Oregon Sea Grant

As the effect of ocean acidification (OA) on marine calcifiers is better understood, a range of potential mitigative strategies have been proposed, many of which are plagued by concerns of scale and feasibility. One oft-cited option is to increase the biomass of photosynthetic organisms to remove CO2 from the water column and facilitate organic carbon burial. Seagrasses show much promise in this regard, owing to their highly refractive tissue. Timescales of carbon burial with respect to this strategy are on the order of years to decades. Recent studies, however, demonstrate that some marine bivalves experience short windows of heightened sensitivity to OA, especially during the early larval and early postmetamorphic “juvenile” stages, which occur on timescales of hours to days. In coastal areas, carbonate chemistry is highly variable on similar timescales, due in part to photosynthetic cycles, the pattern and magnitude of which will vary due to the ecological make-up of the habitat. Therefore, we must consider the highly complex and variable nature of CO2 dynamics in seagrass habitats on short timescales when we consider their potential role in OA mitigation. We examined patterns of growth and survival in the juvenile Pacific Oyster (Crassostrea gigas) outplanted within and outside beds of two different species of seagrass, the native Zostera marina and the non-native Z. japonica, in Netarts Bay, Oregon. Z. marina and Z. japonica differ in the timing and magnitude of their growth and decay cycles and allocation of biomass above or below ground. Z. marina increased both growth and survival of C. gigas spat with the magnitude of the effects decreasing after mid-​season as seagrass growth slows and there is a transition to an upwelling-​dominant hydrodynamic regime. Z. japonica appeared to have a slightly negative, but not statistically significant, effect on juvenile C. gigas growth, compared to the associated bare-sand control site, with the notable exception of a reversal of the trend in June, co-incident with this seagrass’s short period of growth. We have compared bivalve success metrics (growth and survival) with PCO2 measurements within each habitat. Patterns in bivalve success metrics appear to inversely correlate with trends in daily CO2 minima over the course of the season, suggesting that bivalve success may be attributed to compensatory growth during daily low- CO2 periods associated with the seagrass’s respective growing seasons.