The impact of mobile demersal fishing on carbon storage in seabed sediments

Effects of bottom trawling and environmental factors on benthic bacteria, meiofauna and macrofauna communities and benthic ecosystem processes

Soft sediment marine benthic ecosystems comprise a diverse community of bacteria, meiofauna and macrofauna, which together support a range of ecosystem processes such as biogeochemical cycling. These ecosystems are also fishing grounds for demersal species that are often caught using bottom trawling. This fishing method can have deleterious effects on benthic communities by causing injury or mortality, and through alteration of sediment properties that in turn influence community structure. Although the impacts of bottom trawling on macrofauna are relatively well studied, less is known about the responses of meiofauna and bacteria to such disturbances, or how bottom trawling impacts benthic ecosystem processes. Quantifying trawling impacts against a background of natural environmental variability is also a challenge. To address these questions, we examined effects of bottom trawling and a range of environmental variables (e.g. water chemistry and physical and biochemical surface sediment properties) on a) bacterial, meiofaunal and macrofaunal community structure and b) benthic ecosystem processes (nutrient fluxes, extracellular enzyme activities and carbon turnover and degradation rates). We also investigated the link between the benthic macrofauna community and the same ecosystem processes. While there was a significant effect of bottom trawling intensity on macrofaunal community structure, the same was not seen for bacterial or meiofaunal community composition, which were more affected by environmental factors, such as surface sediment properties. The labile component of the surface sediment carbon pool was higher at highly trawled sites. Carbon degradation rates, extracellular enzyme activities, oxygen fluxes and some nutrient fluxes were significantly affected by trawling, but ecosystem processes were also strongly linked to the abundance of key bioturbators (Macoma balthica, Halicryptus spinulosus, Scoloplos armiger and Pontoporeia femorata). Although benthic ecosystems were affected by a combination of trawling and natural variability, disentangling these showed that the anthropogenic effects were clearest on the larger component of the community, i.e. macrofauna composition, and on ecosystem processes related to sedimentary carbon.

Bottom-trawling signals lost in sediment: A combined biogeochemical and modeling approach to early diagenesis in a perturbed coastal area of the southern Baltic Sea

Trawl-fishing is broadly considered to be one of the most destructive anthropogenic activities toward benthic ecosystems. In this study, we examine the effects of bottom-contact fishing by otter trawls on the geochemistry and macrofauna in sandy silt sediment in an area of the Baltic Sea where clear spatial patterns in trawling activity were previously identified by acoustic mapping. We calibrated an early diagenetic model to biogeochemical data from various coring locations. Fitting measured mercury profiles allowed for the determination of the sediment mixing and burial velocity. For all sites, independent of the trawl mark density, good fits were obtained by applying the model with the same organic matter loading and parameter values, while iron fluxes scaled linearly with the burial velocity. A sensitivity analysis revealed that the fitted sulfate reduction rate, solid sulfur contents, ammonium concentration, and both the isotopic composition and concentration of dissolved inorganic carbon provided reliable constraints for the total mineralization rate, which exhibited a narrow range of variability (around ±20 % from the mean) across the sites. Also, the trawling intensity did not significantly correlate with total organic carbon contents in surficial sediment, indicating limited loss of organic matter due to trawling. The fits to the reactive iron, acid volatile sulfur, chromium(II) reducible sulfur contents, and porewater composition demonstrate that sediment burial and mixing primarily determine the redox stratification. The mixing depth did not correlate with trawling intensity and is more likely the result of bioturbation, as the analyzed macrofaunal taxonomy and density showed a high potential for sediment reworking. The extraordinarily long-lived Arctica islandica bivalve dominated the infaunal biomass, despite the expectation that trawling leads to the succession from longer-lived to shorter-lived and bigger to smaller macrofauna. Our results further suggest that a clear geochemical footprint of bottom-trawling may not develop in sediments actively reworked by tenacious macrofauna.

Benthic microbial biogeographic trends in the North Sea are shaped by an interplay of environmental drivers and bottom trawling effort

Microbial composition and diversity in marine sediments are shaped by environmental, biological, and anthropogenic processes operating at different scales. However, our understanding of benthic microbial biogeography remains limited. Here, we used 16S rDNA amplicon sequencing to characterize benthic microbiota in the North Sea from the top centimeter of 339 sediment samples. We utilized spatially explicit statistical models, to disentangle the effects of the different predictors, including bottom trawling intensity, a prevalent industrial fishing practice which heavily impacts benthic ecosystems. Fitted models demonstrate how the geographic interplay of different environmental and anthropogenic drivers shapes the diversity, structure and potential metabolism of benthic microbial communities. Sediment properties were the primary determinants, with diversity increasing with sediment permeability but also with mud content, highlighting different underlying processes. Additionally, diversity and structure varied with total organic matter content, temperature, bottom shear stress and bottom trawling. Changes in diversity associated with bottom trawling intensity were accompanied by shifts in predicted energy metabolism. Specifically, with increasing trawling intensity, we observed a transition toward more aerobic heterotrophic and less denitrifying predicted metabolism. Our findings provide first insights into benthic microbial biogeographic patterns on a large spatial scale and illustrate how anthropogenic activity such as bottom trawling may influence the distribution and abundances of microbes and potential metabolism at macroecological scales.

