Oceanographic Monitoring Research Articles

Integrating Blockchains with the IoT: A Review of Architectures and Marine Use Cases

This review examines the integration of blockchain technology with the IoT in the Marine Internet of Things (MIoT) and Internet of Underwater Things (IoUT), with applications in areas such as oceanographic monitoring and naval defense. These environments present distinct challenges, including a limited communication bandwidth, energy constraints, and secure data handling needs. Enhancing BIoT systems requires a strategic selection of computing paradigms, such as edge and fog computing, and lightweight nodes to reduce latency and improve data processing in resource-limited settings. While a blockchain can improve data integrity and security, it can also introduce complexities, including interoperability issues, high energy consumption, standardization challenges, and costly transitions from legacy systems. The solutions reviewed here include lightweight consensus mechanisms to reduce computational demands. They also utilize established platforms, such as Ethereum and Hyperledger, or custom blockchains designed to meet marine-specific requirements. Additional approaches incorporate technologies such as fog and edge layers, software-defined networking (SDN), the InterPlanetary File System (IPFS) for decentralized storage, and AI-enhanced security measures, all adapted to each application’s needs. Future research will need to prioritize scalability, energy efficiency, and interoperability for effective BIoT deployment.

Oceanographic monitoring in Hornsund fjord, Svalbard

Abstract. Several climate-driven processes take place in the Arctic fjords. These include ice–ocean interactions, biodiversity and ocean circulation pattern changes, and coastal erosion phenomena. Conducting long-term oceanographic monitoring in the Arctic fjords is, therefore, essential for better understanding and predicting global environmental shifts. Here we address this issue by introducing a new hydrographic dataset from Hornsund, a fjord located in the southwestern part of the Svalbard archipelago. Hydrographic properties have been monitored with vertical temperature, salinity and depth profiles in several locations across the Hornsund fjord from 2015 to 2023. From 2016 onward, dissolved oxygen and turbidity data are available for the majority of casts. The dataset contributes to the so far infrequent observations, especially in spring and autumn, and extends the observations, typically concentrated in the central fjord, to the areas adjacent to the tidewater glaciers. Because sediment discharge from glaciers and land is an inseparable part of the glacier–ocean interactions, the suspended sediment concentration in the water column and the daily sedimentation rate adjacent to the tidewater glaciers are monitored with regular water sampling and bottom-moored sediment traps. Here we present the planning and execution of the monitoring campaign from the collection of the data to the postprocessing methods. All datasets are publicly available in the repositories referred to in the “Data availability” section of this paper.

Multiannual oceanographic studies in the coastal area of Karadag Nature Reserve and Koktebel Bay (northern Black Sea) in 2004–2021

The study presents the results of the first multiannual oceanographic monitoring in the coastal area of Karadag Nature Reserve and adjacent water areas in May and September in the period from 2004 to 2021. Significant and unprecedentedly high increasing trends were detected for salinity in May. Increasing trends in nitrate concentration in Koktebel Bay were noted both for May and for September. The five-day biochemical oxygen demand (BOD5), ammonium and phosphate concentrations demonstrated mostly negative trends for the measurements in May and positive trends in September. Significant decreasing trends in phytoplankton biomass were noted in September, which fit in the range of the larger-scale phytoplankton trends reported in the literature. The position of the extrema of nutrients in the long-term-average and trend slope maps was confined to the areas of increased hydrodynamic and anthropogenic activity. Multivariate analysis showed significant negative correlations of the quantitative phytoplankton characteristics with salinity, which were explained by the effect of the freshened and nutrient-enriched Azov plume. The absence of significant correlations with nutrients was attributed to the non-conservative and stochastic character of the nutrient inflow on the yearly scale and time lag between the nutrient supply and the phytoplankton growth. The large percentage of occurrences of low BOD5 values at any station indicates the unpolluted status of the area under research, which is confirmed by the low trophic index values and appearance of probability distributions of the nutrient concentrations.

