Publications

Abstract 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 13 C 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.

Certain coastal ecosystems such as mangrove, saltmarsh and seagrass habitats have been identified as significant natural carbon sinks, through the sequestration and storage of carbon in their biomass and sediments, collectively known as 'blue carbon' ecosystems. These ecosystems can often thrive in extreme environments where terrestrial systems otherwise survive at the limit of their existence, such as in arid and desert regions of the globe. To further our understanding of the capability of blue carbon ecosystems to sequester and store carbon in such extreme climates, we measured carbon sediment stocks in 25 sites along the Western Arabian Gulf coast. While seagrass meadows and saltmarsh habitats were widely distributed along the coast, mangrove stands were much reduced as a result of anthropogenic pressures, with 90% of stands having been lost over the last century. Carbon stocks in 1 m deep surface sediments were similar across all three blue carbon habitats, with comparable stocks for saltmarsh (81 ± 22 Mg Corg ha−1), seagrass (76 ± 20 Mg Corg ha−1) and mangroves (76 ± 23 Mg Corg ha−1). We recorded a 38% decrease in carbon stocks between mature established mangrove stands (91 Mg Corg ha−1) and recently planted mangroves (56 Mg Corg ha−1). Mangroves also had the lowest carbon stock per total area owing to their very limited spatial coverage along the coast. The largest stock per total area belonged to seagrass beds as a result of their large spatial coverage within the Gulf. We employed 210Pb dating to determine the sediment accretion rates in each ecosystem and found mangrove habitats to be the most efficient carbon sequesters over the past century, with the highest carbon burial rate of the three ecosystems (19 g Corg m−2 yr−1), followed by seagrass (9 g Corg m−2 yr−1) and saltmarshes (8 g Corg m−2 yr−1). In this work, we describe a comprehensive comparison of sediment stocks in different blue carbon ecosystems within a single marine environment and across a large geographical area, and discuss our results in a global context for other blue carbon ecosystems in the dry tropics.

Abstract. The Maritime Aerosol Network (MAN) has been collecting data over the oceans since November 2006. Over 80 cruises were completed through early 2010 with deployments continuing. Measurements areas included various parts of the Atlantic Ocean, the Northern and Southern Pacific Ocean, the South Indian Ocean, the Southern Ocean, the Arctic Ocean and inland seas. MAN deploys Microtops hand-held sunphotometers and utilizes a calibration procedure and data processing traceable to AERONET. Data collection included areas that previously had no aerosol optical depth (AOD) coverage at all, particularly vast areas of the Southern Ocean. The MAN data archive provides a valuable resource for aerosol studies in maritime environments. In the current paper we present results of AOD measurements over the oceans, and make a comparison with satellite AOD retrievals and model simulations.

Abstract. Diatoms tend to dominate the Arctic spring bloom, a key event in the ecosystem. Large sinking of diatoms is expected at the end of the bloom driven by deteriorated cell status associated to nutrients (silicon) depletion. However, there are few reports on the status of diatoms' health during Arctic blooms and its possible role on sedimentary fluxes. Here we quantify, using the Bottle-Net, Arctic diatom stocks below and above the photic layer and assess their cell health status. The communities were sampled around the Svalbard Islands and encompassed a broad diversity of conditions and bloom stages. About 1/4 (mean±SE 24.2 ± 6.7 %) of the total water column (max. 415 m) diatom stock was found below the photic layer, indicating significant sinking of diatoms in the area. The fraction of living diatom cells in the photic layer averaged 59.4 ± 6.3 % but showed the highest percentages (72.0 %) in stations supporting active blooms. In contrast, populations below the photic layer were dominated by dead cells (20.8 ± 4.9 % living cells). The percentage of diatom’s stock found below the photic layer was negatively related to the percentage of living diatoms in the surface, indicating that healthy populations remained in the surface layer. An experiment on board in a tall (1.35 m) sedimentation column confirmed that dead diatom cells from the Arctic community sink faster that living ones. Also, diatoms cell mortality increased in darkness, showing an averaged half life of 1.025 ± 0.075 d−1. The results conform to a conceptual model where diatoms grow during the bloom until silicic acid stocks are depleted, and support a link between diatom cell health status and sedimentation fluxes in the Arctic. Healthy arctic phytoplankton communities remained at the photic layer, whereas dying communities exported a large fraction of the biomass to the aphotic zone, fuelling carbon sequestration and benthic ecosystems.

