Leibniz Institute for Baltic Sea Research

The permeable silicate sediments which cover more than 50% of the continental shelves are a major, but poorly constrained sink for the vast amount of anthropogenic nitrogen (N) that enters the ocean. Surface-attached microbial communities on sand grains remove fixed-N via denitrification, a process generally restricted to anoxic or low oxygen (O2) environments. Yet, in sands, denitrification also occurs in the centimeters thick well-oxygenated surface layer, which leads to additional and substantial N-loss. So far however, the underlying mechanisms that drive denitrification in oxic sands are poorly resolved. In this study, we applied a non-invasive microfluidic technique to visualize and quantify how sediment-attached microorganisms shape O2 availability on the surface of silicate sand grains. This revealed a remarkable heterogeneity in rates; with colonies of O2 consuming and producing microorganisms situated within micrometers of each other. Using a mechanistic approach to model respiration on the surface of a single silicate sand grain we showed that the high rates of O2 consumption within the microbial colonies on the sand-grain surface outpace O2 supply from the surrounding pore water. As a result anoxic microenvironments develop on the sand grain surface, which so far have been invisible to conventional techniques. The model results indicate that anaerobic denitrification occurring in these anoxic microenvironments can account for up to 74% of denitrification in oxygenated sands, with the remainder occurring in the presence of oxygen. In a preliminary upscaling approach, using a global dataset we estimated that anoxic microenvironments in oxygenated surface layers could be responsible for up to a third of the total N-loss that occurs in silicate shelf sands. Consequently, denitrification in anoxic microenvironments drives substantial anthropogenic-N removal from continental silicate shelf sands.

The Wadden Sea is a complex coastal system where sea level rise (SLR), tidal dynamics, and geomorphology interact non-linearly. Today, the functioning of coastal ecosystems and their services in this region, historically resilient to natural changes in sea level, is at risk due to climate change-induced SLR. This study investigates the changes in tidally induced transport pathways of passive tracers, while providing a comprehensive analysis of tidal inundation, asymmetry, and current velocities under different SLR scenarios projected for 2050. The Sylt-Rømø Bight, a semi-enclosed basin, serves as the study site. Using FESOM-C coastal ocean model with a Lagrangian tracking module on a high-resolution unstructured mesh (up to 2 m resolution in the intertidal zone), we simulate tidal dynamics under SLR scenarios based on projections under low (SSP1-2.6) and high (SSP5-8.5) emission scenarios. Results show submerged areas increase by 2–3%, corresponding to a 4–7% loss of intertidal zones by 2050. As the increased depth and inundation affect the system, tidal channels show contrasting changes in current velocities, suggesting shifts in transport pathways. Meanwhile, weakening tidal asymmetry points to a shift toward a more lagoon-like system, leading to a subsequent reduction in outflowing net transport of passive tracers by up to 10%. This study offers insights into tidal transport patterns in the bight, emphasizing the separation between the southwestern and northeastern regions, both in current and future scenarios. It also provides a methodology for analyzing and explaining SLR-induced changes in the dynamics of shallow, tidally dominated areas. The findings underscore the importance of understanding the complex hydrodynamic response to SLR in coastal areas to meaningfully assess its impacts on tidal ecosystems and to develop efficient mitigation strategies for coastal protection.

Dissolved organic carbon (DOC) in coastal waters is integral to biogeochemical cycling, but global and regional drivers of DOC are still uncertain. In this study we explored spatial and temporal differences in DOC concentrations and stocks across the global coastal ocean, and how these relate to temperature and salinity. We estimated a global median coastal DOC stock of 3.15 Pg C (interquartile range (IQR) = 0.85 Pg C), with median DOC concentrations being 2.2 times higher than in open ocean surface waters. Globally and seasonally, salinity was the main driver of DOC with concentrations correlated negatively with salinity, without a clear relationship to temperature. DOC concentrations and stocks varied with region and season and this pattern is likely driven by riverine inputs of DOC and nutrients that stimulate coastal phytoplankton production. Temporally, high DOC concentrations occurred mainly in months with high freshwater input, with some exceptions such as in Eastern Boundary Current margins where peaks are related to primary production stimulated by nutrients upwelled from the adjacent ocean. No spatial trend between DOC and temperature was apparent, but many regions (19 out of 25) had aligned peaks of seasonal temperature and DOC, related to increased phytoplankton production and vertical stratification at high temperatures. Links of coastal DOC with salinity and temperature highlight the potential for anthropogenic impacts to alter coastal DOC concentration and composition, and thereby ecosystem status.