Interactive impact of residual pyrethroid compounds used in the Chilean salmon farming industry and coastal acidification conditions on the feeding performance of farmed mussels in northern Patagonia

The use of pyrethroids in aquaculture has been an important component of achieving a thriving salmon farming industry in Chile. While the residual presence of such substances is known to depend on environmental conditions, most ecotoxicological studies to date have not considered environmental context. Here, we conducted oceanographic monitoring combined with experiments aiming to estimate the effects of two pyrethroids on the feeding rates of larvae of farmed mussels, Mytilus chilensis. In additional experiments, mussel spats were exposed to both pyrethroids, but under contrasting temperature/pH so as to mimic winter and summer conditions. Experiments mimicking spring conditions revealed that both pyrethroid substances affected the feeding of mussel larvae as a function of concentration. Conversely, significant impact of pyrethroids on adults were not observed with regard to temperature and pH, but a significant impact of low temperature/low pH condition on ingestion rates was confirmed. Given the current status of increasing ocean acidification, the results of this study are expected to provide useful information with regard to achieving sustainable mussel aquaculture, especially considering both activities occur in similar geographic areas, and the expansion of salmon farming areas is ongoing in Chile.

Environmental conditions on the Pacific halibut fishing grounds obtained from a decade of coastwide oceanographic monitoring, and the potential application of these data in stock analyses

Context Establishing baseline environmental characteristics of demersal fish habitat is essential to understanding future distribution changes and to identifying shorter-term anomalies that may affect fish density during monitoring efforts. Aims Our aim was to synthesise environmental data to provide near-bottom oceanographic baseline information on the Pacific halibut fishing grounds, to establish geographic groupings that may be used as co-variates in fish-density modelling and to identify temporal trends in the data. Methods Water-column profiler data were collected from 2009 to 2018 along the North American continental shelf, during summer fishery surveys focused on Pacific halibut. Key results In addition to establishing baseline information on the fishing grounds, this analysis illustrated that environmental variables can be grouped geographically into four regions that correspond to the four biological regions established by the International Pacific Halibut Commission. A spatio-temporal modelling approach is presented as an example of how to describe the relationship between environmental data and Pacific halibut distribution. Conclusions This study has highlighted the efficacy of environmental data in analysing fish distribution and density changes. Implications Oceanographic monitoring provides the ability to detect annual anomalies such as seasonal hypoxic zones that may affect fish density and to establish baseline information for future research.

Phenotypic plasticity and carryover effects in an ecologically important bivalve in response to changing environments

Phenotypic plasticity can improve an organism’s fitness when exposed to novel environmental conditions or stress associated with climate change. Our study analyzed spatiotemporal differences in phenotypic plasticity and offspring performance in Olympia oysters Ostrea lurida. This species is an ecosystem engineer and is of great interest for commercial and restoration aquaculture. We used a multidisciplinary approach to examine acute and long-term physiological differences in O. lurida in response to in situ oceanographic conditions in a dynamic inland sea. We outplanted oysters to different areas in Puget Sound, Washington, affixing cages to anchor lines of oceanographic monitoring buoys. This allowed us to couple high-resolution oceanographic data with organism’s phenotypic response. To assess spatiotemporal differences in oyster physiological performance, we collected oysters after six-months and one year of acclimatization at four field sites. During each collection period we evaluated changes in shell properties, diet, metabolism, and reproduction. Adult growth, δ13C and δ15N isotopic signatures, and gametogenesis were affected by both seasonal and environmental conditions. In the winter, oysters from all sites had higher respiration rates when exposed to acute thermal stress, and lower respiration response to acute pH stress. Lipid content, sex ratio and shell strength were unchanged across locations. Offspring growth rates between sites at experimental temperature 20°C closely reflected parental growth rate patterns. Offspring survival was not correlated with growth rates suggesting different energetic trade-offs in oyster offspring. The metabolic response (respiration) of larvae reached its highest point at 20°C but sharply decreased at 25°C. This indicates that larvae are more sensitive to temperature stress, as adults did not exhibit a reduction in metabolic response at 25°C. By deploying genetically similar oysters into distinct environments and employing a wide range of physiological methodologies to examine performance and fitness, our results indicate that Olympia oysters exhibit a high degree of phenotypic plasticity and show evidence of parental carryover.

