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Water transparency can be measured with optical instruments and estimated with satellite sensors, but such measurements have been widely available for only a few decades. Estimates of water transparency using a white disk called a Secchi disk have been made for over a century and can be used to estimate long-term trends. However, historic in situ measurements of the Secchi depth (ZSd) were irregular in space and time and are difficult to interpret in regular time series due to biases introduced by changing locations and the timing of measurements. Satellite data time series, on the other hand, have consistent resolution in both space and time but cover too short a time to resolve climate-scale trends. We normalized historic ZSd measurements in the Baltic Sea with a satellite-derived mean climatology at 5 d temporal and 4 km spatial resolutions and created a merged time series of ZSd for the last century. The mean ZSd in the Baltic Sea from 1927-2020 decreased by 4.2 ± 0.6 m at a rate of 0.045 ± 0.06 m yr-1. Most of the change happened before 1987, and a further decrease was evident primarily in the satellite data during the 1998-2008 period. After 2008, no significant trend in ZSd and or the coefficient of diffuse light attenuation was detected in the Baltic Sea. However, in some sub-basins of the Baltic Sea, the decrease in ZSd continued even after that. The decrease in spectral water transparency in recent decades was highest in the 412 nm band, indicating an increase in the concentration of chromophoric dissolved organic matter.

Dataset from Uto Atmospheric and Marine Research Station (59°46’55” N, 21°21’27” E) between July 2018 and June 2019 This is a dataset for the publication: Martti Honkanen, Jens Daniel Müller, Jukka Seppälä, Gregor Rehder, Sami Kielosto, Pasi Ylöstalo, Timo Mäkelä, Juha Hatakka, and Lauri Laakso: Diurnal cycle of the CO2 system in the coastal region of the Baltic Sea (Submitted to Ocean Science 2020/11). The dataset consists of 1 hour average values of the variables used in the paper. The data are in a comma separated file that has blanks as the nan values.

In Baltic TRANSCOAST we study the physical, biogeochemical, and biological processes at the land-ocean interface. The coastal zone is heavily impacted by various human activities as well as by geomorphological and climatic processes – on both the land and the sea side. Land-sea interactions at low lying coastal areas that are often dominated by peatlands, and are a common feature along the Baltic Sea coast, are not well understood. The core hypothesis of Baltic TRANSCOAST is that the shallow sea and the terrestrial peatland have a mutual impact on each other with far-reaching consequences for water and energy fluxes, matter cycling and the biota. The interdisciplinary research focuses on the significance of flooding frequency and duration on biogeochemical processes by concentrating the investigations on three comparison sites. We are assessing how changing boundary conditions (such as climate and land use) affect the hydrology, biota and biogeochemical processes in coastal wetlands and the adjacent marine ecosystem.

Autonomous systems measuring the partial pressure of CO2 (pCO2) in surface waters on commercial carrier ships (Voluntary Observing Ship, VOS), which allows for high spatiotemporal data coverage, are a major component of the Ocean Thematic Centre (OTC) data stream. Currently, ICOS operates lines in the Atlantic, North Sea and the Baltic. All lines are determining pCO2 by measuring CO2 in air that has been equilibrated with seawater. As part of the European H2020 project RINGO (https://www.icos-ri.eu/ringo), we are evaluating the possibility of using VOS to expand the atmospheric network. We will provide technical solutions for three different settings and approaches, and assess the added value for the atmospheric observation network. Two systems are designed as stand-alone modules for continuous atmospheric CO2 and CH4 measurements, following the technological requirements defined by the ATC, and will be operated in the Baltic (high anthropogenic influence) and on a line between France and Brazil (clean marine air, large temperature and humidity gradient). A second approach is using the existing instrumentation for seawater measurements (North Atlantic), which we aim to improve in order to make these measurements usable for the atmospheric research community. This is an effort that connects the ocean research community with the Central Analytical Laboratories (CAL; testing an extended range of standard gases, providing flask sampling opportunity), the Atmospheric Thematic Centre (ATC; work on data streams that can be digested by the ATC system), and the modelling community (identifying useful sampling strategies). Here we present a status update of the ongoing work, which is a joined effort of the atmospheric and ocean community within ICOS and relying on the expertise of both fields.

Optic technologies and methods/procedures are established across all areas and scales in limnic and marine research in Germany and develop further continuously. The working group “Aquatic Optic Technologies” (AOT) constitutes a common platform for knowledge transfer among scientists and users, provides a synergistic environment for the national developer community and will enhance the international visibility of the German activities in this field. This document summarizes the AOT-procedures and -techniques applied by national research institutions. We expect to initiate a trend towards harmonization across institutes. This will facilitate the establishment of open standards, provide better access to documentation, and render technical assistance for systems integration. The document consists of the parts: Platforms and carrier systems outlines the main application areas and the used technologies. Focus parameters specifies the parameters measured by means of optical methods/techniques and indicates to which extent these parameters have a socio-political dimension. Methods presents the individual optical sensors and their underlying physical methods. Similarities denominates the common space of AOT-techniques and applications. National developments lists projects and developer groups in Germany designing optical high-technologies for limnic and marine scientific purposes.