In this paper we detail the instantaneous transformation between freshwater and seawater while crossing the ocean boundary. From the kinematic boundary conditions for the seawater continuum we derive the associated flux of squared salinity and link it to an instantaneous mixing of salt (salinity variance decay) at the boundary. Within the water mass transformation framework this mixing occurs across virtual isohalines in salinity space. Our derivations complement the analytical description of seawater and clarify the demixing during freshwater removal.

Filamentous sulfide-oxidizing Beggiatoa spp., which occasionally form extensive white microbial mats, are widespread in marine coastal environments and can achieve significant biomass because of their large size. Their ability to store phosphates in the polymerized form of polyphosphates makes them potential key players in altering the phosphorus (P) cycle at the sediment-water interface. This study examined phosphate uptake and polyphosphate formation in a P-starved culture of Beggiatoa sp. 35Flor strain. Remarkably, even after sustained P starvation over five generations, the mat establishment rate of the examined culture was 46%, demonstrating considerable plasticity in response to different levels of phosphate availability. Under these P-depleted conditions, at least 17% of filaments still contained polyphosphates, highlighting their critical role in their metabolism. Upon reintroduction of phosphate to starved cultures, an extremely rapid phosphate uptake was observed within the first 10 min, with rates reaching up to 12.4 mmol phosphate g ⁻¹ protein h ⁻¹ , which is significantly higher than values previously reported in the literature for similar-sized organisms. The high phosphate uptake capacity of Beggiatoa spp., estimated at 0.6–6 mmol m ⁻² d ⁻¹ for typical densities of filaments in coastal sediments, suggests that under certain environmental conditions, these bacteria could act as a P sink and thus play an important role in benthic P cycling. IMPORTANCE Sulfide-oxidizing bacteria of the genus Beggiatoa occur ubiquitously in marine coastal sediments and have a large potential to influence phosphate fluxes at the sediment-water interface, owing to their ability to accumulate polyphosphate and their large size. However, the extent to which these bacteria can contribute to phosphorus (P) sequestration or release remains poorly assessed. The importance of this study lies in demonstrating the remarkable flexibility in the adaptation of the strain Beggiatoa sp. 35Flor to varying P availability, including prolonged P starvation and its capacity to rapidly uptake and store available phosphate in the form of polyphosphate. When considered on a global scale, these physiological traits could form the basis for Beggiatoa 's role in moderating P fluxes.

Marine pollution, especially from oil spills and litter, poses significant threats to marine ecosystems, aquaculture and fisheries. The proliferation of pollutants requires advanced monitoring techniques to enhance early detection and mitigation efforts. Artificial Intelligence revolutionizes environmental monitoring by enabling rapid and precise pollution detection using remote sensing and machine learning models. This review synthesizes 53 recent studies on Artificial Intelligence applications in marine pollution detection, focusing on different model architectures, sensing technologies and preprocessing methods. The most deployed models of Random Forest, U-Network, Generative Adversarial Networks, Mask Region-based Convolution Neural Network and You Only Look Once demonstrated high prediction rate for detecting oil spills and marine litter. However, challenges remain, including limited training datasets, inconsistencies in sensor data and real-time monitoring constraints. Future research should improve Artificial Intelligence model generalization, integrate multi-sensor data and enhance real-time processing capabilities to create more efficient and scalable marine pollution detection systems.