Flow measurement in the southern coast of the Caspian Sea

Oceanographic monitoring was conducted in the southern basin of the Caspian Sea to evaluate the physical structure of seawater and sea currents. This monitoring aimed to gather data and analyze the characteristics of the seawater, including temperature, salinity, density, and other relevant parameters. Additionally, the monitoring also focused on studying the patterns and dynamics of sea currents in the region. The collected data and analysis from this monitoring period provided valuable insights into the oceanographic conditions of the southern basin of the Caspian Sea. During storm events, a high level of correlation between the current and wind data was observed in the southern basin of the Caspian Sea during the 2017-2018 monitoring period. The measured current data indicated that the predominant directions were north (N) and northwest (NW). It was observed that strong gusts predominantly originated from the north (N) and northwest (NW) directions. Additionally, a smaller portion of the strong gusts was observed to come from the South-Southeast (S-SE) direction. These findings indicate that the prevailing wind patterns during the monitoring period were primarily from the north and northwest, with a lesser contribution from the South-Southeast direction. The current profiles observed during the monitoring period in the southern basin of the Caspian Sea were primarily influenced by the general circulation pattern of the region. This circulation pattern played a significant role in shaping the current profiles observed during the measurements. In terms of the surface current speed, the maximum recorded value was approximately 200 cm/s, which occurred in January. This indicates that there were instances of relatively high-speed currents in the southern basin of the Caspian Sea during that time. The evaluation of inter-annual variability in vertical structure and seawater temperature profiles during the monitoring period confirmed the presence of seasonal stratification in the water column of the southern basin of the Caspian Sea. This stratification was observed to consist of different layers, including the surface mixed layer, thermocline, and deep-water layers. The formation of these layers is indicative of the seasonal variations in temperature and density within the water column. The vertical changes in the water’s physical parameters during the monitoring period revealed the formation of stratification in the southern basin of the Caspian Sea. In March, it was observed that the water temperature decreased from 11.5°C at the surface to 8.5°C at a depth of 100 m, indicating the presence of a temperature gradient. As the monitoring progressed into May, the stratification became stronger, with the surface water temperature reaching around 23°C. By August, the surface layer of the sea water experienced a significant increase in temperature, reaching 29°C. These observations highlight the development of stratification and the seasonal variations in water temperature during the monitoring period in spring and summer seasons. The water temperature beneath the thermocline layer, specifically at a depth of 100 meters, was recorded to be around 8-8.5 oC. Additionally, the water salinity in the water column exhibited fluctuations between 12-12.5 (psu). Monitoring and understanding physical properties variations are crucial for assessing the oceanographic conditions and their potential impact on marine ecosystems in the southern basin of the Caspian Sea.

A high-resolution synthesis dataset for multistressor analyses along the US West Coast

Abstract. Global trends of ocean warming, deoxygenation, and acidification are not easily extrapolated to coastal environments. Local factors, including intricate hydrodynamics, high primary productivity, freshwater inputs, and pollution, can exacerbate or attenuate global trends and produce complex mosaics of physiologically stressful or favorable conditions for organisms. In the California Current System (CCS), coastal oceanographic monitoring programs document some of this complexity; however, data fragmentation and limited data availability constrain our understanding of when and where intersecting stressful temperatures, carbonate system conditions, and reduced oxygen availability manifest. Here, we undertake a large data synthesis to compile, format, and quality-control publicly available oceanographic data from the US West Coast to create an accessible database for coastal CCS climate risk mapping, available from the National Centers for Environmental Information (accession 0277984) at https://doi.org/10.25921/2vve-fh39 (Kennedy et al., 2023). With this synthesis, we combine publicly available observations and data contributed by the author team from synoptic oceanographic cruises, autonomous sensors, and shore samples with relevance to coastal ocean acidification and hypoxia (OAH) risk. This large-scale compilation includes 13.7 million observations from 66 sources and spans 1949 to 2020. Here, we discuss the quality and composition of the synthesized dataset, the spatial and temporal distribution of available data, and examples of potential analyses. This dataset will provide a valuable tool for scientists supporting policy- and management-relevant investigations including assessing regional and local climate risk, evaluating the efficacy and completeness of CCS monitoring efforts, and elucidating spatiotemporal scales of coastal oceanographic variability.

