Theory: Determining spatial and temporal patterns of habitats and processes is a fundamental goal in ecological research (Morin 1999). Habitat maps and models provide spatiotemporal baseline information that critically enhance management and conservation of natural resources. With today’s rapid decline of natural coastal habitats and calls for ecosystem-based coastal management, it is of critical importance to map and quantify spatial distribution of habitats and temporal dynamics at large coastal seascape (km to 100s km) scales (Pittman 2018). Such information is also necessary to understand seascape connectivity over relevant spatiotemporal scales and benefit marine spatial planning processes.

Objective: WP1 will provide spatial maps over coastal seascapes across the Baltic Sea region to build seascape units consisting of data on vegetation cover of blue forest habitats, coastal geomorphology and land-use activities in the land-sea interface.

Material and methods: Seascape units (n = 20) of appropriate size (km to 10s km) will be selected and mapped in different basins all around the Baltic Sea coasts (in Estonia, Lithuania, Poland, Denmark, Finland and Sweden) based on a stratified random design (to consider the entire range of coastal seascape types in the region and also depending on access to maps or monitoring data) and created in a GIS environment (ArcMap). In each seascape unit, we will map areas of vegetation cover (potential blue forest habitats), geomorphological settings and land-use activities (agriculture, forestry, urban areas, etc.). Using satellite imagery, we will also collect retrospective information (from the 1980s or later) on major (habitat) changes in any seascape (or surrounding area) assumed to largely affect blue carbon sequestration. To generate complete seascape maps, we will initially collect existing data from environmental monitoring (from geological survey authorities, consultancies, etc.). Complementary mapping will be performed using different groundtruth methodologies, including dropvideo in combination with echo-sounder and/or drone for shallow-water habitats, and air photos (if available), satellite remote sensing and/or drones for land-based coastal features. Our mapping will focus on blue forest habitats distributed in our seascape units, including coastal forested wetlands, coastal unforested wetlands, reed, submerged rooted macrophytes (including seagrass meadows) and macroalgae (as export to deep and shallow soft bottom substrata).

Theory: Vegetated coastal ecosystems are important blue carbon sinks and thus significant contributors to climate change mitigation (Mcleod et al 2011; Fourqurean et al. 2012; Mcreadie et al. 2019). Along the carbon sink function in these systems, there are other important cobenefits to the ecosystem, including nutrient filtering, biota refuge and coastal protection (Duarte et al. 2003). However, as the organic matter making up the carbon sinks are largely in hypoxic, the turnover of organic matter promotes the production of potent greenhouse gases, nitrous oxide (N2O) and methane (CH4) (García-Lledó, et al. 2011; Mander et al. 2008); this production can however be suppressed by a vegetation cover (Rushingisha, Gullström and Björk, preliminary results). We expect vegetated areas, e.g. submerged rooted macrophytes, to be strong blue carbon sinks. In contrast, habitats with loss of vegetation are expected to be converted to sources, releasing CO2 and other GHGs. Objective: The overall objective of WP2 is to gather field data to assess carbon sink capacity of coastal blue forest habitats (different vegetation groups and habitat categories) and model these for climate mitigation capacity using seascape information from WP1. We will also sample data to compare vegetated habitats with habitats where vegetation has been lost (also based on data from WP1) to evaluate possible losses in carbon stocks and carbon sequestration capacity.

Objective: The overall objective of WP2 is to gather field data to assess carbon sink capacity of coastal blue forest habitats (different vegetation groups and habitat categories) and model these for climate mitigation capacity using seascape information from WP1. We will also sample data to compare vegetated habitats with habitats where vegetation has been lost (also based on data from WP1) to evaluate possible losses in carbon stocks and carbon sequestration capacity.