Optic technologies and methods/procedures are established across all areas and scales in limnic and marine research in Germany and develop further continuously. The working group “Aquatic Optic Technologies” (AOT) constitutes a common platform for knowledge transfer among scientists and users, provides a synergistic environment for the national developer community and will enhance the international visibility of the German activities in this field. This document summarizes the AOT-procedures and -techniques applied by national research institutions. We expect to initiate a trend towards harmonization across institutes. This will facilitate the establishment of open standards, provide better access to documentation, and render technical assistance for systems integration. The document consists of the parts: Platforms and carrier systems outlines the main application areas and the used technologies. Focus parameters specifies the parameters measured by means of optical methods/techniques and indicates to which extent these parameters have a socio-political dimension. Methods presents the individual optical sensors and their underlying physical methods. Similarities denominates the common space of AOT-techniques and applications. National developments lists projects and developer groups in Germany designing optical hightechnologies for limnic and marine scientific purposes.

We present our experiences in monitoring hypoxia and assessing oxygen-related phenomena in aquatic systems, resulting from the EU-FP7 project HYPOX (“In situ monitoring of oxygen depletion in hypoxic ecosystems of coastal and open seas, and land-locked water bodies”, www.hypox.net). For this presentation, we selected approaches and technologies which could be relevant for monitoring fast fluctuations at the pelagic redoxcline, seasonal benthic hypoxia and decadal trends oxycline boundary shifts in the Black Sea. Using novel technologies like e.g., the profiling instrumentation platform GODESS, ARGO floats with oxygen optodes and long-term moorings equipped with ADCP and oxygen optodes, temporal and spatial patterns of water column oxygenation from hours to seasons, and from basin-scale to local-scale patterns were resolved. We present examples from study sites in the Baltic Sea and in the Black Sea. The time series recordings of GODESS and mooring arrays allowed a thorough characterization of oscillating redoxclines in the central Baltic Sea and in the Black Sea off southwestern Crimea as temporally dynamic, three-dimensional systems. For the first time, oxygen sensor equipped ARGO type profiling floats were deployed in the Black Sea and proved to be powerful tools to address seasonal changes in patterns of water column oxygenation on larger spatial scales and emphasize the importance of mesoscale processes for oxygen distribution in the Black Sea basin. A 3-month continuous time series recording of a stand-alone static mooring equipped with optical oxygen sensors, current meters, and turbidity sensors identified summer hypoxia to be a highly dynamic process and provided insights into the controls of hypoxia formation on the north-western Black Sea shelf. Existing multi-decadal time-series monitoring data were used to demonstrate the imprint of climate change and eutrophication on long-term oxygen distributions and, hence, the importance of maintaining long-term commitments to oxygen monitoring programs. Such time series data allow separating out the effects that climatic forcing and eutrophication exert on oxygen depletion i.e., in the Black Sea. Standard CTD measurements in the central Black Sea over the last 90 years reflect the rising of the upper boundary of the suboxic zone in the 1970s and 1980s due to eutrophication, and again in the 1990s and 2000s due to NAO forcing, while eutrophication relaxed. Our comprehensive study within HYPOX was able to address many aspects of hypoxia in aquatic systems and revealed the vital need for dedicated oxygen monitoring at appropriate spatial and temporal scales with appropriate technologies. The variety of hypoxia characteristics and consequences emphasizes also the need for a careful selection of locations and periods of time for oxygen observations in order to adequately address the risk for hypoxia formation and ecosystem response.

The distribution of dissolved methane in the water column of the Baltic Sea was extensively investigated. A strong correlation between the vertical density stratification, the distribution of oxygen, hydrogen sulfide, and methane has been identified. A widespread release of methane from the seafloor is indicated by increasing methane concentrations with water depth. The deep basins in the central Baltic Sea show the strongest methane enrichments in stagnant anoxic water bodies (max. 1086 nM and 504 nM, respectively), with a pronounced decrease towards the pelagic redoxcline and slightly elevated surface water concentrations (saturation values of 206% and 120%, respectively). In general the more limnic basins in the northern part of the Baltic are characterized by lower water column methane concentrations and surface water saturation values close to the atmospheric equilibrium (between 106% and 116%). In contrast, the shallow Western Baltic Sea is characterized by high saturation values up to 746%.