Aerial Drone Imaging in Alongshore Marine Ecosystems: Small-Scale Detection of a Coastal Spring System in the North-Eastern Adriatic Sea

The eastern coastline of the Gulf of Trieste (north-eastern Adriatic Sea, Italy) is characterized by the occurrence of coastal and submarine freshwater springs of karstic origin. In one of these areas, we performed a survey with a drone with a thermal camera installed, in tandem with in situ oceanographic sampling with a CTD. Drone images revealed a small time-space scale (i.e., up to a few meters) phenomenon of freshwater plumes floating over seawater. Comparing sea surface temperature data with those acquired in situ revealed that the phenomenon was not clearly detectable by the classical oceanographic monitoring, this surface spring freshwater layer being too thin. Instead, the drone’s thermal camera detected these dynamics with great accuracy, indicating that aerial drones can be efficiently used for studying fine-scale events involving surface waters (e.g., spills/pollution). The experience gained allowed us to discuss some of the advantages and disadvantages of using drone thermal imaging for monitoring alongshore areas.

A cabled ocean acoustic array to be installed in the Gulf of Maine

It is often noted that we know more about the planets than the processes of our oceans. A network of ocean observatories is improving our knowledge; however, measurements are still relatively scarce and do not provide enough data to validate and refine models of ocean currents, soundscapes, and biological activity. A cabled acoustic array is being developed for deployment in 2024 in the coastal waters of the Gulf of Maine to complement existing oceanographic monitoring infrastructure. The system will make measurements of the nearshore (6 km) environment and shed light on the connections between coastal measurements and the overall Gulf of Maine environment. The system is designed to synoptically collect acoustic, oceanographic, and biogeochemical data, while also remaining flexible enough in design to incorporate new sensors in the future. Measurements made with this ocean asset will serve as a baseline for pattern and trend analyses of changing environmental conditions in an area of high productivity, human use, and species diversity. The data management and visualization team are developing an infrastructure to provide public access to the data and its products. Public data access will permit researchers at all levels to advance our understanding of ocean acoustics and oceanographic processes.

Occurrence and behavioral rhythms of the endangered Acadian redfish (Sebastes fasciatus) in the Sambro Bank (Scotian Shelf)

The genus Sebastes is a morphologically and ecologically diverse genus of rockfish characterized by high longevity, late-maturity and low natural mortality. On the northwest Atlantic continental shelf, the Acadian redfish (Sebastes fasciatus) is the most common rockfish species above 300 m depth. This species has been widely exploited resulting in the depletion or collapse of most of its stocks. Management of long-lived species with intricate life-history characteristics is challenging and requires highly integrated biological and oceanographic monitoring, which allow the identification of environmental drivers and demographic and behavioral trends. The present study uses high-temporal resolution imaging and environmental data, acquired with an autonomous lander deployed for 10-months at the Sambro Bank Sponge Conservation Area (Scotian Shelf) to elucidate S. fasciatus temporal dynamics and behavioral trends in response to near-bed environmental conditions. S. fasciatus, mostly displayed passive locomotion and static behaviors, in common with other shelf-dwelling Sebastes species. Structural complexity provided by sponges positively influenced S. fasciatus presence. Fish used sponges to avoid being dragged by bottom currents. Hydrodynamics appear to act as a synchronizing factor conditioning its swimming behavior. S. fasciatus total counts exhibited a seasonal shift in rhythm’s phase likely reflecting changes in lifestyle requirements. This study provides new insights on S. fasciatus dynamics and behavior. Nonintrusive monitoring approaches, such as the one used in this study, will be key to monitor this threatened species populations. Especially, since it is expected that S. fasciatus will experience distribution shifts to higher latitudes due to future climate stressors.