Material and methods: Field surveys will identify where and when carbon sequestration is most efficient within the coastal seascape, and evaluate (i) carbon stocks of the different habitats, (ii) carbon- and mass accumulation rates, (iii) plant production rates, and (iv) CO2, CH4, and N2O gas balance. Our sampling design is focused on coastal seascapes of blue forest habitats and based on a landscape ecology approach (see WPs 1 and 3). (i) Carbon stocks and sediment accumulation rates (dating >100 yrs back) will be estimated as described in Asplund et al. (2021) by collecting short (0.5 m) and long (1-3 m) sediment cores (using PVC tubes) in the selected habitats. Short cores (n = 3) will be collected in all blue forest habitats of each coastal seascape, while long cores (n = 1) will be sampled in deeper accumulation bottoms off each seascape as well as in land-based and shallow habitats within six seascapes spread across the Baltic Sea area. The sediment cores will be sectioned at 0.5-cmthick intervals throughout and the carbon and nitrogen (Ntot for C/N ratios in WP3) content analyzed using a carbon-nitrogen elemental analyzer. This will yield total carbon (Ctot), organic carbon (Corg %), and by acid treatment also inorganic carbon content (Cinorg). The mass accumulation rates will be assessed after constructing chronologies using a combination of radiometric dating methods (210Pb, 137Cs, 241Pb) on bulk sediments allowing carbon content to be calculated as mass per year (g C m-2 yr-1). (ii) Net productivity of plants (carbon fixation) will be determined according to Deyanova et al. (2017) by assessing CO2 capture potential using areal productivity calculated from continuous measurements of diel photosynthetic rates, and estimates of plant morphology, biomass and productivity/respiration (P/R) ratios. (iii) Measurement of air–water CO2, CH4, and N2O fluxes: A combination of in situ measurements of fluxes using a floating chamber technique (Tokoro et al. 2007) connected to a portable gas analyser (LiCor Ultraportable Greenhouse Gas Analyzer GLA132-GGA or similar, applied for in this proposal) will be used together with static chambers collecting samples in specific habitats. For selected habitats, we will analyse denitrification gas production rates by the Isotope Pairing Technique (Bonaglia et al. 2017).

Theory: In climate research of today, a landscape approach serves management and policy agendas. Policy guidelines and regulatory instruments for spatial explicit planning and management in the Baltic region (and elsewhere) are today conceptualized to follow ecosystembased frameworks, which also include climate change mitigation and adaptation strategies (HELCOM 2013). Sustainable development in coastal environments therefore benefits from being targeted by a broad landscape-scale approach. The discipline of landscape ecology offers a spatially explicit approach of studying spatial ecology, where linkages between ecological patterns and processes are studied across a range of meaningful scales (Turner 1989). A landscape approach seriously underpins governance systems and policies for species and habitat conservation in terrestrial environments (Lui and Taylor 2002), and is today also emerging in marine management (Pittman 2018). However, even though spatial explicit heterogeneity and connectivity are acknowledged as key determinants of coastal processes and ecosystem functioning, there is a critical knowledge gap in understanding landscape-scale effects and seascape connectivity of major processes such as blue carbon sequestration (but see e.g. Gullström et al. 2018, Asplund et al. 2021). The diversity of coastal landforms in the land-sea interface may be strongly linked to coastal geomorphology, in turn affecting sedimentary processes and depositional environments shaped by e.g. rivers or waves (Woodroffe 1990). Given the fundamental gradient-oriented differences in coastal morphology and seawater properties across seascapes of the Baltic Sea (from northern Bothnian Bay to the southern Baltic basin coastlines; Snoeijs and Andrén 2017), habitat diversity at landscape level may significantly influence blue carbon estimates within vegetated habitats and strongly across the various types of coastal seascapes in the land-sea interface (Twilley et al. 2018). Differential glacio-isostatic uplift in the southern and northern parts of the Baltic Sea has influenced both sedimentation processes and the evolution of coasts throughout the Holocene (Rosentau et al., 2017). Ongoing glacial isostatic adjustment partly compensates for local sea-level rise in the Baltic Sea region and the projected rise due to global warming equates to 1.10 m near Hamburg and 0.15 m in the Bothnian Bay in a twenty-first century high-end scenario (Grinsted, 2015). Coastal erosion induced by an increasing number of severe storm surges and sea-level rise will pose a threat to low-lying coastal areas such as salt marshes and wetlands and lead to loss of coastlines (Łabuz, 2015). This will severely affect coastal vegetated habitats leading to blue carbon estimates to change over time.

