Factorial field manipulation reveals multi-stressor effects on a critical habitat-forming shellfish

Ocean acidification and warming are expected to have negative impacts on marine organisms, and particularly on shell-building species. Acidification and warming may operate independently or interactively, amplifying or mitigating impacts on individuals. Previous results have primarily come from lab studies of single species, yet these climate stressors are occurring within naturally dynamic systems with high abiotic and biotic variability. As a result, the impacts of these stressors in situ remains poorly understood. We conducted a 6-month field manipulation to determine the effects of ocean acidification and warming on a habitat-forming shellfish, the Pacific blue mussel (Mytilus trossulus), in a dynamic coastal system. Twenty tide pools were factorially manipulated, including unmanipulated control, CO2 added, warmed, and combined CO2 added and warmed treatments. We measured mussel shell thickness, strength, and corrosion at 0, 3, and 6 months of exposure to treatment conditions. CO2 addition led to a decrease in shell thickness and strength and an increase in shell corrosion. However, we also detected an increase in shell strength for mussels exposed to both CO2 addition and warming. These findings indicate that ocean acidification negatively impacted shellfish in situ and that these effects might be mitigated when exposed concurrently to moderate warming, leading to an interactive effect of acidification and warming on this critical habitat-forming shellfish.

Methods
Temperature Data:

Temperature data was collected at the tide pool level using Onset ® HOBO TidbiT temperature loggers (±0.2 accuracy) that recorded temperature consecutively every 5 min from March to September 2019. Temperature data were summarized for the 28-day periods preceding each collection period (March, July, and September). For each tidepool, we calculated the following thermal parameters: minimum, mean daily minimum, 10th percentile, median, average, range, mean daily range, mean daily average, maximum, mean daily maximum, 90th percentile, mean daily 90th percentile, 95th percentile, 99th percentile, mean daily 90th percentile temperatures, mean daily 95th percentile temperatures, mean daily 99th percentile temperatures. All temperature data was separated into low day and nighttime tides based on known shore heights and observed tide data from  Sitka Harbor, Baranof Island, Sitka, Alaska (tbone tides; https://tide.arthroinfo.org/).

Environmental Tide Pool Data: 

Tide pool parameters were collected using handheld instruments and total alkalinity was determined at UCSC.

Shell Trait Data:

All shell trait data were measured within the laboratory.

Carbonate Chemistry Variables:

pCO2, DIC, and saturation states of calcite and aragonite were calculated using the R package seacarb.