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Macroalgae drive the largest CO2 flux fixed globally by marine macrophytes. Most of the resulting biomass is exported through the coastal ocean as detritus and yet almost no field measurements have verified its potential net sequestration in marine sediments. This gap limits the scope for the inclusion of macroalgae within blue carbon schemes that support ocean carbon sequestration globally, and the understanding of the role their carbon plays within distal food webs. Here, we pursued three lines of evidence (eDNA sequencing, Bayesian Stable Isotope Mixing Modeling, and benthic‐pelagic process measurements) to generate needed, novel data addressing this gap. To this end, a 13‐month study was undertaken at a deep coastal sedimentary site in the English Channel, and the surrounding shoreline of Plymouth, UK. The eDNA sequencing indicated that detritus from most macroalgae in surrounding shores occurs within deep, coastal sediments, with detritus supply reflecting the seasonal ecology of individual species. Bayesian stable isotope mixing modeling [C and N] highlighted its vital role in supporting the deep coastal benthic food web (22–36% of diets), especially when other resources are seasonally low. The magnitude of detritus uptake within the food web and sediments varies seasonally, with an average net sedimentary organic macroalgal carbon sequestration of 8.75 g C·m−2·yr−1. The average net sequestration of particulate organic carbon in sediments is 58.74 g C·m−2·yr−1, the two rates corresponding to 4–5% and 26–37% of those associated with mangroves, salt marshes, and seagrass beds, systems more readily identified as blue carbon habitats. These novel data provide important first estimates that help to contextualize the importance of macroalgal‐sedimentary connectivity for deep coastal food webs, and measured fluxes help constrain its role within global blue carbon that can support policy development. At a time when climate change mitigation is at the foreground of environmental policy development, embracing the full potential of the ocean in supporting climate regulation via CO2 sequestration is a necessity.

Abstract. The dissolution of anthropogenically emitted excess carbon dioxide lowers the pH of the world's ocean water. The larvae of mass spawning marine fishes may be particularly vulnerable to such ocean acidification (OA), yet the generality of earlier results is unclear. Here we show the detrimental effects of OA on the development of a commercially important fish species, the Atlantic herring (Clupea harengus). Larvae were reared at three levels of CO 2 : today (0.0385 kPa), end of next century (0.183 kPa), and a coastal upwelling scenario (0.426 kPa), under near-natural conditions in large outdoor tanks. Exposure to elevated CO 2 levels resulted in stunted growth and development, decreased condition, and severe tissue damage in many organs, with the degree of damage increasing with CO 2 concentration. This complements earlier studies of OA on Atlantic cod larvae that revealed similar organ damage but at increased growth rates and no effect on condition.

The impact of CO 2 -acidified seawater (pH 7.8, 7.6, or 6.5, control = pH 8) on the health of Mytilus edulis was investigated during a 60 d mesocosm experiment. Mussel health was determined using the neutral red retention (NRR) assay for lysosomal membrane stability and from histopathological analysis of reproductive, digestive and respiratory tissues. Seawater acidification was shown to significantly reduce mussel health as measured by the NRR assay, and it is suggested that this impact is due to elevated levels of calcium ions (Ca 2+ ) in the haemolymph, generated by the dissolution of the mussels' calcium carbonate shells. No impact on tissue structures was observed, and it is concluded that M. edulis possess strong physiological mechanisms by which they are able to protect body tissues against short-term exposure to highly acidified seawater. However, these mechanisms come at an energetic cost, which can result in reduced growth during long-term exposures. Consequently, the predicted long-term changes to seawater chemistry associated with ocean acidification are likely to have a more significant effect on the health and survival of M. edulis populations than the short-lived effects envisaged from CO 2 leakage from sub-seabed storage. Ocean acidification could reduce the general health status of this commercially and ecologically important marine species.

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