Potential Shell Hash Mitigation of Coastal Acidification for Juvenile Oysters Reared in Upwellers

Decreased pH in nearshore seawater due to increased CO₂ in the atmosphere and other land-sea interface processes, termed ocean and coastal acidification (OCA), is harmful to the development, growth, and survival of the larvae of many bivalve species. In the aquaculture industry, oyster hatcheries on either U.S. coast are already implementing seawater buffer treatments to mitigate these OCA impacts. But, we know little about the vulnerability of older, juvenile stages of oysters, grown-out in untreated upwelling systems, for which there is currently no mitigation strategy. Here we use a controlled experiment exposing juvenile oysters to ‘acidified’ seawater to test the mitigation potential of shell hash, finely ground shell by-product (500 μm) and determine whether hash can raise seawater Ω and oyster growth in miniature upweller mimics. Juveniles were subjected to four treatments that naturally varied in pH levels: T1= an ambient pH control, T2= -0.2 pH below the ambient, T3= -0.4 pH, and T4= -0.4 + 500 μm hash. We measured calcification rates, tissue weight and shell height weekly over 5 weeks, as well as water quality. Saturation state in T4 was slightly raised relative to T3 by dissolution of shell hash at an average rate of 0.12 g/day. Calcification rates in T3 and T4 were lower than T1 and T2 during weeks 3, 4 and 5, though T4 had greater average shell weight than T3 in week 4, but not in week 5. T4 also had lower tissue weights than all other treatments, except T3 in the later weeks. Chlorophyll a concentration in the outflow monotonically increased across treatments, indicating lower consumption rates at higher acidity. Our data suggest 500 μm shell hash may buffer ‘acidified’ seawater and increase calcification rates for early juvenile stages, but not past 4 weeks, and not equal to present day conditions. More research is needed to refine hash delivery mechanisms to maximize buffering potential and minimize impediments to food delivery in upwelling systems.