Cetacean interactions can be diverse and complex, spanning agonistic to affiliative behaviors. While killer whales are known for complex hunting strategies and a wide range of prey worldwide, they also engage in various non‐predatory interspecific interactions. This study describes two encounters off south Iceland, on the 23rd of June 2022 and the 20th of June 2023, where neonate pilot whales were observed with killer whale groups. No other pilot whales were sighted in the area for the duration of these encounters. The killer whale groups, of mixed age and sex classes, engaged in slow travel, foraging and social behaviors throughout the encounters. The pilot whale calves were seen surfacing in front of killer whales or in echelon position, and at times being lifted out of the water. Photo‐identification revealed that, while different individual whales participated in each event, all were likely fish‐eating specialists. The interactions with the calves displayed no clear signs of predation. We explore several hypotheses for the function of these interactions, which appear suggestive of play behavior (whether affiliative or agonistic in nature), or possibly practice hunting, or epimeletic behavior. However, because neither interaction between killer whales and a pilot whale neonate was observed from start to end, the true motivation and function of the interactions remain unknown. Our observations not only highlight the complexity of these interactions but also underscore the broader importance of investigating interspecific encounters among cetaceans and other mammals, where such behaviors can offer insights into ecological and social processes.

Marine carbon dioxide removal (mCDR) options could potentially play an important role in future CDR policy portfolios. They include, for example, ocean alkalinity enhancement, blue carbon projects such as mangrove cultivation, as well as sub-seabed storage of captured atmospheric CO2. In this paper we present a novel assessment framework designed for mCDR options. The framework provides important conceptual advancements to existing frameworks currently used to assess climate options: It clearly distinguishes between and allows for the assessment of both the feasibility and desirability of mCDR options, it makes explicit the evaluative standards upon which the assessment is based and it separates the descriptive listing of information from the evaluation of said information. The assessment framework aims to advance the debate on what role mCDR can and should play in responding to the climate crisis by providing a tool for both policymakers and stakeholders to assess mCDR options in a transparent and comprehensive way.

The Humboldt Current System (HCS) off southwest America is known for its strong upwelling and the resulting high primary production and associated oxygen minimum zones (OMZs). Macrozoobenthic species represent a group of organisms that are affected by the low oxygen concentrations in the OMZ. In January 2023, benthic diversity was investigated at 8 stations on a transect off Concepción, central Chile (in the centre of the OMZ) in a water depth range from 56 to 912 m. The measured oxygen values ranged from 0 µmol/L in the OMZ to 144.64 µmol/L outside the OMZ. At each station, 3 van Veen grabs were taken, the species identified, counted and weighed. The mean abundance, biomass and diversity were calculated for each station. This analysis provided an overview of the changes in the species communities at different oxygen concentrations. The species communities at the stations with low oxygen levels differed greatly from those with higher oxygen levels. Species diversity at the stations increased during the transition from low (<2 µmol/L) to higher oxygen levels (>100 µmol/L). In contrast, species abundance and, to a lesser extent, biomass tended to be higher at low oxygen concentrations. The species composition at the various stations showed a high occurrence of polychaetes. The spionid polychaete Paraprionospio pinnata played an important role as a central key species within the OMZ. In addition to Paraprionospio, Ampelisca araucana, Magelona phyllisae, Nephtys ferruginea and Cossura chilensis were found in high abundance in the oxygen minimum zone (50–200 m water depth). At the edge and presumably below the oxygen minimum zone (300–912 m), where the oxygen concentration rises again, the dominance of individual species decreased, and the total number of species increased. In addition, the species composition changed and the abundance of other polychaete families (Cirratulidae, Amphinomidae, Oweniidae and Capitellidae) amplified. The proportion of polychaetes in the total abundance decreased from almost 100% at the low-oxygen stations to around 60% at the stations below the oxygen minimum zone. Bivalvia of the families Thyasiridae, Nuculidae and Yoldiidae were of particular importance at the deeper stations with a share of up to 20% of the total abundance. The study of benthic communities is of central importance to better understand the future changes in the structure and function of marine ecosystems in hypoxic waters.

