Modelling terrigenous DOC across the north west European Shelf: Fate of riverine input and impact on air-sea CO2 fluxes.

Terrigenous carbon in aquatic systems is increasingly recognised as an important part of the global carbon cycle. Despite this, the fate and distribution of terrigenous dissolved organic carbon (tDOC) in coastal and oceanic systems is poorly understood. We have implemented a theoretical framework for the degradation of tDOC across the land to ocean continuum in a 3D hydrodynamical-biogeochemical model on the North West European Shelf. A key feature of this model is that both photochemical and bacterial tDOC degradation rates are age dependant constituting an advance in our ability to describe carbon cycling in the marine environment. Over the time period 1986-2015, 182±17 Gmol yr-1 of riverine tDOC is input to the shelf. Results indicate that bacterial degradation is by far the most important process in removing tDOC on the shelf, contributing to 73±6 % (132±11 Gmol yr-1) of the total removal flux, while 21±3 % (39±6 Gmol yr-1) of riverine tDOC was advected away from the shelf and photochemical degradation removing 5±0.5 % of the riverine flux. Explicitly including tDOC in the model decreased the air-sea carbon dioxide (CO2) flux by 112±8 Gmol yr-1 (4±0.4 %), an amount approximately equivalent to the CO2 released by the UK chemical industry in 2020. The reduction is equivalent to 62 % of the riverine tDOC input to the shelf while approximately 17 % of riverine input is incorporated into the foodweb. This work can improve the assumptions of the fate of tDOC by Earth System Models and demonstrates that the inclusion of tDOC in models can impact ecosystem dynamics and change predicted global carbon budgets for the ocean.

Mussel power: Scoping a nature-based solution to microplastic debris.

Microplastics are a prolific environmental contaminant. Curbing microplastic pollution requires an array of globally relevant interventions, including source-reduction and curative measures. A novel, nature-based solution to microplastics is proposed, in which mussels are deployed in aquatic ecosystems to act as microplastic biofilters, removing waterborne microplastics and repackaging them into biodeposits that are subsequently captured and removed. Blue mussels (Mytilus edulis) were used to establish the feasibility of such an approach. In the laboratory, mussels were exposed to representative microplastics in a flume tank; at an initial concentration of 1000 microplastics L-1, mussels reduced waterborne microplastic concentrations at an average rate of 40,146 microplastics kg-1 h-1. Mussel faeces sank irrespective of microplastic content, with average sinking velocities of 223-266 m day-1. Modelling predicts ∼3 × 109 mussels deployed on ropes at the mouths of estuaries could remove 4% of waterborne microplastics discharged from rivers. Mussels were successfully deployed in a prototype biodeposit collection system in an urban marina, with 5.0 kg of mussels removing and repackaging 239.9 ± 145.9 microplastics and anthropogenic particles day-1 into their faeces. These results provide impetus for further development of nature-based solutions targeting plastic debris.

Harmful Algal Blooms and their impacts on shellfish mariculture follow regionally distinct patterns of water circulation in the western English Channel during the 2018 heatwave.

Harmful algal blooms (HABs) can have severe ecological, societal and economic impacts upon marine ecosystems, human health and the seafood industry. We evaluated changes in marine plankton communities with prevailing physico-chemical conditions throughout an exceptionally warm summer (2018), to elucidate key factors governing HABs and their impacts on shellfish mariculture in the western English Channel. Despite warm, stable weather conditions and widespread seasonal stratification throughout the summer, divergent plankton community compositions were observed at two rope-grown mussel (Mytilus edulis) farms (St Austell Bay and Lyme Bay) and a long-term ecological research LTER site (Plymouth L4). There were significant differences between sites in the abundances of HAB species, including Dinophysis spp. and Karenia mikimotoi, whose cell counts bloomed in excess of UK Food Standards Agency (FSA) advisory 'trigger' levels at Plymouth L4 and St Austell Bay, but not at the Lyme Bay site. The K. mikimotoi bloom occurred over two weeks in August and comprised up to 88% of the standing phytoplankton biomass in St Austell Bay. Dinophysis spp. also bloomed here from May to September, constituting up to 28% of phytoplankton biomass. This protracted bloom resulted in concentrations of Dinophysis toxins 1 & 2 and pectenotoxins and okadaic acid in shellfish, which closed shellfish harvesting operations on farms located in St Austell Bay, and other shellfish sites in the west of the western English Channel (but not in the east of the region). Inter-site differences in the abundances of these and other HAB species were associated with variations in water circulation and co-occurring phytoplankton and zooplankton communities. Furthermore, plankton monitoring data obtained from the L4 site over the past 3 decades showed HAB species (including Dinophysis spp.) with abundances commonly occurring above advisory trigger levels during warmer periods, such as that coinciding with our study. Under projected climate warming these blooms are likely to continue to be governed by regionally distinct patterns of water circulation, which need to be taken into account in marine spatial planning, when assessing the suitability of new shellfish mariculture sites.