Objective: Using a multiscale landscape ecology approach (Turner 1989; Wu and Hobbs 2002), we aim to assess seascape connectivity and the effects of seascape configuration, composition and change (i.e. contemporary context of land use partitioning and degraded ecosystem areas) on stocks (data from WP2), dynamics and fate (through export) of blue carbon in distinguished coastal seascape mosaics across the Baltic Sea.

Material and methods: (i) Carbon export across coastal seascapes: Carbon transport between habitats will be estimated within each coastal seascape by performing additional analyses on sediment cores already collected in the field surveys of WP1. To assess sink-source fluxes of organic matter (i.e. autochthonous and allochthonous carbon deposition), samples from sediment cores (collected and dated in WP2) will be analysed for stable isotopes (δ13C, δ15N), C/N ratios (analysed in WP2) and using environmental DNA mini-barcoding for marine macrophytes (Ortega et al. 2020), aquatic freshwater vascular plants (Coghlan et al. 2020) and terrestrial plants (Reef et al. 2014). Material of potential organic matter sources (from relevant blue forest habitats and other environments at each coastal seascape) will be collected to provide the isotopic signatures of these organic matter end-members, forming the basis to estimate their relative contribution using a mixed model. (ii) Influence of seascape configuration on carbon sequestration: The seascape maps (n = 20) produced in WP1 will be used to assess the influence of seascape configuration, based on a selection of landscape ecology-derived metrics, on blue carbon sequestration. We will use spatial metrics of relevance to be able to understand how seascape context links to blue carbon sink hotspots. The landscape assessments will assess how carbon sequestration relates to spatial arrangement (diversity and richness) of habitats (and habitat patches), proportion of major blue carbon habitats, coastal morphology, catchment area, habitat types (following divisions in the Habitats Directive 92/43/EEC) and seascape change (incorporating urban areas, agriculture areas, forestry areas, etc.). Landscape metric analyses will be performed using the spatial pattern analysis program FRAGSTATS (as a plug-in to ArcGIS, McGarigal et al. 2012) and by modelling of projections to latent structures using partial least squares (PLS) regression analysis (Wold et al. 2001).

Theory: Climate change mitigation is today a priority topic on local to global management agendas all over the world. In this respect, reducing risks of net GHG emissions from ecosystems is vital. Aquatic systems are active interacting components of the carbon cycle, and considerable research have focused on constraining carbon fluxes along the aquatic continuum (Webb et al. 2019). Integrated terrestrial and aquatic ecosystem carbon budgets are rare, but fully integrated carbon budgets are important as realistic accounting of aquatic and terrestrial carbon fluxes are needed to estimate source-sink status of ecosystems.

Objective: The overall objective of WP4 is to create carbon budgets for the main coastal seascape configurations (WPs 1 and 3) of the Baltic Sea.

