Oysters and eelgrass: potential partners in a high <scp>pCO</scp>₂ ocean

Abstract Climate change is affecting the health and physiology of marine organisms and altering species interactions. Ocean acidification ( OA ) threatens calcifying organisms such as the Pacific oyster, Crassostrea gigas . In contrast, seagrasses, such as the eelgrass Zostera marina , can benefit from the increase in available carbon for photosynthesis found at a lower seawater pH . Seagrasses can remove dissolved inorganic carbon from OA environments, creating local daytime pH refugia. Pacific oysters may improve the health of eelgrass by filtering out pathogens such as Labyrinthula zosterae ( LZ ), which causes eelgrass wasting disease ( EWD ). We examined how co‐culture of eelgrass ramets and juvenile oysters affected the health and growth of eelgrass and the mass of oysters under different pCO 2 exposures. In Phase I, each species was cultured alone or in co‐culture at 12 ° C across ambient, medium, and high pCO 2 conditions, (656, 1,158 and 1,606 μatm pCO 2 , respectively). Under high pCO 2 , eelgrass grew faster and had less severe EWD (contracted in the field prior to the experiment). Co‐culture with oysters also reduced the severity of EWD . While the presence of eelgrass decreased daytime pCO 2 , this reduction was not substantial enough to ameliorate the negative impact of high pCO 2 on oyster mass. In Phase II , eelgrass alone or oysters and eelgrass in co‐culture were held at 15 ° C under ambient and high pCO 2 conditions, (488 and 2,013 μatm pCO 2 , respectively). Half of the replicates were challenged with cultured LZ . Concentrations of defensive compounds in eelgrass (total phenolics and tannins), were altered by LZ exposure and pCO 2 treatments. Greater pathogen loads and increased EWD severity were detected in LZ exposed eelgrass ramets; EWD severity was reduced at high relative to low pCO 2 . Oyster presence did not influence pathogen load or EWD severity; high LZ concentrations in experimental treatments may have masked the effect of this treatment. Collectively, these results indicate that, when exposed to natural concentrations of LZ under high pCO 2 conditions, eelgrass can benefit from co‐culture with oysters. Further experimentation is necessary to quantify how oysters may benefit from co‐culture with eelgrass, examine these interactions in the field and quantify context‐dependency.

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