Sampling and analytical procedures:

To determine the effect of current and future seasonal differences in carbonate chemistry on abalone bioenergetics, we exposed juvenile, non-reproductive (36 ± 5 mm) H. kamschatkana to three pH/pCO2 levels (i.e., pHT 8.1, 7.8, 7.5) for four months in a flow-through system at the Sitka Sound Science Center (SSSC) from June-October 2017. We focused on abalone that were close to transitioning to reproductive maturity (~50 mm; Busch et al., 2014 and references therein) so that we could best capture energetic trade-offs during an energetically costly life history period. The four-month long exposure to stable pH conditions was meant to capture seasonal exposures to relatively constant carbonate chemistry conditions during summer and winter that was demonstrated by our environmental monitoring. The pH values were chosen to capture the current seasonal fluctuations in Sitka Sound (7.8-8.2 pHT for winter-summer, respectively) as well as future projections for winter based on RCP 8.5 (i.e., 7.8 – 0.3 = 7.5 pHT; (Mathis et al., 2015)). The temperature and salinity were not manipulated and reflected the seasonal conditions in Sitka Sound from June-October.

At the end of the experiment, we measured the consumption rate and wet weight for the individual abalone. The abalone were starved for two days. After this, half of the abalone in each aquarium were assigned a M. pyrifera only diet, and the other half were assigned a mixed algal diet. Each individual was given four equal sized, pre-weighed discs/pieces of either (1) M. pyrifera, or (2) M. pyrifera, H. nigripes, A. clathratum, and C. ruprechtiana collected from the field just prior to the assay. The trials were stopped before the resources were exhausted, after which the time was noted and the algae were reweighed. We also included resource controls (macroalgae without abalone) to determine if there were any losses of algal biomass not associated with consumption. To account for differences in abalone sizes on grazing rates, mass was included as a covariate in the analysis (see below). At the end of the experiment, we assessed growth via measurements of wet weight.