Editorial: Towards a Unifying Pan-Arctic Perspective of the Contemporary and Future Arctic Ocean

An international symposium addressing pan-Arctic perspectives of the marine ecosystems of the Arctic Ocean took place in October 2017 and this editorial introduces the publications that derived from the conference. The symposium focused in particular upon physical forcing and biogeochemical cycling in surface waters of the Arctic Ocean, connectivity between surface and deep waters in the central basins and adjacent slopes and the ecology of the lesser-known shelf ecosystems. The symposium was the fourth in a sequence that has pan-Arctic integrations of Arctic Ocean ecosystems at its core. The series started in 2002 and its first volume was published under the title Structure and function of contemporary food webs on Arctic shelves (Wassmann, 2006). At the 2002-meeting, a suite of marine Arctic researchers from the main nations that work in the Arctic Ocean started applying the now-ubiquitous term pan-Arctic. The term underlined that the applied research goals and directions were more than a circumarctic perspective, but distinctly considered the entire expanse of the Arctic Ocean. Based upon this exercise, increased interest in the Arctic and some of the scientific endeavors of the 4th International Polar Year central projects and key oceanographers operating in the pan-Arctic region convened at the 2nd pan-Arctic integration symposium, entitled Arctic Marine Ecosystems in an Era of Rapid Climate Change in 2009 (Wassmann, 2011). After a decade of pan-Arctic research and building upon the foundation presented in Wassmann (2006, 2011) a 3rd conference was initiated in 2012, entitled Overarching perspectives of contemporary and future ecosystems in the Arctic Ocean (Wassmann, 2015). This Research Topic brings together 13 publications from the 4th pan-arctic integration symposium held in 2017, entitled Toward a Unifying Pan-Arctic Perspective of the Contemporary and Future Arctic Ocean. We, the editors of the Research Topic, are delighted with the breadth, quality and diversity of the papers. We introduce the essence of the publications under three, summarizing headlines • Physical connectivity, yet regionality • What shapes pan-Arctic primary production • The fate of production. Toward the end we incorporate the knowledge presented in this volume into the overall progress. and status of pan-Arctic marine ecosystem integration that has been achieved, so far, through the four pan-Arctic integration symposia

Expanding Greenland seagrass meadows contribute new sediment carbon sinks

The loss of natural carbon sinks, such as seagrass meadows, contributes to grenhouse gas emissions and, thus, global warming. Whereas seagrass meadows are declining in temperate and tropical regions, they are expected to expand into the Arctic with future warming. Using paleoreconstruction of carbon burial and sources of organic carbon to shallow coastal sediments of three Greenland seagrass (Zostera marina) meadows of contrasting density and age, we test the hypothesis that Arctic seagrass meadows are expanding along with the associated sediment carbon sinks. We show that sediments accreted before 1900 were highly 13C depleted, indicative of low inputs of seagrass carbon, whereas from 1940’s to present carbon burial rates increased greatly and sediment carbon stocks were largely enriched with seagrass material. Currently, the increase of seagrass carbon inputs to sediments of lush and dense meadows (Kapisillit and Ameralik) was 2.6 fold larger than that of sparse meadows with low biomass (Kobbefjord). Our results demonstrate an increasing important role of Arctic seagrass meadows in supporting sediment carbon sinks, likely to be enhanced with future Arctic warming

Ecosystem metabolism of benthic and pelagic zones of a shallow productive estuary : spatio-temporal variability