Abstract. The temperature dependence of planktonic metabolism in the Subtropical North Atlantic Ocean was assessed on the basis of measurements of gross primary production (GPP), community respiration (CR) and net community production (NCP), as well as experimental assessments of the response of CR to temperature manipulations. Metabolic rates were measured at 68 stations along three consecutive longitudinal transects completed during the Malaspina 2010 Expedition, in three different seasons. Temperatures gradients were observed in depth and at basin and seasonal scale. The results showed seasonal variability in the metabolic rates, being the highest rates observed during the spring transect. The overall mean integrated GPP/CR ratio was of 1.39 ± 0.27 decreasing from winter to summer and the NCP for the Subtropical North Atlantic Ocean during this cruises, was net autotrophy (NCP > 0) in about two-thirds of the total sampled communities (68.2%). Here, we reported the activation energies describing the temperature-dependence of planktonic community metabolism, which generally was higher for CR than for GPP in the Subtropical North Atlantic Ocean, as the metabolic theory of ecology predicts. Also, we performed an assessment of the activation energies describing the responses to in situ temperature at field (EaCR = 1.64 ± 0.36 eV) and those derive experimentally by temperature manipulations (EaCR = 1.45 ± 0.6 eV), which showed a great consistency.

Current emission reduction pledges under the 2015 Paris Agreement are insufficient to keep global temperature “well below +2°C” in 2100 relative to pre-industrial levels and to reach targets of the United Nations Sustainable Development Goals. Increased political ambition is therefore required, as well as enhanced efforts in terms of both mitigation and ecosystem and human adaptation. There is growing evidence highlighting both the role the ocean plays in mitigating anthropogenic climate change (i.e., absorption of atmospheric heat and anthropogenic carbon), and the cascading consequences on its chemistry and physics (i.e., ocean warming, acidification, deoxygenation, sea-level rise), ecosystems and ecosystem services. In such a context, a critical question arises: what are the ocean-based opportunities for climate action? In other words, what is the potential of the ocean and its ecosystems to reduce the causes of climate change and its impacts? This document summarises the main findings of The Ocean Solutions Initiative1 that assessed the potential of 13 ocean-based measures.

“Blue carbon” ecosystems, which include tidal marshes, mangrove forests, and seagrass meadows, have large stocks of organic carbon (C org ) in their soils. These carbon stocks are vulnerable to decomposition and – if degraded – can be released to the atmosphere in the form of CO 2 . We present a framework to help assess the relative risk of CO 2 emissions from degraded soils, thereby supporting inclusion of soil C org into blue carbon projects and establishing a means to prioritize management for their carbon values. Assessing the risk of CO 2 emissions after various kinds of disturbances can be accomplished through knowledge of both the size of the soil C org stock at a site and the likelihood that the soil C org will decompose to CO 2 .