Nitrous oxide and methane in a changing Arctic Ocean.

Human activities are changing the Arctic environment at an unprecedented rate resulting in rapid warming, freshening, sea ice retreat and ocean acidification of the Arctic Ocean. Trace gases such as nitrous oxide (N2O) and methane (CH4) play important roles in both the atmospheric reactivity and radiative budget of the Arctic and thus have a high potential to influence the region's climate. However, little is known about how these rapid physical and chemical changes will impact the emissions of major climate-relevant trace gases from the Arctic Ocean. The combined consequences of these stressors present a complex combination of environmental changes which might impact on trace gas production and their subsequent release to the Arctic atmosphere. Here we present our current understanding of nitrous oxide and methane cycling in the Arctic Ocean and its relevance for regional and global atmosphere and climate and offer our thoughts on how this might change over coming decades.

The role of a changing Arctic Ocean and climate for the biogeochemical cycling of dimethyl sulphide and carbon monoxide.

Dimethyl sulphide (DMS) and carbon monoxide (CO) are climate-relevant trace gases that play key roles in the radiative budget of the Arctic atmosphere. Under global warming, Arctic sea ice retreats at an unprecedented rate, altering light penetration and biological communities, and potentially affect DMS and CO cycling in the Arctic Ocean. This could have socio-economic implications in and beyond the Arctic region. However, little is known about CO production pathways and emissions in this region and the future development of DMS and CO cycling. Here we summarize the current understanding and assess potential future changes of DMS and CO cycling in relation to changes in sea ice coverage, light penetration, bacterial and microalgal communities, pH and physical properties. We suggest that production of DMS and CO might increase with ice melting, increasing light availability and shifting phytoplankton community. Among others, policy measures should facilitate large-scale process studies, coordinated long term observations and modelling efforts to improve our current understanding of the cycling and emissions of DMS and CO in the Arctic Ocean and of global consequences.

Regional adaptation defines sensitivity to future ocean acidification.

Physiological responses to temperature are known to be a major determinant of species distributions and can dictate the sensitivity of populations to global warming. In contrast, little is known about how other major global change drivers, such as ocean acidification (OA), will shape species distributions in the future. Here, by integrating population genetics with experimental data for growth and mineralization, physiology and metabolomics, we demonstrate that the sensitivity of populations of the gastropod Littorina littorea to future OA is shaped by regional adaptation. Individuals from populations towards the edges of the natural latitudinal range in the Northeast Atlantic exhibit greater shell dissolution and the inability to upregulate their metabolism when exposed to low pH, thus appearing most sensitive to low seawater pH. Our results suggest that future levels of OA could mediate temperature-driven shifts in species distributions, thereby influencing future biogeography and the functioning of marine ecosystems.

Modelling impacts and recovery in benthic communities exposed to localised high CO2.