Material and methods: Carbon budget estimates will be made for individual terrestrial and aquatic ecosystems along catchment gradients, with focus on understanding vegetation transport and deposition. We will assess the climatological drivers and geomorphological processes (linked to WPs 1-3) of organic carbon transport over the last century, including extreme weather and climate events (e.g. storms, droughts, floods), to develop predictive models representing various coastal seascapes. From the new understanding of the role of climate forcing on the carbon budget and transport, we will estimate the whole terrestrialaquatic carbon budget for the Baltic Sea under various future scenarios (RCPs 2.6 to 8.5), providing the contribution of the different coastal seascapes. Subsequently, we will assess how climate change affects the mitigation capacity of sensitive coastal seascapes (i.e. hotspots). We will calculate full carbon budgets for the analysed seascape types and their adjoining terrestrial environments, to determine the source or sink strength of each land/seascape by looking at the overall net green-blue carbon budget (NGBCB). Net ecosystem production (NEP, accounting for both NPP and soil respiration) will represent the amount of carbon available within the terrestrial environment for potential storage or export (e.g. Lovett et al. 2006). For the aquatic environment, the total carbon flux is calculated from the amount of carbon released or retained via lateral exchange (or by absorption) to the atmosphere, and accumulation within the aquatic environment. The NGBCB is derived from the total of inputs and outputs from all physical, biological, and anthropogenic sources within a defined spatial boundary, where the physical configuration, including land use, and climatic influence need to be included. Meteorological observations, land-use histories and leaf sediment cores where carbon mass accumulation rates are represented will be used to create conceptual models of the short- and medium-term impacts of climate on NGBCB in the analysed coastal seascapes. Subsequently, using output from regional climate model (RCM) ensembles, including REMO (Jacob 2001) and HCLIM38 (Belusic et al. 2020), the impact of climate change on future Baltic Sea region NGBCB will be estimated for different RCPs (2.6 to 8.5), providing indications of future carbon budget changes of the analysed seascapes, such as changes from sink to source or vise versa. RCMs are obtained by downscaling global climate models over a limited-area domain to provide much higher resolution, providing more robust results on landscape level.

Theory: In WP5, we will synthesize the findings from the empirical research in WPs 1-3 in order to weigh the ability of Baltic coastal seascapes for climate mitigation (including future threats), against principles laid forth in international climate agreements. Pathways for climate governance will be analysed from the point of view of the climate justice struggles identified and studied in the project (partly in WP4), as well as the implementation of solutions and suggestions regarding the results of WPs 1-4. Theoretically, WP5 aligns with the broader international discussion on climate governance (e.g. Okerere et al. 2009; Bäckstrand et al. 2017, Dorsch and Flachsland 2017, Ostrom 2014), not least emphasizing the polycentric approach and the ‘deliberative systems approaches’ (Stevenson and Dryzek 2014). While not explicitly pointing to the role of civil society and its organizations, these approaches point to the advantages of encouraging experimental efforts at multiple levels, the importance of segmenting the global tasks into what can be attempted by small and medium sized groups, which have a better understanding of specific risks and opportunities (Pacheco et.al. 2014), and the role of local innovation (Contipelli 2018). The co-production of scientific knowledge and policy recommendations as a communicative approach between knowledge in civil society and the ones emerging from expertise and power are guiding throughout the different WPs to be synthesized in WP5. Specific attention will be paid to how the different actor’s ability to achieve or affect outcomes related to just transformation is affected by the power they got in relation to resources, capacities and their positions in macro-societal structures (Marquardt 2017), in this context applied to the potential impact of using digital tools to build up a leadership with a critical mass of followers in the civil society (Torney 2019). WP5, however, is action-oriented, aiming to produce the recommendations necessary for actual social change, plus a toolkit for stakeholders, inspired by the model for policy recommendations and toolkits developed by the International Panel for Social Progress (ipsp.org) (Couldry et al. 2018a, see also Couldry et al 2018b, and Bolin 2019).

Objective: The overall aim of WP5 is to formulate policy recommendations for a just and sustainable climate governance, and create a lasting platform for dialogue with policy- and decision makers within the private and public sectors.

Material and methods: We will (a) map current status of monitoring, conservation, policy making and potential user conflicts/synergies (conservation vs. human use) of coastal blue forest habitats in the different Baltic Sea countries, (b) identify areas of concern (in GIS environment) to illustrate blue forest habitats and seascape types representing all from blue carbon hotspots to high-risk (potentially regional-based) seascape settings using spatial risk assessment methodology developed and applied by our project team (Perry et al. 2020), and (c) develop a user-friendly blue carbon ecosystem-based management tool explicitly for policy and decision-making in the different countries.