Impact of a Turbulent Ocean Surface on Laser Beam Propagation.

The roughness of the ocean surface significantly impacts air-to-sea imaging, oceanographic monitoring, and optical communication. Most current and previous methods for addressing this roughness and its impact on optical propagation are either entirely statistical or theoretical, or are ‘mixed methods’ based on a combination of statistical models and parametric-based physical models. In this paper, we performed experiments in a 50-foot-wave tank on wind-generated waves, in which we varied the wind speed to measure how the surface waves affect the laser beam propagation and develop a geometrical optical model to measure and analyze the refraction angle and slope angle of the laser beam under various environmental conditions. The study results show that the laser beam deviations/distortions and laser beam footprint size are strongly related to wind speed and laser beam incidence angle.

Environmental changes in Krossfjorden, Svalbard, since 1950: Benthic foraminiferal and stable isotope evidence

ABSTRACT Environmental changes for the past ca. 50 years were studied in a short sediment core from inner Krossfjorden, Svalbard, investigating benthic foraminifera and stable isotopes (δ18O, δ13C). A depth–age model based on anthropogenic 137Cs time markers indicates that record covers the period from 1955 to 2007 and has sediment accumulation rates of ca. 0.3 to 1 cm/year. The benthic foraminifera are arctic and/or common in glaciomarine environments. Six fauna assemblages were identified using stratigraphically constrained cluster analysis. Benthic foraminiferal fauna assemblages are mainly dominated by Cassidulina reniforme. Elphidium clavatum is dominant from 1973 to 1986 and 2002 to 2007, likely due to greater turbidity in the water column. We interpret the increased percentages of Spiroplectammina biformis over the same intervals to reflect a slightly lower salinity probably caused by meltwater. During a short time period, 1970 to 1973, Stainforthia concava dominates the benthic foraminiferal fauna interpreted to reflect increased productivity within a marginal ice zone. Other species as Islandiella norcrossi, Nonionellina labradorica, Islandiella helenae, and Melonis barleanus also indicate more nutrient-rich waters are present but not very abundant throughout the record probably due to the glacier proximal position of the study site. The stable isotope record (δ18O) shows lighter values from 2001 to 2007, which seem to correlate well with oceanographic monitoring data showing increasing core temperatures of West Spitsbergen Current.

Monitoring and Locating Oceanic Sailors Through Wireless Sensor Network

A systematic oceanographic monitoring and management requires an effective marine tracking system. It helps the localization of different marine assets, their border control and in encountering emergency situations. Thus, a reliable marine tracking system is always required to ensure a safe marine environment for shippers, fishers, sailors and other marine assets. Nowadays due to the evolution in wireless sensor network (WSN) technology, their easy and inexpensive deployment can be seen in different fields such as agriculture, medical, industries and IoT. The implementation of WSN in the marine environment comes with various issues due to the complex behavior of the marine environment and thus affects the reliability of the system. In this paper, a WSN-based marine tracking protocol sailing is presented, which identifies the location of marine assets reliably through the genetic firefly (GEFIR) algorithm. The system comprises sensor nodes, sink nodes, server-client terminals and a base station. The protocol uses the application and contact layers to create a group of beacon nodes (boats) based on the geographical location to conserve energy and increase data reliability. GEFIR algorithm is implemented to give a precise and more convenient data to the WSN for the mariners. The results show a better performance for latency calculation through the WSN- and GEFIR-based sailing protocol. The system generates the alert to nodes (sailors) in case they are moving out from a specified region and sends their location for further communication.