Marine carbon dioxide removal (mCDR) and geological carbon storage in the marine environment (mCS) promise to help mitigate global climate change alongside drastic emission reductions. However, the implementable potential of mCDR and mCS depends, apart from technology readiness, also on site‐specific conditions. In this work, we explore different options for mCDR and mCS, using the German context as a case study. We challenge each option to remove 10 Mt CO2 yr⁻¹, accounting for 8%–22% of projected hard‐to‐abate and residual emissions of Germany in 2045. We focus on the environmental, resource, and infrastructure requirements of individual mCDR and mCS options at specific sites, within the German jurisdiction when possible. This serves as an entry point to discuss main uncertainty factors and research needs beyond technology readiness, and, where possible, cost estimates, expected environmental effects, and monitoring approaches. In total, we describe 10 mCDR and mCS options; four aim at enhancing the chemical carbon uptake of the ocean through alkalinity enhancement, four aim at enhancing blue carbon ecosystems' sink capacity, and two employ geological off‐shore storage. Our results indicate that five out of 10 options would potentially be implementable within German jurisdiction, and three of them could potentially meet the challenge. Our exercise serves as an example on how the creation of more tangible and site‐specific CDR options can provide a basis for the assessment of socio‐economic, ethical, political, and legal aspects for such implementations. The approach presented here can easily be applied to other regional or national CDR capacity considerations.

Sandy beaches are ecologically important coastal ecosystems that are increasingly threatened by plastic pollution. This pollution disrupts their ecological balance and reduces their ability to provide ecosystem services. This study case at Joaquina Beach, Santa Catarina Island, southern Brazil, aimed to assess the spatiotemporal contamination by macro and mesoplastics concerning meteorological and anthropogenic variations, and to identify potential plastic sources for the region. Over 18 months (December 2018 to March 2020), monthly collections of macroplastics and mesoplastics were performed at 12 fixed sampling points. The amount of mesoplastics found was 216 items, with an average of 2.18 items m-2 (range: 0-17.33 items m-2), a higher density than that of macroplastics, of which, 1069 items were found, at an average of 0.32 items m-2 (range: 0-2.2 items m-2). Fragments were the predominant plastic type in both size categories. The region was assessed as “very clean” only once during the monitoring, with the Clean-Coast Index classifying it as “clean” in 59% of the months. March 2019 had the highest macroplastic amount, followed by April 2019 and February 2020. Meanwhile, mesoplastic quantity was highest in April 2019, December 2018, and January 2019. For both categories, beach users were identified as the main possible source of plastic litter, with a smaller contribution from fishing activities. However, meteorological conditions, like wind direction, can also contribute to plastic accumulation in the area. The months with the highest concentration of macro and meso occurrences had a prevailing pattern of southern winds. This study contributes to the knowledge addressing macro and mesoplastics, providing useful information to bridge scientific and management gaps regarding the distribution of different plastic sizes.

Bransfield Strait has been identified as a climate hotspot for understanding regional environmental changes with global impact. This study focuses on enhancing the understanding of carbon cycle dynamics and its interactions with hydrographic variables in Bransfield Strait, located on the northern Antarctic Peninsula. The stable carbon isotopes of dissolved inorganic carbon (δ¹³CDIC) were investigated in the study region during comprehensive sampling in 2023 along the major ocean basins. Bransfield Strait is influenced by two main source water masses: the Circumpolar Deep Water (CDW), which intrudes into the region from the Antarctic Circumpolar Current meander, and Dense Shelf Water (DSW), which is advected by coastal currents from the Weddell Sea continental shelf. The study reveals CDW’s dominant role in 2023, accounting for ~60% of the water mass mixture in the region and limiting the highest contribution of DSW to the deep layer of the central basin. The spatial variation of δ¹³CDIC signatures showed that biogeochemical processes predominantly shape the δ¹³CDIC distribution along the water column. Photosynthesis enriched the surface waters with the heavier carbon isotope, with signatures ranging from 2 to 1.5‰, while organic matter remineralization depleted it below the mixed layer (ranging from 0 to − 2‰). Horizontally, δ¹³CDIC distribution was influenced by the higher contribution of each source water mass. Thermodynamic fractionation contributed to the enrichment of δ¹³CDIC (~ 1 to 1.5‰) in the CDW layer in Bransfield Strait. Conversely, the predominance of younger and colder DSW exhibited a depletion of δ¹³CDIC (− 1 to − 2‰). Therefore, δ¹³CDIC is identified as an additional tracer to provide new insights into the biogeochemical and hydrodynamic processes of Bransfield Strait.

