OS2.1

We describe estimates of overall transport across three contrasted sectors of the north-west European shelf edge: the Celtic Sea south-west of Britain, the Malin-Hebrides shelf west of Scotland and the West Shetland shelf north of Scotland.  The estimates derive from a variety of measurements in the project FASTNEt (Fluxes across sloping topography of the North East Atlantic): drifters and moored current meters, effective “diffusivity” from drifter dispersion and salinity surveys, other estimates of velocity variance contributing to exchange.  Process contributions include transport by along-slope flow, internal waves and their Stokes drift, tidal pumping, eddies and Ekman transports, in a wind-driven surface layer and in a bottom boundary layer.   

Estimated overall exchange across the shelf edge is several m²/s (Sverdrups per 1000 km) and thereby large compared with many other locations, large compared with oceanic transports if extrapolated globally and potentially important to the shelf-sea and adjacent oceanic budgets.  However, the large majority of this is in tides and other motion with periods of order one day or less; such exchange is only effective for water properties that evolve on time-scales of a day or less.  Nevertheless, cross-slope fluxes, and exchange due to motion with periods exceeding two days, are large by global standards and also very variable.  Flux values nearest the shelf break were in the range 0.3 – 3 m²/s, and exchanges were 0.8 – 4 m²/s.  Deeper longer-term moorings and drifters crossing the 500 m depth contour gave much larger fluxes and exchanges up to 20 m²/s.  Significance of these transports depends on distinctive properties of the water, or its contents, and on internal shelf-sea circulation affecting the further progress of these transports.  For the NW European shelf, transports across the shelf edge enable its disproportionately strong CO₂ “pump”.

The small scales of numerous processes enabling cross-slope transports, and the complex context, imply a need for models.  Measurements remain limited in extent and duration, but a wide variety of contexts, particular conditions, events and behaviours is now available for model validation, especially around the north-west European continental shelf edge.  Variability continues to render observations insufficient for stable estimates of transports and exchanges, especially if partitioned by sector and season; indeed, there may be significant inter-annual differences.   Validated fine-resolution models give the best prospect of coverage and of estimating shelf-sea sensitivities to the adjacent ocean.

How to cite: Huthnance, J. M., Hopkins, J. E., Inall, M., Holt, J., and team, F.: Ocean Shelf Exchange, NW European Shelf Seas: measurements, estimates and comparisons., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2954, https://doi.org/10.5194/egusphere-egu21-2954, 2021.

High-impact ocean weather events and climate extremes can have devastating effects on coastal zones and small islands. Marine Disaster Risk Reduction (DRR) is a systematic approach to such events, through which the risk of disaster can be identified, assessed and reduced via direct observations, thus improving ocean and atmosphere prediction models and the development of efficient early warnings systems. A common user request during disaster remediation actions is for high-resolution information, which can be derived from easily deployable numerical models nested into operational larger-scale ocean models.

The Structured and Unstructured Relocatable Ocean Model for Forecasting (SURF) has been designed to provide operational ocean forecasting communities with the means to rapidly deploy a nested high-resolution numerical model into larger-scale ocean forecasts. Rapidly downscaling the current, sea level and temperature, and salinity fields is critical in supporting emergency response and DRR planning, which are typically related to very localized areas in the world’s oceans. The first and most important requirement in a relocatable modelling capability is to ensure all of the interfaces have been tested through low-resolution operational ocean analyses, forecasts and atmospheric forcing. The provision of continuous ocean circulation forecasts through the Copernicus Marine Environment Monitoring Service (CMEMS) enables this testing. High-resolution SURF ocean circulation forecasts can then be accessed through specific numerical application model interfaces that require the knowledge of meteo-oceanographic conditions, such as oil spill forecasting, search and rescue modelling, and ship routing modelling for safe navigation.

SURF was used to downscale CMEMS circulation analyses in four world ocean regions, and the high-resolution currents it can simulate for specific applications are examined. The SURF downscaled circulation fields show that the marine current resolutions affect the quality of the application models to be used for assessing disaster risks, particularly near coastal areas where the coastline geometry must be resolved through a numerical grid, and high-frequency coastal currents must be accurately simulated.

How to cite: Trotta, F., Federico, I., Pinardi, N., Coppini, G., Causio, S., Jansen, E., Iovino, D., and Masina, S.: A relocatable ocean modelling platform for downscaling to shelf-coastal areas to support disaster risk reduction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5420, https://doi.org/10.5194/egusphere-egu21-5420, 2021.

