Without mitigation strategies, ocean acidification lessens U.S. shellfisheries profitability by weakening shells and byssal threads and disrupting larval development. Cultivated seaweed can raise seawater pH in the immediate vicinity of a sheltered coastal seafarm, but this remediation capacity is unclear when considering a warmer and more acidic ocean. We performed a two-factorial tank experiment with contemporary conditions (~12±2 °C, 400±100 µatm pCO2) and projected conditions (+4°C, +600 µatm pCO2) for the Gulf of Maine. Within these treatments, blue mussels were grown with (~150g) and without (0 g) sugar kelp, Saccharina latissima for 50 days (n=6 tanks). Custom 35L flow-through tanks (1.5L/min) each housed 10 juvenile mussels (~2-4 cm shell length) and ~100 spat (0.5 cm). Reported are resultant seawater carbonate chemistry conditions (e.g., total alkalinity and dissolved inorganic carbon), mussel fitness (growth, shell morphometrics, meat mass) and kelp growth (wet weight, linear extension). Shell thickness for juvenile mussels increased 1.62 to 5.84% when co-cultivated with kelp in both ambient and future conditions and meat mass increased 6.47% in future conditions. Only meat mass of spat increased 14.72% with kelp, and only in future conditions. Gut contents are currently being analyzed with recently developed qPCR primers (COI gene) to quantify gene copy numbers of S. latissima therein to identify mechanisms of kelp influence on mussel productivity, be they a result of phytoremediation of CO2-driven acidification or kelp detritus feed subsidies. Kelp-mussel co-cultivation may improve mussel fitness now, and in the future, with variable impacts across mussel size classes.