Spatio-temporal variation in stable isotope and elemental composition of key-species reflect environmental changes in the Baltic Sea

Carbon and nitrogen stable isotope ratios are increasingly used to study long-term change in food web structure and nutrient cycling. We retrospectively analyse elemental composition (C, N and P) and stable isotopes (δ13C, δ15N) in archived monitoring samples of two important taxa from the bottom of the food web; the filamentous ephemeral macroalgae Cladophora spp. and the blue mussel Mytilus edulis trossulus from three contrasting regions in the Baltic Sea (coastal Bothnian Sea and Baltic Proper, open sea central Baltic). The aim is to statistically link the observed spatial and interannual (8–24 years’ time-series) variability in elemental and isotope baselines with their biomass trends and to the oceanographic monitoring data reflecting the ongoing environmental changes (i.e., eutrophication and climate) in this system. We find clear differences in isotope baselines between the two major Baltic Sea basins. However, the temporal variation in Mytilus δ13C was similar among regions and, at the open sea station, mussels and algae δ13C also correlated over time, likely reflecting a global (Suess) effect, whereas δ15N of both taxa varied with local and regional dissolved nitrogen concentrations in water. δ15N in source amino acids allowed detection of diazotrophic N in Mytilus, which was masked in bulk δ15N. Finally, Cladophora N:P reflected regional nutrient levels in the water while P%, which differed for both taxa, was linked to food quality for Mytilus. This study highlights the potential of a multi-taxa and multi-stable isotope approach to understand nutrient dynamics and monitor long-term environmental changes.

Application of AHP and GIS for Determination of Suitable Wireless Sensor Network Zones for Oceanographic Monitoring in the South Caribbean Sea Upwelling Zone

The monitoring and analysis of oceanographic variables is important for several research areas in marine sciences, such as marine spatial planning and integrated coastal management, among others. But, the high costs of monitoring equipment, its installation, and maintenance, damage, destruction, theft, and loss, make it difficult to monitor the maritime territory. Equipment installed in areas with environmental risks or anthropic activities showing the lack of analysis. Therefore, this paper determined the feasibility of installing a Wireless Sensor Network (WSN) for measurement of oceanographic variables in the South Caribbean Sea upwelling zone, using Hierarchy Analytic Process (AHP) and GIS tools (Geographic Information System). Marine ecosystems, boat traffic, fishing zones, and bathymetry criteria were used. The paired comparison matrix analysis shows the most important criterion is the buoy-type monitoring system (54.82%) far away from the marine ecosystems, while the bathymetric zone is the least relevant criterion (10.75%). It was possible to find that 62.36% of the study area is highly favorable to install the monitoring network and where it is advisable to do it, within which it is possible to avoid various risks and maximize the utility of the information, considering the different ecosystems and uses of the maritime territory. A restrictive variable to do replicable this work is the presence of GSM, GPRS or satellite coverage, otherwise, there would be no way to transmit data in real-time, involving transfers to collect the data, increasing the project’s costs. The method and results allow replicating this study in any coastal marine environment.

Metabolic pathways inferred from a bacterial marker gene illuminate ecological changes across South Pacific frontal boundaries

Global oceanographic monitoring initiatives originally measured abiotic essential ocean variables but are currently incorporating biological and metagenomic sampling programs. There is, however, a large knowledge gap on how to infer bacterial functions, the information sought by biogeochemists, ecologists, and modelers, from the bacterial taxonomic information (produced by bacterial marker gene surveys). Here, we provide a correlative understanding of how a bacterial marker gene (16S rRNA) can be used to infer latitudinal trends for metabolic pathways in global monitoring campaigns. From a transect spanning 7000 km in the South Pacific Ocean we infer ten metabolic pathways from 16S rRNA gene sequences and 11 corresponding metagenome samples, which relate to metabolic processes of primary productivity, temperature-regulated thermodynamic effects, coping strategies for nutrient limitation, energy metabolism, and organic matter degradation. This study demonstrates that low-cost, high-throughput bacterial marker gene data, can be used to infer shifts in the metabolic strategies at the community scale.