Strong upwelling driven by the NNW winds was detected off the eastern middle Adriatic coast in May 2017. High resolution CTD data revealed thermocline doming by about 20 m at approximately 20 km from the coast. Main characteristics of the upwelling event are reproduced in the realistic ROMS model simulation. Adriatic scale ROMS model having 2.5 km horizontal resolution, forced by the air-sea fluxes calculated using surface fields from operational weather forecast model ALADIN-HR (Tudor et al., 2013; Termonia et al., 2018), river discharges, tides and water mass exchange through the Strait of Otranto, reproduces cold water dome and two-layer offshore flow in accordance with CTD and shipborne ADCP measurements. Significant improvement in the upwelling simulations is obtained using increased drag coefficient. The location of upwelling is correctly modelled, although with somewhat lower upper layer temperatures if compared with measurements. Moreover, the surface cyclonic circulation indicated by ADCP measurements along the cross-Adriatic transect is also evident in the model results. In order to improve understanding of the upwelling mechanism, several schematized numerical experiments are conducted. Wind fields from dynamical adaptation (Zagar and Rakovec, 1999; Ivatek-Sahdan and Tudor, 2004) of ALADIN-HR8 (8 km horizontal grid spacing) wind forecast to 2 km grid, are decomposed by the Natural Helmholtz-Hodge Decomposition (HHD) into divergence-free (incompressible), rotation-free (irrotational), and harmonic (translational) component (Bhatia et al., 2014). The components thus obtained and their combinations are used for calculation of the wind stress instead of the total wind field. Simulations with decomposed wind stress are conducted in the Adriatic domains with both flat bottom and realistic topography. Schematized simulations reveal that the positive rotational wind component is responsible for the rising of thermocline through Ekman pumping and it is more pronounced in the flat bottom basin. In the simulations with divergent wind component, the thermocline doming disappears and only coastal upwelling is reproduced. Additional idealised simulations with homogeneous NW wind stress are performed assuming both two-layer and uniform initial density field.

How to cite: Beg Paklar, G., Pasaric, Z., Orlic, M., and Stanesic, A.: A close look at the middle Adriatic upwelling: schematized ROMS model simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12014, https://doi.org/10.5194/egusphere-egu21-12014, 2021.

Cold Intermediate Layer (CIL) is apparent in the thermohaline structure of the Baltic Sea every year, typically from April to December. Within the CIL, water temperature, salinity, oxygen content, and other parameters are highly inhomogeneous in vertical, reflecting a complicated process of its formation. The core of the CIL (the layer of the coldest waters) has its T,S-index) allowing to identify the south-western part of the sea as the source of these waters. At the beginning of spring warming, a combination of environmental factors favors the subduction of the cold surface waters into the intermediate layers of the Baltic Proper, where they adjust to the density field, making up the coldest layer right above the permanent pycnocline.

For spring 2006, CTD measurements from 2 expeditions of research vessels “Professor Shtokman” of the Shirshov Institute of Oceanology and “Gauss” of Leibniz Institute for Baltic Sea Research in Warnemünde (IOW) were analyzed, along with the CTD measurements from ICES open database, and meteorological information. Remote sensing data provide observations of the abrupt transformation of SST field in the Bornholm Basin in early spring 2006, when the coldest surface water occurred within the coastal zones and its temperature was close to or below the temperature of maximum density (Tmd). The beginning of spring warming in the region and further heating of the cold surface water from temperature below the Tmd induce horizontal exchange, which favors the penetration of winter-cold (1.1–2.1 °C) surface waters of moderate salinity (7.6-8.1) into the intermediate layers in March. This water was observed in the Gdansk and Gotland basins in April-May 2006 as the core of the CIL. On the basis of vertical T,S-profiles and T,S-diagrams, the range of parameters of the CIL core waters in spring 2006 was determined (T: 1.4–2.1 °C; S: 7.6–8.1), which corresponds to the upper mixed layer in the vicinity of the Bornholm Island in March, 2006. Since this relation has already been confirmed for other years, and having in mind the importance of the process of the CIL formation for the entire Baltic Sea conveyor belt, we suggest to term waters of the CIL core as the Bornholm Intermediate Waters (BIW). Obviously, the T,S-index of the BIW shall vary from year to year, reflecting the severity of the past winter and the conditions of the particular spring. However, the BIW location right above the pycnocline, the lowest (for the current year) temperature, and its characteristic salinity of 7.6-8.1 seem to be repeatedly confirmed by field observations in the Baltic Proper in spring.

Investigations are supported by the Russian Foundation for Basic Research, grant No. 19-05-00717 (in part of the data analysis) and the State Assignment No. 0149-2019-0013 (in part of satellite data collecting and processing).

How to cite: Bukanova, T., Lobchuk, O., and Chubarenko, I.: The core of the Baltic CIL: shall we introduce the Bornholm Intermediate Water?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9930, https://doi.org/10.5194/egusphere-egu21-9930, 2021.

The Skagerrak and Kattegat are a narrow and shallow channel separating the North Sea and Baltic Sea. This highly dynamic area plays a key role in transforming water masses which flow into and oxygenate deep regions of the Baltic. This site is also a region of important carbon export, through advection down into and through the Norwegian Trench. This rich and productive ecosystem is strained by intensive human activity and shows strong coupling between biological and physical processes at a range of scales. 

