Climate‐driven shifts in kelp forest composition reduce carbon sequestration potential

Abstract

The potential contribution of kelp forests to blue carbon sinks is currently of great interest but interspecific variance has received no attention. In the Northeast Atlantic, kelp forest composition is changing due to climate-driven poleward range shifts of cold temperate Laminaria digitata and L. hyperborea and warm temperate L. ochroleuca. To understand how this might affect the carbon sequestration potential of these ecosystems, we quantified interspecific differences in carbon export and decomposition alongside changes in detrital photophysiology and biochemistry. We found that while warm temperate kelp exports up to 71% more carbon per plant, it decomposes up to 155% faster than its boreal congeners. Elemental stoichiometry and polyphenolic content cannot fully explain faster carbon turnover, which may be attributable to contrasting tissue toughness or unknown biochemical and structural defences. Faster decomposition causes the detrital photosynthetic apparatus of L. ochroleuca to be overwhelmed after 20 d and lose integrity after 36 d, while detritus of cold temperate species maintains carbon assimilation. Depending on the photoenvironment, detrital photosynthesis could further exacerbate interspecific differences in decomposition via a potential positive feedback loop. Via compositional change such as the predicted prevalence of L. ochroleuca, ocean warming will therefore likely reduce the carbon sequestration potential of these temperate marine forests. Keywords biogeography; carbon budget uncertainty; carbon flux; climate change; C:N; decay; degradation; ecophysiology; erosion; photosynthesis Introduction Over the last decade humans have emitted around 11 billion tons of carbon per year (Canadell et al., 2021). In the Paris Agreement, 193 countries pledged to reduce this emission and enhance carbon sink capacity. Ocean-based biological carbon dioxide removal (CDR) is now acknowledged as an integral part of fulfilling this goal (Canadell et al., 2021). Such CDR may be facilitated by marine macrophytes, whose role in carbon sequestration was first recognised four decades ago (Smith, 1981) but has only recently come to mainstream attention as blue carbon (Canadell et al., 2021). Despite the potential involvement of marine vegetated habitats in CDR and their location within national jurisdiction, few ocean-based nationally determined contributions (NDCs) have been put forward by affluent Annex I parties such as the UK, US and Australia (Gallo et al., 2017) which hold some of the highest blue carbon wealth (Bertram et al., 2021). In part, this may be due to the ongoing debate on the blue carbon status of temperate kelp forests that dominate the coasts of these countries (Krause-Jensen et al., 2018). Therefore, identifying the magnitude and fate of carbon assimilated by these marine plants is key to our understanding of their carbon sequestration potential (CSP) and their consequent inclusion in blue carbon frameworks (Krause-Jensen et al., 2018). CSP is a function of carbon export and fate (Duarte and Cebriá...

Standing Crop, Turnover, and Production Dynamics of Macrocystis pyrifera and Understory Species Hedophyllum nigripes and Neoagarum fimbriatum in High Latitude Giant Kelp Forests

“… 2021 , Wright et al. 2022 ). Research investigating how such changes in the marine environment will impact the carbon and nitrogen mass productivity of these coastal primary producers will be a crucial next step for predicting the future carbon sequestration potential of high latitude kelp forest communities (Harley et al.…”

Carbon sequestration and climate change mitigation using macroalgae: a state of knowledge review

“…The biochemical composition of detritus (Smith & Foreman, 1984), as well as environmental variables (e.g. temperature, oxygen, light), are important drivers of macroalgae decomposition (Pedersen et al, 2021;Filbee-Dexter et al, 2022a;Wright, Table 1. Degradation rates of macroalgae dissolved organic carbon (DOC) and remaining (recalcitrant) carbon (as a percentage of the original DOC concentration) under different experimental conditions.…”

“…The biochemical composition of detritus (Smith & Foreman, 1984), as well as environmental variables (e.g. temperature, oxygen, light), are important drivers of macroalgae decomposition (Pedersen et al ., 2021; Filbee‐Dexter et al ., 2022 a ; Wright, Pessarrodona & Foggo, 2022).…”

“…herbivory), and genetic fortification of wild populations with warm‐adapted individuals or beneficial microbes, may offer opportunities to maintain or prevent carbon stock losses (Wood et al ., 2019). In temperate regions, warming is expected to decrease kelp carbon fixation, increase detrital remineralization, and ultimately diminish carbon sequestration potential (Pessarrodona et al ., 2018 a , b ; Filbee‐Dexter et al ., 2022 a ; Wright et al ., 2022), especially in warm range‐edge populations (Pessarrodona et al ., 2018 b ). In Australia, marine heatwaves have resulted in 120,000 ha of macroalgal forests being lost in just two decades, which represents an estimated loss of 0.4 Tg C in standing stock and of 0.05 Tg C year −1 in sequestration capacity (0.2–0.6% and 0.6–1.1% of the total stock and capacity, respectively) (Filbee‐Dexter & Wernberg, 2020).…”

“…Pessarrodona & Foggo, 2022). The transport of available POC beyond macroalgal habitats is influenced by particle size and buoyancy, as well asBiological Reviews 98 (2023) 1945-1971 © 2023 The Authors.…”