Long-term deterioration of water quality is known to reduce the importance of benthic ecosystem metabolism in shallow coastal ecosystems, but drivers of spatial and short-term variability in ecosystem metabolism are poorly understood. We addressed this knowledge gap through detailed seasonal measurements of ecosystem metabolism across depth gradients from shallow (2 to 3 m) eelgrass-dominated to deeper (4 to 5 m) muddy regions of a shallow, productive estuary. Combined measurements of gross primary production (GPP), respiration (R) and, by difference, net ecosystem production (NEP) by the open-water diel oxygen technique and in situ chamber incubations showed high importance of shallow eelgrass habitats for metabolism at the system scale. Seasonal variations in GPP, R and NEP increased with light availability and temperature with highest NEP in all habitats during the warm and sunny mid-summer. The shallow eelgrass-dominated and neighboring habitats were seasonally net autotrophic (NEP = 0.54 and 0.31 mg O2 m-2 d-1, respectively), compared to net heterotrophy (NEP = -0.26 mg O2 m-2 d-1) at the deeper muddy site. Detailed studies along depth gradients further confirmed the role of eelgrass as a key driver of spatial differences in ecosystem metabolism across the estuary. Strong northerly winds (>8 m s-1) caused short-term (<24 h) periods of similar oxygen dynamics and similar apparent productivity in shallow and deeper waters, indicative of efficient lateral mixing, while calm periods (<4 m s-1) enabled formation of ‘pockets’, i.e. water masses with limited connectivity, which exacerbated the metabolic differences between shallow and deep sites.Peer reviewe

Major expansion of marine forests in a warmer Arctic

Accelerating warming and associated loss of sea ice are expected to promote the expansion of coastal marine forests (macrophytes) along the massive Arctic coastlines. Yet, this region has received much less attention compared to other global oceans. The available future projections of Arctic macrophytes are still limited to few species and regions, and mostly focused at lower latitude ranges, thus precluding well-informed IPCC impact assessments, conservation and management. Here we aim to quantify potential distributional changes of Arctic intertidal and subtidal brown macroalgae and eelgrass by the year 2100, relative to present. We estimate habitat suitability by means of species distribution modeling, considering changes in seawater temperature, salinity, nutrients and sea ice cover under two greenhouse gas emission scenarios, one consistent with the Paris Agreement (RCP 2.6) and the other representing limited mitigation strategies (RCP 8.5). As data on substrate conditions do not exist, the models were restricted to the depth range supporting Arctic macrophytes (down to 5 m for eelgrass and 30 m for brown macroalgae). Models projected major expansions of Arctic macrophytes between 69,940 and 123,360 km2, depending on the climate scenario, with polar distribution limits shifting northwards by up to 1.5 latitude degrees at 21.81 km per decade. Such expansions in response to changing climate will likely elicit major changes in biodiversity and ecosystem functions in the future Arctic. Expansions are, however, less intense than those already realized over the past century, indicating an overall slowing down despite accelerated warming as habitats become increasingly occupied..This study was supported by the Independent Research Fund Denmark through the project “CARMA” (8021- 00222B) and the European Union through the project “FACE-IT” to DK-J, the Foundation for Science and Technology (FCT) through projects UID/Multi/04326/2020 to CCMAR and PTDC/BIA-CBI/6515/2020, the transitional norm DL57/2016/CP1361/CT0035 to JA and the fellowship SFRH/BD/144878/2019 to EF, and a Pew Marine Fellowship to ES.info:eu-repo/semantics/publishedVersio

Seagrass sedimentary deposits as security vaults and time capsules of the human past

Seagrass meadows form valuable ecosystems, but are considered to have low cultural value due to limited research efforts in this field. We provide evidence that seagrass deposits play a hitherto unrealized central role in preserving valuable submerged archaeological and historical heritage across the world, while also providing an historical archive of human cultural development over time. We highlight three case studies showing the significance of seagrass in protecting underwater cultural heritage in Denmark, the Mediterranean and Australia. Moreover, we present an overview of additional evidence compiled from the literature. We emphasize that this important role of seagrasses is linked to their capacity to form thick sedimentary deposits, accumulating over time, thereby covering and sealing submerged archaeological heritage. Seagrass conservation and restoration are key to protecting this buried heritage while also supporting the role of seagrass deposits as carbon sinks as well as the many other important ecosystem functions of seagrasses. © 2018, The Author(s)