Divers as Citizen Scientists: Response Time, Accuracy and Precision of Water Temperature Measurement Using Dive Computers

There is a lack of depth-resolved temperature data, especially in coastal areas, which are often commonly dived by SCUBA divers. Many case studies have demonstrated that citizen science can provide high quality data, although users require more confidence in the accuracy of these data. This study examined the response time, accuracy and precision of water temperature measurement in 28 dive computers plus three underwater cameras, from 12 models. A total of 239 temperature response times (τ) were collected from 29 devices over 11 chamber dives. Mean τ by device ranged from (17 ± 6) to (341 ± 69) s, with significant between-model differences found for τ across all models. Clear differences were found in τ by pressure sensor location and material, but not by size. Two models had comparable τ to designed-for-purpose aquatic temperature loggers. 337 mean data points were collected from equilibrated temperatures in hyperbaric chamber (n = 185) and sea (n = 152) dives, compared with baseline mean temperature from Castaway CTDs over the same time period. Mean bias, defined as mean device temperature minus baseline temperature, by model ranged from (0.0 ± 0.5) to (−1.4 ± 2.1) °C and by device from (0.0 ± 0.6) to (−3.4 ± 1.0) °C. Nine of the twelve models were found to have “good” accuracy (≤0.5 °C) overall. Irrespective of model, the overall mean bias of (−0.2 ± 1.1) °C is comparable with existing commonly used coastal temperature data sets, and within global ocean observing system accuracy requirements for in situ temperature. Our research shows that the quality of temperature data in dive computers could be improved, but, with collection of appropriate metadata to allow assessment of data quality, some models of dive computers have a role in future oceanographic monitoring.

Tracking the spatiotemporal variability of the oxic–anoxic interface in the Baltic Sea with broadband acoustics

AbstractAnoxic zones, regions of the water column completely devoid of dissolved oxygen, occur in open oceans and coastal zones worldwide. The Baltic Sea is characterized by strong salinity-driven stratification, maintained by occasional water inflows from the Danish Straights and freshwater input from rivers. Between inflow events, the stratification interface between surface and deep waters hinders mixing and ventilation of deep water; consequently, the bottom waters of large regions of the Baltic are anoxic. The onset of the anoxic zone is closely coincident with the depth of the halocline and, as a result, the interface between oxic and anoxic waters corresponds to a strong impedance contrast. Here, we track acoustic scattering from the impedance contrast utilizing a broadband split-beam echosounder in the Western Gotland Basin and link it to a dissolved oxygen level of 2 ml/l using ground truth stations. The broadband acoustic dataset provides the means to remotely observe the spatiotemporal variations in the oxic–anoxic interface, map out the extent of the anoxic zone with high resolution, and identify several mechanisms influencing the vertical distribution of oxygen in the water column. The method described here can be used to study other systems with applications in ongoing oceanographic monitoring programs.

Fishing Gear as a Data Collection Platform: Opportunities to Fill Spatial and Temporal Gaps in Operational Sub-Surface Observation Networks

While the open oceans are well covered by automated ocean measurement instruments, there is a lack of sub-surface oceanographic observation in coastal and shelf seas. Commercial fishing gear such as bottom trawls, pots, traps and long lines can act as a platform for sensors, which collect physical oceanographic data concurrently with normal fishing operations. The lack of observed in-situ ocean data in coastal and shelf seas limits operational oceanography, weather forecasting, maritime industries, and climate change monitoring. In addition, using fishing gear as an ocean observation platform has auxiliary benefits for fisheries management including stakeholder involvement. This study quantifies and compares the existing sub-surface in-situ data coverage with the spatial distribution of fishing activities. The results show that integration with fishing could contribute to filling some of the most pressing gaps in existing ocean observation systems in coastal and shelf seas. There are limitations inherent to opportunistic data collection, mainly related to spatial and temporal heterogeneity of fishing activities. However, we make the case that fishery-based observations have the potential to complement existing ocean observing systems in areas where oceanographic data are lacking and needed most in order to ensure long term sustainability of oceanographic monitoring.