Regulations pertaining to carbon dioxide capture with offshore storage (CCS) require an understanding of the potential localised environmental impacts and demonstrably suitable monitoring practices. This study uses a marine ecosystem model to examine a comprehensive range of hypothetical CO2 leakage scenarios, quantifying both impact and recovery time within the benthic system. Whilst significant mortalities and long recovery times were projected for the larger and longer term scenarios, shorter-term or low level exposures lead to reduced projected impacts. This suggests that efficient monitoring and leak mitigation strategies, coupled with appropriate selection of storage sites can effectively limit concerns regarding localised environmental impacts from CCS. The feedbacks and interactions between physiological and ecological responses simulated reveal that benthic responses to CO2 leakage could be complex. This type of modelling investigation can aid the understanding of impact potential, the role of benthic community recovery and inform the design of baseline and monitoring surveys.

Scaling up experimental ocean acidification and warming research: from individuals to the ecosystem.

Understanding long-term, ecosystem-level impacts of climate change is challenging because experimental research frequently focuses on short-term, individual-level impacts in isolation. We address this shortcoming first through an interdisciplinary ensemble of novel experimental techniques to investigate the impacts of 14-month exposure to ocean acidification and warming (OAW) on the physiology, activity, predatory behaviour and susceptibility to predation of an important marine gastropod (Nucella lapillus). We simultaneously estimated the potential impacts of these global drivers on N. lapillus population dynamics and dispersal parameters. We then used these data to parameterize a dynamic bioclimatic envelope model, to investigate the consequences of OAW on the distribution of the species in the wider NE Atlantic region by 2100. The model accounts also for changes in the distribution of resources, suitable habitat and environment simulated by finely resolved biogeochemical models, under three IPCC global emissions scenarios. The experiments showed that temperature had the greatest impact on individual-level responses, while acidification had a similarly important role in the mediation of predatory behaviour and susceptibility to predators. Changes in Nucella predatory behaviour appeared to serve as a strategy to mitigate individual-level impacts of acidification, but the development of this response may be limited in the presence of predators. The model projected significant large-scale changes in the distribution of Nucella by the year 2100 that were exacerbated by rising greenhouse gas emissions. These changes were spatially heterogeneous, as the degree of impact of OAW on the combination of responses considered by the model varied depending on local-environmental conditions and resource availability. Such changes in macro-scale distributions cannot be predicted by investigating individual-level impacts in isolation, or by considering climate stressors separately. Scaling up the results of experimental climate change research requires approaches that account for long-term, multiscale responses to multiple stressors, in an ecosystem context.

Vulnerability of coastal ecosystems to changes in harmful algal bloom distribution in response to climate change: projections based on model analysis.

Harmful algal blooms (HABs), those proliferations of algae that can cause fish kills, contaminate seafood with toxins, form unsightly scums, or detrimentally alter ecosystem function have been increasing in frequency, magnitude, and duration worldwide. Here, using a global modeling approach, we show, for three regions of the globe, the potential effects of nutrient loading and climate change for two HAB genera, pelagic Prorocentrum and Karenia, each with differing physiological characteristics for growth. The projections (end of century, 2090-2100) are based on climate change resulting from the A1B scenario of the Intergovernmental Panel on Climate Change Institut Pierre Simon Laplace Climate Model (IPCC, IPSL-CM4), applied in a coupled oceanographic-biogeochemical model, combined with a suite of assumed physiological 'rules' for genera-specific bloom development. Based on these models, an expansion in area and/or number of months annually conducive to development of these HABs along the NW European Shelf-Baltic Sea system and NE Asia was projected for both HAB genera, but no expansion (Prorocentrum spp.), or actual contraction in area and months conducive for blooms (Karenia spp.), was projected in the SE Asian domain. The implications of these projections, especially for Northern Europe, are shifts in vulnerability of coastal systems to HAB events, increased regional HAB impacts to aquaculture, increased risks to human health and ecosystems, and economic consequences of these events due to losses to fisheries and ecosystem services.

Biomass changes and trophic amplification of plankton in a warmer ocean.

