Connected macroalgal‐sediment systems: blue carbon and food webs in the deep coastal ocean

Abstract

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.

Carbon on the Northwest European Shelf: Contemporary Budget and Future Influences

“…Most POC delivered to the sea bed is thought to be oxidized through aerobic respiration and sulfate reduction (Krumins et al, 2013) but observational measurements of POC deposition and especially DIC release are scarce. Recent data suggests that these fluxes can be highly variable seasonally and that POC burial may be greater than expected from our current modeling, depending on processes varying temporally and spatially (Snelgrove et al, 2018;Queirós et al, 2019). Furthermore, a substantial proportion of the empirical benthic carbon data is derived from cohesive sediment studies, with the exception of a few (e.g., Boudreau et al, 2001;Precht and Huettel, 2003;Rao et al, 2008;Burt et al, 2014;Aldridge et al, 2017), yet the wide spatial coverage of permeable sediments on the shelf means there is likely to be an underestimation of the influence of advective flow on the carbon dynamics within benthic stocks (Boudreau et al, 2001;de Beer et al, 2005).…”

“…Only carbon in the top 0.1 m of sediment was considered when quantifying benthic and coastal carbon stocks, following the protocol used in previous studies (Burrows et al, 2017;Diesing et al, 2017;Luisetti et al, 2019;Queirós et al, 2019). This depth does not reflect the vertical limit of carbon storage, which is likely to vary across coastal and benthic habitats with sediment type, degree of bioturbation, and sedimentation rate.…”

“…Recent work has highlighted the potential role of macroalgae in contributing to carbon sequestration (Krause-Jensen and Krause-Jensen et al, 2018;Queirós et al, 2019). The dominant group of macroalgae in Europe is kelp (Araujo et al, 2016), which is thought to contribute more to carbon sequestration than smaller, shorter-lived groups of macroalgae (Krause-Jensen et al, 2018).…”

“…The wide range of net benthic burial estimates (−2.2 to 6.0 Tmol year −1 ; Supplementary Material Part A) reflects our limited knowledge of pelagic-benthic carbon fluxes, including their spatial distribution (Stahl et al, 2004;Burrows et al, 2017;Diesing et al, 2017;Queirós et al, 2019). Most POC delivered to the sea bed is thought to be oxidized through aerobic respiration and sulfate reduction (Krumins et al, 2013) but observational measurements of POC deposition and especially DIC release are scarce.…”

“…However, the proportion of this organic matter which is remineralized and released back into the water column compared to the proportion which is buried in the sediment is largely unknown. Organic matter from kelp forests is also exported to other coastal habitats where it can enter benthic carbon stocks though this process is still largely unconstrained given the limited availability of existing data (Krause-Jensen et al, 2018;Smale et al, 2018;Queirós et al, 2019). Without a better understanding of the fate of this organic matter, it is difficult to accurately quantify its contribution to the net coastal to pelagic carbon flux and the net burial of kelp in the sediment.…”