Sequestration of macroalgal carbon: the elephant in the blue carbon room

Macroalgae form the most extensive and productive benthic marine vegetated habitats globally but their inclusion in Blue Carbon (BC) strategies remains controversial. We review the arguments offered to reject or include macroalgae in the BC framework, and identify the challenges that have precluded macroalgae from being incorporated so far. Evidence that macroalgae support significant carbon burial is compelling. The carbon they supply to sediment stocks in angiosperm BC habitats is already included in current assessments, so that macroalgae are de facto recognized as important donors of BC. The key challenges are (i) documenting macroalgal carbon sequestered beyond BC habitat, (ii) tracing it back to source habitats, and (iii) showing that management actions at the habitat lead to increased sequestration at the sink site. These challenges apply equally to carbon exported from BC coastal habitats. Because of the large carbon sink they support, incorporation of macroalgae into BC accounting and actions is an imperative. This requires a paradigm shift in accounting procedures as well as developing methods to enable the capacity to trace carbon from donor to sink habitats in the ocean

Differentiation in fitness-related traits in response to elevated temperatures between leading and trailing edge populations of marine macrophytes

The nature of species distribution boundaries is a key subject in ecology and evolution. Edge populations are potentially more exposed to climate-related environmental pressures. Despite research efforts, little is known about variability in fitness-related traits in leading (i.e., colder, high latitude) versus trailing (i.e., warmer, low latitude) edge populations. We tested whether the resilience, i.e. the resistance and recovery, of key traits differs between a distributional cold (Greenland) and warm (Portugal) range edge population of two foundation marine macrophytes, the intertidal macroalga Fucus vesiculosus and the subtidal seagrass Zostera marina. The resistance and recovery of edge populations to elevated seawater temperatures was compared under common experimental conditions using photosynthetic efficiency and expression of heat shock proteins (HSP). Cold and warm edge populations differed in their response, but this was species specific. The warm edge population of F. vesiculosus showed higher thermal resistance and recovery whereas the cold leading edge was less tolerant. The opposite was observed in Z. marina, with reduced recovery at the warm edge, while the cold edge was not markedly affected by warming. Our results confirm that differentiation of thermal stress responses can occur between leading and trailing edges, but such responses depend on local population traits and are thus not predictable just based on thermal pressures.FCT (Portuguese Science Foundation) [BIODIVERSA/0004/2015, PTDC/MAR-EST/6053/2014, UID/Multi/04326/2013, SFRH/BPD/63/03/2009, SFRH/BPD/107878/2015, SFRH/BD/74436/2010]; European Commission (ATP) [226248

Fingerprinting blue carbon: Rationale and tools to determine the source of organic carbon in marine depositional environments

Blue carbon is the organic carbon in oceanic and coastal ecosystems that is captured on centennial to millennial timescales. Maintaining and increasing blue carbon is an integral component of strategies to mitigate global warming. Marine vegetated ecosystems (especially seagrass meadows, mangrove forests, and tidal marshes) are blue carbon hotspots and their degradation and loss worldwide have reduced organic carbon stocks and increased CO2 emissions. Carbon markets, and conservation and restoration schemes aimed at enhancing blue carbon sequestration and avoiding greenhouse gas emissions, will be aided by knowing the provenance and fate of blue carbon. We review and critique current methods and the potential of nascent methods to track the provenance and fate of organic carbon, including: bulk isotopes, compound-specific isotopes, biomarkers, molecular properties, and environmental DNA (eDNA). We find that most studies to date have used bulk isotopes to determine provenance, but this approach often cannot distinguish the contribution of different primary producers to organic carbon in depositional marine environments. Based on our assessment, we recommend application of multiple complementary methods. In particular, the use of carbon and nitrogen isotopes of lipids along with eDNA have a great potential to identify the source and quantify the contribution of different primary producers to sedimentary organic carbon in marine ecosystems. Despite the promising potential of these new techniques, further research is needed to validate them. This critical overview can inform future research to help underpin methodologies for the implementation of blue carbon focused climate change mitigation schemes

Sinking seaweed in the deep ocean for carbon neutrality is ahead of science and beyond the ethics

Sinking vast amounts of seaweed in the deep ocean is currently being proposed as a promising ocean carbon dioxide removal strategy as well as a natural-based solution to mitigate climate change. Still, marketable carbon offsets through large-scale seaweed sinking in the deep ocean lack documentation and could involve unintended environmental and social consequences. Managing the risks requires a number of urgent actions