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Seaweed (macroalgae) has attracted attention globally given its potential for climate change mitigation. A topical and contentious question is: Can seaweeds' contribution to climate change mitigation be enhanced at globally meaningful scales? Here, we provide an overview of the pressing research needs surrounding the potential role of seaweed in climate change mitigation and current scientific consensus via eight key research challenges. There are four categories where seaweed has been suggested to be used for climate change mitigation: 1) protecting and restoring wild seaweed forests with potential climate change mitigation co-benefits; 2) expanding sustainable nearshore seaweed aquaculture with potential climate change mitigation co-benefits; 3) offsetting industrial CO2 emissions using seaweed products for emission abatement; and 4) sinking seaweed into the deep sea to sequester CO2. Uncertainties remain about quantification of the net impact of carbon export from seaweed restoration and seaweed farming sites on atmospheric CO2. Evidence suggests that nearshore seaweed farming contributes to carbon storage in sediments below farm sites, but how scalable is this process? Products from seaweed aquaculture, such as the livestock methane-reducing seaweed Asparagopsis or low carbon food resources show promise for climate change mitigation, yet the carbon footprint and emission abatement potential remains unquantified for most seaweed products. Similarly, purposely cultivating then sinking seaweed biomass in the open ocean raises ecological concerns and the climate change mitigation potential of this concept is poorly constrained. Improving the tracing of seaweed carbon export to ocean sinks is a critical step in seaweed carbon accounting. Despite carbon accounting uncertainties, seaweed provides many other ecosystem services that justify conservation and restoration and the uptake of seaweed aquaculture will contribute to the United Nations Sustainable Development Goals. However, we caution that verified seaweed carbon accounting and associated sustainability thresholds are needed before large-scale investment into climate change mitigation from seaweed projects.

Trophic cascade theory predicts that predator effects should extend to influence carbon cycling in ecosystems. Yet, there has been little empirical evidence in natural ecosystems to support this hypothesis. Here, we use a naturally-occurring trophic cascade to provide evidence that predators help protect sedimentary organic carbon stocks in coral reef ecosystems. Our results show that predation risk altered the behavior of herbivorous fish, whereby it constrained grazing to areas close to the refuge of the patch reefs. Macroalgae growing in “riskier” areas further away from the reef were released from grazing pressure, which subsequently promoted carbon accumulation in the sediments underlying the macroalgal beds. Here we found that carbon stocks furthest away from the reef edge were ~24% higher than stocks closest to the reef. Our results indicate that predators and herbivores play an important role in structuring carbon dynamics in a natural marine ecosystem, highlighting the need to conserve natural predator-prey dynamics to help maintain the crucial role of marine sediments in sequestering carbon.

Both habitat patchiness and behaviorally-mediated indirect effects (BMIEs; predator- induced changes in prey behavior that affect the prey's resources) are important in many food webs, but the relationships between these 2 factors have yet to be investigated. To explore effects of habitat patchiness and variation in perceived risk of predation on food-web dynamics, we conducted a factorial experiment in a model aquatic food chain of predator-prey-resource using 2 contrasting predators (adult blue crab Callinectes sapidus and toad fish Opsanus tau), juvenile blue crab as prey, and mussel Geukensia demissa as resource. Both predator presence and habitat patchiness influenced the prey's preference for consuming resources at patch edges instead of interiors. The preference of prey for consuming resources at habitat edges was 4 times stronger in continuous oyster reef habitat than in smaller habitat patches. This suggests that interior resources in continuous habitat experience a refuge from consumption, but this refuge is largely lost in patchy habitat. The mere presence of predators reduced the prey's preference for consuming resources at habitat edges. This BMIE was significant for the ambush predator (toadfish) and the treatment containing both predators, but not for the actively hunting predator (adult blue crab). We conclude that habitat patchiness and predator presence can jointly affect resource distribution by inducing shifts in prey foraging behavior, revealing a need to incorporate BMIEs into habitat fragmentation studies. This conclusion has broad and growing relevance as anthropogenic factors increasingly modify predator abundances and fragment coastal habitats. © Inter-Research 2012.

Provides detailed methods and results for data collection and analyses.

Shallow-water seagrasses capture and store globally significant quantities of organic carbon (OC), often referred to as ‘blue carbon’; however, data are lacking on the importance of deep-water (greater than 15 m) seagrasses as blue carbon sinks. We compared OC stocks from deep-, mid- and shallow-water seagrasses at Lizard Island within the Great Barrier Reef Lagoon. We found deep-water seagrasses (Halophila species) contained similar levels of OC as shallow-water species (e.g. Halodule uninervis) (0.64 ± 0.08% and 0.9 ± 0.1 mg C cm3, 0.87 ± 0.19% and 1.3 ± 0.3 mg C cm3, respectively), despite being much sparser and smaller in stature. Deep-water seagrasses sediments contained significantly higher levels (approx. 9-fold) of OC than surrounding bare areas. Inorganic carbon (CaCO3) levels were relatively high in deep-water seagrass sediments (8.2 ± 0.4%) and, if precipitated from epiphytes within the meadow, could offset the potential CO2-sink capacity of these meadows. The δ13C signatures of sediment samples varied among depths and habitats (−10.9 and −17.0), reflecting contributions from autochthonous and allochthonous sources. If the OC stocks reported in this study are similar to deep-water Halophila meadows elsewhere within the GBR lagoon (total area 31 000 km2), then OC bound within this system is roughly estimated at 27.4 million tonnes.

Provides detailed methods and results for data collection and analyses.

This Marine and Coastal Horizon Scan was funded by Oceankind. S.N.R.B. is supported by EcoStar (DM048) and Cefas (My time). R.C. acknowledges FCT/MCTES for the financial support to CESAM (UIDP/50017/2020, UIDB/50017/2020, LA/P/0094/2020) through national funds. O.D. is supported by CSIC Uruguay and Inter-American Institute for Global Change Research. J.E.H.-R. is supported by the Whitten Lectureship in Marine Biology. S.A.K. is supported by a Natural Environment Research Council grant (NE/S00050X/1). P.I.M. is supported by an Australian Research Council Discovery Grant (DP200100575). D.M.P. is supported by the Marine Alliance for Science and Technology for Scotland (MASTS). A.R.P. is supported by the Inter-American Institute for Global Change Research. W.J.S. is funded by Arcadia. A.T. is supported by Oceankind. M.Y. is supported by the Deep Ocean Stewardship Initiative and bioDISCOVERY. The biodiversity of marine and coastal habitats is experiencing unprecedented change. While there are well-known drivers of these changes, such as overexploitation, climate change and pollution, there are also relatively unknown emerging issues that are poorly understood or recognized that have potentially positive or negative impacts on marine and coastal ecosystems. In this inaugural Marine and Coastal Horizon Scan, we brought together 30 scientists, policymakers and practitioners with transdisciplinary expertise in marine and coastal systems to identify new issues that are likely to have a significant impact on the functioning and conservation of marine and coastal biodiversity over the next 5–10 years. Based on a modified Delphi voting process, the final 15 issues presented were distilled from a list of 75 submitted by participants at the start of the process. These issues are grouped into three categories: ecosystem impacts, for example the impact of wildfires and the effect of poleward migration on equatorial biodiversity; resource exploitation, including an increase in the trade of fish swim bladders and increased exploitation of marine collagens; and new technologies, such as soft robotics and new biodegradable products. Our early identification of these issues and their potential impacts on marine and coastal biodiversity will support scientists, conservationists, resource managers and policymakers to address the challenges facing marine ecosystems. Postprint