Below the water column depths, marine sediments harbor a vibrant tapestry of life that underpins a variety of ecological balances. From the tiniest microbes to complex multicellular organisms, benthic communities play pivotal roles in nutrient cycling, energy flow, and habitat formation. Understanding these intricate ecosystems is paramount, especially as they face intensive evolutionary fluctuation and natural succession. Changing climate conditions and the increasing influence of other threats from human activities are putting pressure on communities in benthic marine ecosystems and require adaptations within short periods of time. In this Special Issue, we wanted to delve into the multifaceted world of benthic biodiversity, exploring the dynamic interactions among its diverse communities and their impacts on marine ecosystem functioning. We were lucky to collect 13 articles, totaling 252 pages, that have been co-authored by 65 researchers from 8 countries. The contributions span a broad spectrum of topics, beginning with microbial communities that form the foundation of benthic ecosystems. Advanced metabarcoding techniques have unveiled the astonishing diversity and versatility of benthic protists, shedding light on their roles in biogeochemical processes and food webs and addressing their resilience to environmental perturbations. Transitioning to meiofauna and macrofauna, several studies investigate species distribution patterns in response to natural gradients and anthropogenic pressures like bottom trawling, sea level rise, or organic and pollutant influences from the river plume, suggesting species that suffer most from these perturbations and spotting those that are tolerant. These findings underscore the sensitivity of benthic organisms to habitat alterations, emphasizing the need for comprehensive monitoring and conservation strategies.

Millennial-scale variations in the strength and position of the Antarctic Circumpolar Current exert considerable influence on the global meridional overturning circulation and the ocean carbon cycle. The mechanistic understanding of these variations is still incomplete, partly due to the scarcity of sediment records covering multiple glacial-interglacial cycles with millennial-scale resolution. Here, we present high-resolution current strength and sea surface temperature records covering the past 790,000 years from the Cape Horn Current as part of the subantarctic Antarctic Circumpolar Current system, flowing along the Chilean margin. Both temperature and current velocity data document persistent millennial-scale climate variability throughout the last eight glacial periods with stronger current flow and warmer sea surface temperatures coinciding with Antarctic warm intervals. These Southern Hemisphere changes are linked to North Atlantic millennial-scale climate fluctuations, plausibly involving changes in the Atlantic thermohaline circulation. The variations in the Antarctic Circumpolar Current system are associated with atmospheric CO2 changes, suggesting a mechanistic link through the Southern Ocean carbon cycle.

Next-Generation Sequencing methods like DNA metabarcoding enable the generation of large community composition datasets and have grown instrumental in many branches of ecology in recent years. However, the sparsity, compositionality, and high dimensionality of metabarcoding datasets pose challenges in data analysis. In theory, feature selection methods improve the analyzability of eDNA metabarcoding datasets by identifying a subset of informative taxa that are relevant for a certain task and discarding those that are redundant or irrelevant. However, general guidelines on selecting a feature selection method for application to a given setting are lacking. Here, we report a comparison of feature selection methods in a supervised machine learning setup across 13 environmental metabarcoding datasets with differing characteristics. We evaluate workflows that consist of data preprocessing, feature selection and a machine learning model by their ability to capture the ecological relationship between the microbial community composition and environmental parameters. Our results demonstrate that, while the optimal feature selection approach depends on dataset characteristics, feature selection is more likely to impair model performance than to improve it for tree ensemble models like Random Forests. Furthermore, our results show that calculating relative counts impairs model performance, which suggests that novel methods to combat the compositionality of metabarcoding data are required.