Measurements of CH4 concentrations in the bottom water during two discrete sampling periods in subsequent years above different cold seeps at the Pacific margin off Costa Rica indicate large-scale variations of CH4 release. CH4 is emitted from mud extrusions and a slide scar at 1000–2300 m water depth. Maximum CH4 concentrations were found to be lower above all investigated sites in autumn 2003 than in autumn 2002 although seep sites are up to 300 km apart. Tidal and current changes were observed but found to apply only to individual seep sites. Increased seismic activity connected to the moment magnitude (M W ) 6.4 earthquake offshore Costa Rica in June 2002 could have had an impact on all seep sites and thereby caused an increase in CH4 emission. This is supported by the largest variations of CH4 concentration found above mud extrusions located above faults likely more strongly affected by tectonic movements. Even though our data indicate a relation between seismicity and CH4 seepage, the relation is not proven, and future work is needed to comprehensively test this hypothesis.

To constrain the fluxes of methane (CH4) in the water column above the accretionary wedge along the Cascadia continental margin, we measured methane and its stable carbon isotope signature (δ13C-CH4). The studies focused on Hydrate Ridge (HR), where venting occurs in the presence of gas-hydrate-bearing sediments. The vent CH4 has a light δ13C-CH4 biogenic signature (−63 to −66‰ PDB) and forms thin zones of elevated methane concentrations several tens of meters above the ocean floor in the overlying water column. These concentrations, ranging up to 4400 nmol L−1, vary by 3 orders of magnitude over periods of only a few hours. The poleward undercurrent of the California Current system rapidly dilutes the vent methane and distributes it widely within the gas hydrate stability zone (GHSZ). Above 480 m water depth, the methane budget is dominated by isotopically heavier CH4 from the shelf and upper slope, where mixtures of various local biogenic and thermogenic methane sources were detected (−56 to −28‰ PDB). The distribution of dissolved methane in the working area can be represented by mixtures of methane from the two primary source regions with an isotopically heavy background component (−25 to −6‰ PDB). Methane oxidation rates of 0.09 to 4.1% per day are small in comparison to the timescales of advection. This highly variable physical regime precludes a simple characterization and tracing of “downcurrent” plumes. However, methane inventories and current measurements suggest a methane flux of approximately 3 × 104 mol h−1 for the working area (1230 km2), and this is dominated by the shallower sources. We estimate that the combined vent sites on HR produce 0.6 × 104 mol h−1, and this is primarily released in the gas phase rather than dissolved within fluid seeps. There is no evidence that significant amounts of this methane are released to the atmosphere locally.

METEOR 66/2b Corinto - Caldera Wochenberichte Zweiter Wochenbericht, 9.10 – 16.10. 2005

Submarine high‐intensity methane seeps have been surveyed in the Sorokin Trough and Paleo Dnepr Area in the Black Sea from May to June, 2003 to estimate the sea‐air methane flux. The Sorokin Trough mud volcano area in around 2080 m water depth shows no direct effects on the methane concentration in the surface water and the atmosphere (average methane saturation ratios (SR) of 143%). The average sea‐air methane flux can be determined as 0.2–0.57 nmol m−2 s−1, using two different sea‐air gas exchange models; mean wind speed were extraordinary low throughout the cruise (1.16 m s−1). The investigations in the Paleo Dnepr Area (60 to 800 m water depth) reflects a more diverse pattern. Spots of high methane concentrations in the surface water have been recorded above a seep location in around 90 m water depth (SR up to 294%). The air‐sea methane flux above this seep site (0.96–2.32 nmol m−2 s−1) is 3 times higher than calculated for the surrounding shelf (0.32–0.77 nmol m−2 s−1) and 5 times higher than assessed for open Black Sea waters (water depth > 200 m, 0.19–0.47 nmol m−2 s−1).

In samples of pure natural gas hydrates from Hydrate Ridge, Cascadia Margin, virtually no helium and neon components are present providing evidence that the light noble gases are not incorporated into the structure of natural methane hydrates. In contrast, the hydrates contain significant amounts of argon, krypton and xenon. These gases show a distinct fractionation pattern, with the heavier ones preferentially incorporated into the gas hydrate structure. The hydrate methane is devoid of 14C indicating that there is no contribution of a recent (14C-active) organic carbon reservoir to the hydrate carbon pool. On the basis of the δ13C and δ2H signature, it appears that microbial CO2-reduction is the dominant CH4 production pathway.

Methane in surface waters and marine air off Oregon (44°24′N–44°54′N, 124°36′W–125°24′W) was continuously surveyed in July 1999. During a high-resolution survey after a period of steady winds from the north, CH4 concentrations were high in the northeastern region, near the shelf edge. The highest CH4 concentrations were 2.5 times higher than equilibrium with the atmospheric partial pressure. In contrast, concentrations were near equilibrium in the western part of the survey area, the Hydrate Ridge. The increase in CH4 from southwest to northeast correlates with a drop in sea surface temperature (SST), from 16.5°C to