Description: The impact of ocean acidification on benthic habitats is a major preoccupation of the scientific community. However, the natural variability of pCO2 and pH in those habitats remains understudied, especially in temperate areas. In this study we investigated temporal variations of the carbonate system in nearshore macrophyte meadows of the western Baltic Sea. These are key benthic ecosystems, providing spawning and nursery areas as well as food to numerous commercially important species. In situ pCO2, pH (total scale), salinity and PAR irradiance were measured with a continuous recording sensor package dropped in a shallow macrophyte meadow (Eckernförde bay, western Baltic Sea) during three different weeks in July (pCO2 and PAR only), August and September 2011.The mean (± SD) pCO2 in July was 383±117 µatm. The mean (± SD) pCO2 and pHtot in August were 239±20 µatm and 8.22±0.1, respectively. The mean (± SD) pCO2 and pHtot in September were 1082±711 µatm and 7.83±0.40, respectively. Daily variations of pCO2 due to photosynthesis and respiration (difference between daily maximum and minimum) were of the same order of magnitude: 281±88 µatm, 219±89 μatm and 1488±574 µatm in July, August and September respectively. The observed variations of pCO2 were explained through a statistical model considering wind direction and speed together with PAR irradiance. At a time scale of days to weeks, local upwelling of elevated pCO2 water masses with offshore winds drives the variation. Within days, primary production is responsible. The results demonstrate the high variability of the carbonate system in nearshore macrophyte meadows depending on meteorology and biological activities. We highlight the need to incorporate these variations in future pCO2 scenarios and experimental designs for nearshore habitats.

Description: Inland waters transport and transform substantial amounts of carbon and account for 18% of global methane emissions. Large reservoirs with higher areal methane release rates than natural waters contribute significantly to freshwater emissions. However, there are millions of small dams worldwide that receive and trap high loads of organic carbon and can therefore potentially emit significant amounts of methane to the atmosphere. We evaluated the effect of damming on methane emissions in a central European impounded river. Direct comparison of riverine and reservoir reaches, where sedimentation in the latter is increased due to trapping by dams, revealed that the reservoir reaches are the major source of methane emissions (0.23 mmol CH4 m–2 d–1 vs 19.7 mmol CH4 m–2 d–1, respectively) and that areal emission rates far exceed previous estimates for temperate reservoirs or rivers. We show that sediment accumulation correlates with methane production and subsequent ebullitive release rates and may therefore be an excellent proxy for estimating methane emissions from small reservoirs. Our results suggest that sedimentation-driven methane emissions from dammed river hot spot sites can potentially increase global freshwater emissions by up to 7%

Description: Highlights: • Three different types of pCO2 sensors detected sedimentary artificial CO2 leaks in the water column. • Distribution of leaked CO2 in the water column featured high temporal and spatial heterogeneity. • Clear effect of CO2 leakage on the water column was visible only at high flow rates and low tides. • Fast recovery of the water column pCO2 was observed after the CO2 release was stopped. • Multivariate statistics can help to distinguish between anthropogenic and natural CO2 sources. Abstract: This work is focused on results from a recent controlled sub-seabed in situ carbon dioxide (CO2) release experiment carried out during May–October 2012 in Ardmucknish Bay on the Scottish west coast. Three types of pCO2 sensors (fluorescence, NDIR and ISFET-based technologies) were used in combination with multiparameter instruments measuring oxygen, temperature, salinity and currents in the water column at the epicentre of release and further away. It was shown that distribution of seafloor CO2 emissions features high spatial and temporal heterogeneity. The highest pCO2 values (∼1250 μatm) were detected at low tide around a bubble stream and within centimetres distance from the seafloor. Further up in the water column, 30–100 cm above the seabed, the gradients decreased, but continued to indicate elevated pCO2 at the epicentre of release throughout the injection campaign with the peak values between 400 and 740 μatm. High-frequency parallel measurements from two instruments placed within 1 m from each other, relocation of one of the instruments at the release site and 2D horizontal mapping of the release and control sites confirmed a localized impact from CO2 emissions. Observed effects on the water column were temporary and post-injection recovery took 〈7 days. A multivariate statistical approach was used to recognize the periods when the system was dominated by natural forcing with strong correlation between variation in pCO2 and O2, and when it was influenced by purposefully released CO2. Use of a hydrodynamic circulation model, calibrated with in situ data, was crucial to establishing background conditions in this complex and dynamic shallow water system.

Description: Carbon dioxide sequestration in sub-seafloor aims to store CO2 inside geological trapping structures below the seafloor. However there are concerns related to the possibility of leakage from the storage sites and potential consequences on the marine environment. In order to develop safe and reliable methods for CO2 monitoring, field studies were conducted in a natural analogue–an area where there is a natural release of CO2 from the seafloor. Due to the very high volume of gas emitted, this natural analogue could be considered as the worst-case scenario for a possible leakage from a sub-seabed storage site. Sampling procedures for free and dissolved gas and measuring techniques of the main physical and chemical parameters were developed for use both from the surface and directly underwater by scientific scuba divers. The first results of the research indicate that high levels of CO2 released in the marine realm strongly affect the local environmental conditions with a generalized acidification of the seawater. The experience gained in this study allows further development of a more accurate and suitable monitoring suite that will integrate sensors for measuring pH, dissolved CO2, and eventually, acoustic systems for the detection, monitoring and quantification of gas bubbles. The monitoring system could be deployed on the seafloor for long-term monitoring or could be carried onboard movable platforms such as ROV’s (Remote Operated Vehicles) or AUV’s (Autonomous Underwater Vehicles) for systematic surveys of the sub-seabed storage areas.