China is facing intense coastal eutrophication. Large-scale seaweed aquaculture in China is popular, now accounting for over 2/3's of global production. Here, we estimate the nutrient removal capability of large-scale Chinese seaweed farms to determine its significance in mitigating eutrophication. We combined estimates of yield and nutrient concentration of Chinese seaweed aquaculture to quantify that one hectare of seaweed aquaculture removes the equivalent nutrient inputs entering 17.8 ha for nitrogen and 126.7 ha for phosphorus of Chinese coastal waters, respectively. Chinese seaweed aquaculture annually removes approximately 75,000 t nitrogen and 9,500 t phosphorus. Whereas removal of the total N inputs to Chinese coastal waters requires a seaweed farming area 17 times larger than the extant area, one and a half times more of the seaweed area would be able to remove close to 100% of the P inputs. With the current growth rate of seaweed aquaculture, we project this industry will remove 100% of the current phosphorus inputs to Chinese coastal waters by 2026. Hence, seaweed aquaculture already plays a hitherto unrealized role in mitigating coastal eutrophication, a role that may be greatly expanded with future growth of seaweed aquaculture.

A dataset containing the results of a systematic assessment of the literature on the effects of human-altered soundscapes on marine life. This identified 538 peer-reviewed papers that consider how changes in anthrophony, geophony, and biophony affect marine fauna. Papers on the effects of anthrophony are categorised by species, taxonomic group, effect category, statistical outcome, and whether mitigation strategies were tested. Papers considering changes in bio- or geo-phony contain details on the ecosystem where responses were measured, the effect shown, and the statistical outcome.

Coral reefs are essential marine ecosystems that support a vast array of biodiversity and provide numerous benefits, including fisheries, tourism, and coastal protection. However, these ecosystems are increasingly threatened by various factors, including anthropogenic noise from activities such as shipping and coastal development. Traditional acoustic methods of monitoring reef health, such as hydrophones, are limited by their point-based sensing, reliance on batteries, and need for manual data retrieval, which can be labor-intensive and costly. In this study, we explore the application of fiber-optic distributed acoustic sensing (DAS) for real-time marine reef monitoring, a new application compared to its previous use in deep-sea soundscape monitoring. We deployed a fiber-optic DAS system in a reef area on the coast of the Central Red Sea, alongside a conventional hydrophone for comparison. The experiment was conducted in a degraded inshore reef near the KAUST shoreline, characterized by sand, macroalgae, scattered boulders, and encrusting sponges. This site was selected as a proxy for coral reef monitoring due to its biological activity, including snapping shrimp and the presence of reef-related fish species. Our observations revealed significant acoustic activity within the 1.5 to 5 kHz range, with snapping shrimp sounds increasing after the onshore lights were switched off, consistent with known behavioral patterns of increased acoustic activity during low-light conditions. Additionally, we detected various fish vocalizations, including drums and impulses, within the 100 to 1000 Hz range. The DAS system also successfully tracked the timing and trajectory of scuba diver movements along the reef. These findings demonstrate the potential of fiber-optic DAS technology to provide high-resolution spatial mapping of reef soundscapes, offering a comprehensive and cost-effective solution for continuous reef monitoring, thereby demonstrating the feasibility of DAS for real-time acoustic monitoring in reef environments.

Coastal wetlands are critical blue carbon reservoirs, yet the depth-resolved impacts of warming on belowground carbon dynamics remain poorly understood. Over the course of an 8-year in situ experiment, we investigated plant-derived carbon inputs, soil carbon losses via respiration, and microbially mediated carbon fixation across a 60 cm soil profile under a projected 2°C atmospheric warming scenario. Plant carbon fixation (above- and belowground net primary productivity) and soil respiration exhibited synchronized responses to warming, with an initial increase, followed by a decline in the mid-term, and no significant response in the later stages. Soil and microbial respiration stabilized after prolonged exposure to elevated temperatures, as these processes were constrained by substrate availability. In contrast, phospholipid fatty acid profiling, amino sugar biomarkers, and metagenome-assembled genomes consistently indicated a greater than one-third reduction in microbial carbon fixation within subsoils (40-60 cm). Our fully factorial, depth-stratified warming design reveals the particular vulnerability of deep soil microbial carbon retention to long-term climate warming, independent of shifts in plant input or respiratory carbon loss. This work highlights underappreciated pathways influencing soil blue carbon dynamics in a changing world.