Ocean acidification (OA) is expected to have a major effect on the marine carbonate system and predicting the response of marine ecosystems is a central objective for management and protection of marine communities. While many experimental studies have explored the impact of OA on the physiological performance of marine calcifiers by using future predicted atmospheric CO2 levels to parameterize laboratory experiments, less appreciated is the natural environmental variation within these systems, particularly in terms of pH. This natural variation may inform laboratory experiments and improve our ability to predict responses to a changing ocean climate.

The central objective of this dissertation is to use an interdisciplinary approach to examine how marine larvae living in coastal regions may be impacted by future ocean acidification. This is accomplished using two distinct approaches: (1) organismal-level culturing experiments to test the thresholds of physiological response in larvae to altered seawater chemistry and (2) oceanographic observations of pH variation and water mass properties to identify ecosystem-specific patterns of natural variation.

Biochemical, morphometric, and gene expression-based tools were used to examine how early life history stages of two sea urchins (Strongylocentrotus purpuratus and S. droebachiensis) respond to elevated pCO2 conditions. These results suggest that developing larvae may either prioritize endogenous energy towards development and physiological function at the expense of growth, or that reduced larval length may be strictly due to higher costs of growth under OA conditions. In addition, the magnitude of physiological responses, as quantified by gene expression, may be influenced by the parental adult habitat.

Recent advances in pH sensor technology allowed our first glimpses at high-frequency pH variation in nearshore ecosystems and the ability to identify potential drivers of natural variation. Mooring-based observations in McMurdo Sound indicate that this ecosystem experiences low levels of diel pH variation (< 0.1 units) during austral Spring. Relatively greater pH variation was observed at shallow coastal sites as opposed to those in deeper water. This variation is likely not driven by water mass mixing, but may be due to photosynthesis by under ice algae in shallow coastal sites.