Oil spills and marine litter pose significant threats to marine ecosystems, necessitating innovative real-time monitoring solutions. This research presents a novel AI-driven multisensor system that integrates RGB, thermal infrared, and hyperspectral radiometers to detect and classify pollutants in dynamic offshore environments. The system features a dual-unit design: an overview unit for wide-area detection and a directional unit equipped with an autonomous pan-tilt mechanism for focused high-resolution analysis. By leveraging multi-hyperspectral data fusion, this system overcomes challenges such as variable lighting, water surface reflections, and environmental interferences, significantly enhancing pollutant classification accuracy. The YOLOv5 deep learning model was validated using extensive synthetic and real-world marine datasets, achieving an F1-score of 0.89 and an mAP of 0.90. These results demonstrate the robustness and scalability of the proposed system, enabling real-time pollution monitoring, improving marine conservation strategies, and supporting regulatory enforcement for environmental sustainability.

Seagrass meadow ecosystems offer several valuable ecosystem services in coastal regions around the world. Recent studies have suggested that one such important service is reduction of pathogenic bacteria and specifically Vibrio spp. in adjacent waters. The specific mechanisms of pathogen reduction remain unclear, although increased sedimentation has been suggested as one likely process for pathogens to be quenched from the water column. Whether Vibrio spp. persist in the sediment or in other compartments of the seagrass meadow is currently not known, but it has been shown that marine surface biofilms can function as reservoirs of pathogenic vibrios. This general feature may also apply to seagrass and sediment surfaces. In this study, we investigated the relative abundance and community ecology of Vibrio spp. bacteria in Baltic Sea seagrass meadows using both culturing and culture-independent methods. While we did not detect a significant reduction of Vibrio spp. in the water column above unvegetated sites as compared to seagrass meadows, we observed high relative abundances of Vibrio spp. on seagrass roots This supports previous observations that marine surfaces are selectively colonized by Vibrio spp., implying that these habitats are important for the persistence and possibly release of Vibrio spp. into the water column. Our results emphasize the need to understand the interactions of pathogenic bacteria with coastal habitats, including interactions with host organisms such as seagrasses that provide biofilm microenvironments, in order to understand how diseases associated with these organisms develop.

On behalf of our ocean community of authors and readers, the editors at JGR‐Oceans would like to sincerely thank all those who gave their valuable time to review manuscripts for the journal in 2024. Thank YOU!

Eutrophication in the Baltic Sea has caused an imbalance in the inorganic nitrogen (N) to phosphorus (P) ratio, leaving excess phosphate (PO4) after the phytoplankton spring bloom that terminates after N depletion. Using monitoring data, we demonstrated that the PO4 concentration has continued to increase in the outermost Gulf of Finland during past decades. We further investigated the fate of such excess PO4 in a two‐week mesocosm (1.2 m³) experiment. The starting concentration of PO4 was 0.66 μM, and treatments included a non‐treated control (control), nitrate addition (N‐add; 3.6 μM), glucose addition (C‐add; 36 μM) and combined nitrate and glucose addition (N + C‐add). The addition of N, both in N‐add and N + C‐add treatments, stimulated nano‐ and microphytoplankton, while the picophytoplankton abundance increased after N depletion. Also, the copepod biomass was positively affected by the N addition. N2‐fixing cyanobacteria were present but in low abundance. Carbon addition did not enhance heterotrophic bacterial uptake of PO4 contrary to our expectations, nor did it affect the phyto‐ or zooplankton community composition. The PO4 concentration was reduced to ~ 0.4 μM in the control and C‐add treatments and to 0.16 μM in the two N‐amended treatments, with an inorganic N : P uptake ratio of 6.7. These results underscore the role of picophytoplankton in reducing the excess PO4 pool after the spring bloom, a function traditionally ascribed to bloom‐forming filamentous cyanobacteria in the Baltic Sea.