Mary Margaret Stoll (^1,3)*, Robert Holmberg (¹⁾, Aaron Honig (^1,2), and Dr. Robyn Hannigan (^1)*

¹ School for the Environment, University of Massachusetts Boston, Boston, MA, 02125, USA
² Biology Department, University of Massachusetts Boston, Boston, MA, 02125, USA
³ Chemistry and Environmental Studies Departments, Amherst College, Amherst, MA, 01002, USA

Background
Ocean acidification is the process in which surplus atmospheric carbon dioxide (CO₂(g)) transfers across the ocean-atmosphere boundary and becomes CO₂ (aq). This process changes the carbonate system balance leading towards increased [H⁺] and decreased [CO₃^2-], ultimately causing increased acidity of the water. Furthermore, this changes the saturation state of carbonate minerals, shifting away from stability towards dissolution. As a result, carbonate biominerals such as those in bivalve shells become thermodynamically less stable and may dissolve or shift towards a more stable form.

Methods
We grew the juvenile blue mussel, Mytilus edulis, under different CO₂-induced low pH conditions to explore the effect of ocean acidification on growth. We used a pH-stat CO₂-dosing system designed for ocean acidification research with four replicates per treatment (n=4, control-outside room: pH=8.1, control: pH=8.1, treatment 3: pH=7.6, treatment 4: pH=7.3). We monitored carbonate chemistry parameters including pH, salinity, temperature, and total alkalinity. Juveniles were fed T-Isochrysis algae. We changed the water in each tank everyday and counted algal cells to estimate algal density.

Findings
At the end of the one-week exposure, we measured survivorship, shell length and width. We evaluated shell morphometrics using a Matlab script to determine circularity, area and perimeter. There was no difference in mussel growth between treatments (p>0.1). However, there were differences in the circularity of the mussel shells between treatments (p<0.1). Scanning electron microscopy and associated image analysis of the mussel shells indicated variations in the mineralogy and structure of the shell between treatments.

Conclusions
There was no difference in mussel growth between treatments. However, there were differences in the circularity of the shells between treatments. Analysis of the shell structure and mineralogy showed some impacts as well. Though this is a preliminary study, research is on-going to explore these impacts across life stages.