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

The conservation, restoration, and improved management of terrestrial forests significantly contributes to mitigate climate change and its impacts, as well as providing numerous co-benefits. The pressing need to reduce emissions and increase carbon removal from the atmosphere is now also leading to the development of natural climate solutions in the ocean. Interest in the carbon sequestration potential of underwater macroalgal forests is growing rapidly among policy, conservation, and corporate sectors. Yet, our understanding of whether carbon sequestration from macroalgal forests can lead to tangible climate change mitigation remains severely limited, hampering their inclusion in international policy or carbon finance frameworks. Here, we examine the results of over 180 publications to synthesise evidence regarding macroalgal forest carbon sequestration potential. We show that research efforts on macroalgae carbon sequestration are heavily skewed towards particulate organic carbon (POC) pathways (77% of data publications), and that carbon fixation is the most studied flux (55%). Fluxes leading directly to carbon sequestration (e.g. carbon export or burial in marine sediments) remain poorly resolved, likely hindering regional or country-level assessments of carbon sequestration potential, which are only available from 17 of the 150 countries where macroalgal forests occur. To solve this issue, we present a framework to categorize coastlines according to their carbon sequestration potential. Finally, we review the multiple avenues through which this sequestration can translate into climate change mitigation capacity, which largely depends on whether management interventions can increase carbon removal above a natural baseline or avoid further carbon emissions. We find that conservation, restoration and afforestation interventions on macroalgal forests can potentially lead to carbon removal in the order of 10's of Tg C globally. Although this is lower than current estimates of natural sequestration value of all macroalgal habitats (61-268 Tg C year-1 ), it suggests that macroalgal forests could add to the total mitigation potential of coastal blue carbon ecosystems, and offer valuable mitigation opportunities in polar and temperate areas where blue carbon mitigation is currently low. Operationalizing that potential will necessitate the development of models that reliably estimate the proportion of production sequestered, improvements in macroalgae carbon fingerprinting techniques, and a rethinking of carbon accounting methodologies. The ocean provides major opportunities to mitigate and adapt to climate change, and the largest coastal vegetated habitat on Earth should not be ignored simply because it does not fit into existing frameworks.

Adapting the marine stewardship council's risk-based framework to assess the impact of towed bottom fishing gear on blue carbon habitats

Wild capture fisheries are of economic and social importance, providing a primary source of protein to people globally. There is a broad research base on the environmental impacts of fishing gears and processing methods yet, the impact on the global CO2 budget is less well studied. Evaluating the risk that wild capture fisheries pose to ecosystem health is vital to sustainably managing fishing practices to meet increasing global nutritional needs and reverse declines in marine biodiversity. At the same time meeting net-zero ambitions by reducing direct and indirect GHG emissions is vital. Ecological risk assessments, trait-based assessments, and vulnerability assessments have long supported fisheries management systems globally but do not yet provide any representation regarding the impacts that fishing gears have on the ability of the habitat to capture and store carbon. Considering the importance of accessibility and transparency in approaches necessary for fisheries sustainability certifications, this paper describes a method to integrate habitat carbon capacity attributes into the Marine Stewardship Council (MSC) Consequence and Spatial Analysis (CSA) framework. Applying the CSA carbon extension developed herein produces different CSA risk scores compared to the MSC CSA that does not account for carbon. This has potential consequences for certification schemes as carbon becomes more important in the fisheries sustainability conversation. The CSA carbon extension tool developed here is an important first step in incorporating carbon indicators into evaluations of fisheries that consider fishery carbon impacts.

Toward a network perspective in coastal ecosystem management

Environmental management in coastal ecosystems has been challenged by the complex cumulative effects that occur when many small issues result in large ecological shifts. Current environmental management of these spaces focuses on identifying and limiting problematic stressors via a series of assessment techniques. Whilst there is a strong desire among managers to consider complexity in ecological responses to cumulative effects, current approaches for assessing risk focus on breaking down the issues into multiple cause and effect relationships. However, uncertainty arises when data and information for a place are limited, as is commonly the case, and this creates decision paralysis while more information is generated. Here, we discuss how ecological understanding of network interactions in coastal marine ecosystems can be used as a lens to bring together multiple lines of evidence and create actions. We list and describe four characteristics of marine ecosystem interaction networks including the possibility for; 1) indirect effects, 2) effects that emerge as stressor magnitude increases the number of network components implicated, 3) network interactions that amplify these indirect effects, and 4) feedbacks that reinforce or stabilise against indirect effects. We then link these four characteristics to three case studies of common coastal environmental issues to demonstrate how a general understanding of ecological interaction networks can enhance priorities for stressor management that can be applied even when specific data is limited.

