Sonarlogger: Enabling long-term underwater sonar observations.

Coastal seas are under increasing pressure from extreme weather events and sea level rise, resulting in impacts such as changing hydrodynamic conditions, coastal erosion, and marine heat waves. To monitor changes in coastal marine habitats, such as reefs and macrophytes meadows, which add to the resilience of our coasts, consistent, medium- to long-term seafloor observations are needed. This project aims to deliver repeated, high-frequency sonar surveys on a stationary seabed mooring of a specific target area over a period of up to several months. A new stand-alone subsea system, the Sonarlogger, based on a battery pack, low-power logger and a high-resolution scanning sonar, was developed. It allows for long-term deployments with a customisable battery pack, WI-FI download and configurable sleep state. The system has been tested for over 130 days in dynamic coastal environments off the Belgian coast. Combined with auxiliary sensors, such as for measuring currents, waves and turbidity, this system enables comprehensive studies of morphologic changes and changing benthic ecosystems. Moreover, this system has the capacity to provide measurements of coastal environments during storms, where conventional systems may fall short, providing insights into event-based changes of the seafloor.

Long-term patterns of hydrocarbon biodegradation and bacterial community composition in epipelagic and mesopelagic zones of an Arctic fjord.

Oil spill attenuation in Arctic marine environments depends on oil-degrading bacteria. However, the seasonally harsh conditions in the Arctic such as nutrient limitations and sub-zero temperatures limit the activity even for bacteria capable of hydrocarbon metabolism at low temperatures. Here, we investigated whether the variance between epipelagic (seasonal temperature and inorganic nutrient variations) and mesopelagic zone (stable environmental conditions) could limit the growth of oil-degrading bacteria and lead to lower oil biodegradation rates in the epipelagic than in the mesopelagic zone. Therefore, we deployed absorbents coated with three oil types in a SW-Greenland fjord system at 10-20 m (epipelagic) and 615-650 m (mesopelagic) water depth for one year. During this period we monitored the development and succession of the bacterial biofilms colonizing the oil films by 16S rRNA gene amplicon quantification and sequencing, and the progression of oil biodegradation by gas chromatography - mass spectrometry oil fingerprinting analysis. The removal of hydrocarbons was significantly different, with several polycyclic aromatic hydrocarbons showing longer half-life times in the epipelagic than in the mesopelagic zone. Bacterial community composition and density (16S rRNA genes/ cm2) significantly differed between the two zones, with total bacteria reaching to log-fold higher densities (16S rRNA genes/cm2) in the mesopelagic than epipelagic oil-coated absorbents. Consequently, the environmental conditions in the epipelagic zone limited oil biodegradation performance by limiting bacterial growth.

A mobile observatory powered by sun and wind for near real time measurements of atmospheric, glacial, terrestrial, limnic and coastal oceanic conditions in remote off-grid areas.

Climate change is rapidly altering the Arctic environment. Although long-term environmental observations have been made at a few locations in the Arctic, the incomplete coverage from ground stations is a main limitation to observations in these remote areas. Here we present a wind and sun powered multi-purpose mobile observatory (ARC-MO) that enables near real time measurements of air, ice, land, rivers, and marine parameters in remote off-grid areas. Two test units were constructed and placed in Northeast Greenland where they have collected data from cabled and wireless instruments deployed in the environment since late summer 2021. The two units can communicate locally via WiFi (units placed 25 km apart) and transmit near-real time data globally over satellite. Data are streamed live and accessible from (https://gios.org). The cost of one mobile observatory unit is c. 304.000€. These test units demonstrate the possibility for integrative and automated environmental data collection in remote coastal areas and could serve as models for a proposed global observatory system.

In situ biodegradation, photooxidation and dissolution of petroleum compounds in Arctic seawater and sea ice.

In pristine sea ice-covered Arctic waters the potential of natural attenuation of oil spills has yet to be uncovered, but increasing shipping and oil exploitation may bring along unprecedented risks of oil spills. We deployed adsorbents coated with thin oil films for up to 2.5 month in ice-covered seawater and sea ice in Godthaab Fjord, SW Greenland, to simulate and investigate in situ biodegradation and photooxidation of dispersed oil. GC-MS-based chemometric methods for oil fingerprinting were used to identify characteristic signatures for dissolution, biodegradation and photooxidation. In sub-zero temperature seawater, fast degradation of n-alkanes was observed with estimated half-life times of ∼7 days. PCR amplicon sequencing and qPCR quantification of bacterial genes showed that a biofilm with a diverse microbial community colonised the oil films, yet a population related to the psychrophilic hydrocarbonoclastic gammaproteobacterium Oleispira antarctica seemed to play a key role in n-alkane degradation. Although Oleispira populations were also present in sea ice, we found that biofilms in sea ice had 25 to 100 times lower bacterial densities than in seawater, which explained the non-detectable n-alkane degradation in sea ice. Fingerprinting revealed that photooxidation, but not biodegradation, transformed polycyclic aromatic compounds through 50 cm-thick sea ice and in the upper water column with removal rates up to ∼1% per day. Overall, our results showed a fast biodegradation of n-alkanes in sea ice-covered seawater, but suggested that oils spills will expose the Arctic ecosystem to bio-recalcitrant PACs over prolonged periods of time.

Sea ice breakup and marine melt of a retreating tidewater outlet glacier in northeast Greenland (81°N).

Rising temperatures in the Arctic cause accelerated mass loss from the Greenland Ice Sheet and reduced sea ice cover. Tidewater outlet glaciers represent direct connections between glaciers and the ocean where melt rates at the ice-ocean interface are influenced by ocean temperature and circulation. However, few measurements exist near outlet glaciers from the northern coast towards the Arctic Ocean that has remained nearly permanently ice covered. Here we present hydrographic measurements along the terminus of a major retreating tidewater outlet glacier from Flade Isblink Ice Cap. We show that the region is characterized by a relatively large change of the seasonal freshwater content, corresponding to ~2 m of freshwater, and that solar heating during the short open water period results in surface layer temperatures above 1 °C. Observations of temperature and salinity supported that the outlet glacier is a floating ice shelf with near-glacial subsurface temperatures at the freezing point. Melting from the surface layer significantly influenced the ice foot morphology of the glacier terminus. Hence, melting of the tidewater outlet glacier was found to be critically dependent on the retreat of sea ice adjacent to the terminus and the duration of open water.