Ocean warming can modify the ecophysiology and distribution of marine organisms, and relationships between species, with nonlinear interactions between ecosystem components potentially resulting in trophic amplification. Trophic amplification (or attenuation) describe the propagation of a hydroclimatic signal up the food web, causing magnification (or depression) of biomass values along one or more trophic pathways. We have employed 3-D coupled physical-biogeochemical models to explore ecosystem responses to climate change with a focus on trophic amplification. The response of phytoplankton and zooplankton to global climate-change projections, carried out with the IPSL Earth System Model by the end of the century, is analysed at global and regional basis, including European seas (NE Atlantic, Barents Sea, Baltic Sea, Black Sea, Bay of Biscay, Adriatic Sea, Aegean Sea) and the Eastern Boundary Upwelling System (Benguela). Results indicate that globally and in Atlantic Margin and North Sea, increased ocean stratification causes primary production and zooplankton biomass to decrease in response to a warming climate, whilst in the Barents, Baltic and Black Seas, primary production and zooplankton biomass increase. Projected warming characterized by an increase in sea surface temperature of 2.29 ± 0.05 °C leads to a reduction in zooplankton and phytoplankton biomasses of 11% and 6%, respectively. This suggests negative amplification of climate driven modifications of trophic level biomass through bottom-up control, leading to a reduced capacity of oceans to regulate climate through the biological carbon pump. Simulations suggest negative amplification is the dominant response across 47% of the ocean surface and prevails in the tropical oceans; whilst positive trophic amplification prevails in the Arctic and Antarctic oceans. Trophic attenuation is projected in temperate seas. Uncertainties in ocean plankton projections, associated to the use of single global and regional models, imply the need for caution when extending these considerations into higher trophic levels.

Tidal downwelling and implications for the carbon biogeochemistry of cold-water corals in relation to future ocean acidification and warming.

Cold-water coral (CWC) reefs are recognized as ecologically and biologically significant areas that generate habitats and diversity. The interaction between hydrodynamics and CWCs has been well studied at the Mingulay Reef Complex, a relatively shallow area of reefs found on the continental shelf off Scotland, UK. Within 'Mingulay Area 01' a rapid tidal downwelling of surface waters, brought about as an internal wave, is known to supply warmer, phytoplankton-rich waters to corals growing on the northern flank of an east-west trending seabed ridge. This study shows that this tidal downwelling also causes short-term perturbations in the inorganic carbon (CT ) and nutrient dynamics through the water column and immediately above the reef. Over a 14 h period, corresponding to one semi-diurnal tidal cycle, seawater pH overlying the reef varied by ca. 0.1 pH unit, while pCO2 shifted by >60 µatm, a shift equivalent to a ca. 25 year jump into the future, with respect to atmospheric pCO2 . During the summer stratified period, these downwelling events result in the reef being washed over with surface water that has higher pH, is warmer, nutrient depleted, but rich in phytoplankton-derived particles compared to the deeper waters in which the corals sit. Empirical observations, together with outputs from the European Regional Shelf Sea Ecosystem Model, demonstrate that the variability that the CWC reefs experience changes through the seasons and into the future. Hence, as ocean acidification and warming increase into the future, the downwelling event specific to this site could provide short-term amelioration of corrosive conditions at certain times of the year; however, it could additionally result in enhanced detrimental impacts of warming on CWCs. Natural variability in the CT and nutrient conditions, as well as local hydrodynamic regimes, must be accounted for in any future predictions concerning the responses of marine ecosystems to climate change.

Nutrient budgets for European seas: a measure of the effectiveness of nutrient reduction policies.

Socio-economic development in Europe has exerted increasing pressure on the marine environment. Eutrophication, caused by nutrient enrichment, is evident in regions of all European seas. Its severity varies but has, in places, adversely impacted socio-economic activities. This paper aims to evaluate the effectiveness of recently adopted policies to reduce anthropogenic nutrient inputs to European seas. Nitrogen and phosphorus budgets were constructed for three different periods (prior to severe eutrophication, during severe eutrophication and contemporary) to capture changes in the relative importance of different nutrient sources in four European seas suffering from eutrophication (Baltic Proper, coastal North Sea, Northern Adriatic and North-Western Black Sea Shelf). Policy success is evident for point sources, notably for P in the Baltic and North Seas, but reduction of diffuse sources has been more problematic.