Mapping marine benthic biological traits to facilitate future sustainable development

Escalating societal demands placed on the seabed mean there has never been such a pressing need to align our understanding of the relationship between the physical impact of anthropogenic activities (e.g., installation of wind turbines, demersal fishing) and the structure and function of the seabed assemblages. However, spatial differences in benthic assemblages based on empirical data are currently not adequately incorporated into decision-making processes regarding future licensable activities or wider marine spatial planning frameworks. This study demonstrates that, through harnessing a Big Data approach, large-scale, continuous coverage maps revealing differences in biological traits expressions of benthic assemblages can be produced. We present independent maps based on a suite of response traits (depicting differences in responses to natural or anthropogenically induced change) and effects traits (reflecting different functional potential), although maps derived using single traits or combinations of a range of traits are possible. Models predicting variations in response traits expression provide greater confidence than those predicting effects traits. We discuss how such maps may be used to assist in the decision-making process for the licensing of anthropogenic activities and as part of marine spatial planning approaches. The confidence in such maps to reflect spatial variations in marine benthic trait expression may, in the future, inherently be improved through (1) the inclusion of more empirical macrofaunal assemblage field data; (2) an improved knowledge of marine benthic taxa trait expression; and (3) a greater understanding of the traits responsible for determining a taxon's response to an anthropogenic pressure and a taxon's functional potential.

The natural capital of seagrass beds in the Caribbean: evaluating their ecosystem services and blue carbon trade potential

Seagrass beds provide tremendous services to society, including the storage of carbon, with important implications for climate change mitigation. Prioritizing conservation of this valuable natural capital is of global significance, and including seagrass beds in global carbon markets through projects that minimize loss, increase area or restore degraded areas represents a mechanism towards this end. Using newly available Caribbean seagrass distribution data, we estimated carbon storage in the region and calculated economic valuations of total ecosystem services and carbon storage. We estimated the 88 170 km2 of seagrass in the Caribbean stores 1337.8 (360.5-2335.0, minimum and maximum estimates, respectively) Tg carbon. The value of these seagrass ecosystems in terms of total ecosystem services and carbon alone was estimated to be $255 billion yr-1 and $88.3 billion, respectively, highlighting their potential monetary importance for the region. Our results show that Caribbean seagrass beds are globally substantial pools of carbon, and our findings underscore the importance of such evaluation schemes to promote urgently needed conservation of these highly threatened and globally important ecosystems.

Linking Species Functional Traits to Specific Biogeochemical Processes under Trawling Pressure

Simple Summary

Bottom trawls when fishing move over large areas with different parts of the gears physically impacting the sea bottom, including the trawling wires, doors, ground rope and net. In this way, the trawl nets remove animals from bottom waters, the sediment surface and shallow sub-surface. The animals that live in the sea bottom with their activities and lifestyle play an important role in major ecosystem processes such as nutrient cycling. In this study, we investigated the relationship between species functional characteristics and ecosystem functions under trawling pressure. Our results indicated that under trawling, more opportunistic lifestyles and deposit feeders were associated with the ecosystem processes while in the undisturbed areas these processes were connected with bioturbating and burrowing species. Finding these links helps scientists and policy makers to better predict the impact of fishing disturbance on marine environment and set appropriate thresholds for marine ecosystem impacts.

Abstract

The impact of otter trawling on the relationship between functional traits of benthic invertebrates and specific biogeochemical processes were investigated in the oligotrophic Cretan Sea. The fishery is managed through a seasonal closure during the summer. During two seasons (winter and summer) replicate samples were taken from the field from a commercial trawl ground and an adjacent control area. Environmental parameters related to sediment biogeochemistry were measured including particulate organic carbon, sedimentary organic carbon, bottom water and sedimentary chlorophyll a and phaeopigment concentrations as well as benthic oxygen consumption. A significant impact of trawling was recorded only for bottom water chlorophyll and sedimentary organic carbon. Furthermore, the links between species traits and specific ecosystem processes were affected by trawling, highlighting the importance of unique functional modalities on ecosystem functioning. The traits that mostly influenced benthic biogeochemistry in the control sites were related to bioturbation and burrowing activities. In contrast, in the trawled sites, the associated traits were related to more opportunistic lifestyles and deposit feeding species that do not act as bioturbators. Thus, under trawling disturbance, this shift can decouple the species-sediment relations and affect nutrient cycling.