natural fluctuations of pH as a result of biological processes that exceed the end-of-the century predictions, and as such these natural fluctuations need to be considered. The aim of this study was to investigate which part of the natural variability cycle, minimum pH experienced or duration of exposure under fluctuating conditions, drives the biological response of green sea urchin larvae (Strongylocentrotus droebachiensis), which are residents in the Gullmar Fjord. It was hypothesized that both intensity and duration of different pH exposures contributes to the stress experienced by an organism and further that (1) the level of stress is dependent on both intensity and duration of exposure in a cumulative manner (intensity*time); (2) for a given intensity, the negative effect on sea urchin larvae will increase with the duration of exposure; and (3) for a given duration of exposure, the negative effect on sea urchin larvae will increase with the intensity. The main results were that overall fluctuating conditions were beneficial to growth relative to constant conditions, intensity*time predicted stress response for body length growth, ignoring the role of modulating effects on pH can overestimate biological response to OA, and an overall change in shape was observed under fluctuating conditions. Future studies should further investigate this change in shape and also assess biological response in the context of natural fluctuations combined with other global change stressors.