Ocean acidification (OA) and harmful algal blooms (HABs) are two significant threats for bivalves in coastal zones. While the open ocean will acidify over the coming decades, a multitude of coastal processes including eutrophication, upwelling, and river discharge can cause ephemeral and seasonal bouts of coastal ocean acidification that can create levels of pH and pCO2 that are not expected in open waters until late next century. In parallel, the impacts of HABs on aquaculture have been expansive in recent decades and can range from causing slowed growth and mortality to contaminating bivalves with biotoxins and thereby restricting sale of product. While there have been decades of study regarding the mitigation of HABs and OA, approaches investigated to date have had negative consequences for bivalves and/or ecosystems, work at temporal or spatial scales irrelevant to bivalve aquaculture, or have simply been ineffective. One exception has been the aquaculture of seaweeds. During the past decade, our group has documented the ability of temperate seaweeds including sugar kelp (Saccharina latissima) , Irish Moss (Chondrus crispus) , Gracilaria tikvahiae , Porphyra spp., and Ulva spp., to mitigate both ocean acidification and HABs.
Regarding ocean acidification, faster growing seaweeds were found to be most impactful in altering carbonate chemistry to the benefit of bivalves. Laboratory studies with S. latissima and Ulva spp. grown at aquaculture densities (0.1 - 3 g L-1) have documented their ability to rapidly increase the pH , alkalinity, and saturation state of calcium carbonate (?) while lowing pCO2 concentrations, primarily due to photosynthetic activity and secondarily due to nitrate assimilation. While experimentally elevated levels of pCO2 and lreduced ? significantly (p<0.05) reduced the growth rates of commonly cultured bivalves (hard clams , Mercenaria mercenaria; eastern oysters , Crassostrea virginica; bay scallop s, Argopecten irradians, and blue mussels, Mytilus edulis), the co-culture of these bivalves with Ulva spp. (all bivalves ) or S. latissima (C. virginica, M. edulis, M. mercenaria) under the same CO2 delivery rates ‘rescued ’ the bivalves from OA, yielding growth rates identical to control conditions. Beyond the lab, deployment of C. virginica with sugar kelp on an oyster farm experiencing acidification resulted in growth rates that were significantly faster than oysters grown in the same location without kelp. These results suggest that that the aquaculture of macroalgae in acidified environments can serve as a refuge for calcifying bivalves that may otherwise be negatively impacted by OA.
Regarding HABs, all seaweeds were shown to significantly reduce the densities of different HABs primarily due to the release of allelochemicals by seaweeds and secondarily due to nutrient removal and/or pH elevation. Of greatest significance has been the ability of S. latissima and Ulva spp. to cause lysis of the saxitoxin-producing dinoflagellate, Alexandrium catenella , and in turn, reduce the accumulation of saxitoxin in co-cultured blue mussels to levels below the USFDA closure limit. Collectively, these studies demonstrate that the co-culture of bivalves with seaweeds can create a ‘halo effect’ around bivalves to mitigate the harmful effects of OA and HABs.