We present data from a new autonomous observatory funded by the Voice of the Ocean Foundation. We compare data obtained from two underwater gliders and autonomous surface vehicles with that collected through regional monitoring programmes. Empirical variograms highlight the strong coupling between biological and physical parameters and the prevalence of small-scale processes not usually resolved in this region. We also present event-scale case studies showing variability in the coastal current and small scale export events. Finally, we outline the technical infrastructure and innovations of the Voice of the Ocean observatories and how to access its open data. 

How to cite: Queste, B., Swart, S., and Biddle, L.: Observing Baltic Sea exchanges: results from a new multi-platform autonomous observatory, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14094, https://doi.org/10.5194/egusphere-egu21-14094, 2021.

The Sea of Azov is a small, shallow, and freshened sea that receives a large freshwater discharge. Under certain external forcing conditions brackish water from the Sea of Azov flow into the north-eastern part of the Black Sea through the narrow Kerch Strait and form a surface-advected buoyant plume. Water flow in the Kerch Strait also regularly occurs in the opposite direction, which results in the spreading of an advected plume of saline and dense water from the Black Sea into the Sea of Azov. Using a regional Black Sea Azov Sea model based on NEMO we study physical mechanisms that govern water exchange through the Kerch Strait and analyze the dependence of its direction and intensity on external forcing conditions. We show that water exchange in the Kerch Strait is governed by a wind-induced barotropic pressure gradient. Water flow through the shallow and narrow Kerch Strait is a one-way process for the majority of the time. Outflow from the Sea of Azov to the Black Sea is induced by moderate and strong northerly winds, while flow into the Sea of Azov from the Black Sea is induced by southerly winds. The direction and intensity of water exchange have wind-governed synoptic and seasonal variability, and they do not depend on the variability of river discharge rate to the Sea of Azov on an intraannual timescale.

How to cite: Sedakov, R., Bernard, B., Molines, J.-M., and Mershavka, A.: Study of water exchange in Kerch strait using NEMO model of Black Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15878, https://doi.org/10.5194/egusphere-egu21-15878, 2021.

The Patos Lagoon, located in the Southern Brazil, is the largest freshwater lagoon in the World (area is 10 360 km2). It is connected with the Atlantic Ocean by a narrow strait, through which saline sea waters inflows to the lagoon and fresh waters of Patos outflows to the sea. Todos os Santos is the second large bay in the Brazil, which area is 1223 km2. It is located in the Northern Brazil, connected with Atlantic Ocean and remains saline during the whole year. Study of these basins represent difference in water exchange mechanisms between small and large estuarine lagoon.
Based on year-long in situ data from sea mooring and river gauge stations, as well as wind and precipitation reanalysis data, the influence of local meteorological and hydrological conditions on water exchange of these basins was studied.
It was revealed that the distinct seasonal variability of water exchange in Patos is defined mostly by the seasonal river discharge variability, while the variability of local atmospheric circulation does not influence it. Outflows of lagoon waters to the sea are typical during the high river discharge period, while inflows of sea waters to the lagoon are rare and occur under specific wind conditions. During the low river discharge periods, inflows of sea waters to the lagoon are typical, while short-term outflows are induced by increase of river discharge.
Meantime, it was found that synoptical salinity variation in Todos-os-Santos is mostly caused by tides, while seasonal water exchange variability is almost generally wind-driven. 

How to cite: Gordey, A. and Osadchiev, A.: Water exchange between estuarine lagoon and sea through a narrow strait: case study of two Brazilian lagoons, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1728, https://doi.org/10.5194/egusphere-egu21-1728, 2021.

Many estuaries are characterized by a mixture of clay, silt and sand. The erosion, (re-)suspension and transport of these sediments determine the bathymetry and stability of an estuary. Net estuarine sediment transport is the result of multiple processes. In stratified estuaries, gravitational circulation may lead to an inland near-bed sediment transport, which is directed opposite to the net sediment transport higher in the water column. Considering that coarse material is often transported near the bed, while suspended sediment usually consists of finer particles, gravitational circulation may cause a seaward flux of fine sediment and a landward flux of coarse sediment. The New Waterway in the Rotterdam Port area (The Netherlands) is such a stratified channel. Repeated channel deepening has intensified stratification, resulting in a strong salt-wedge type of flow. The channel is continuously dredged for navigation purposes, while the channel would naturally be gaining sediment (Cox et al., 2020). The amount of sediment entering the channel from sea and upstream, and the contribution of different sediment fractions however remain unclear. In this research, we combine  data analysis with numerical modelling to better understand and quantify sediment transport in stratified estuarine channels.

As a first step, we set up a field campaign which combines flow measurements with determination of suspended sediment characteristics. A measurement frame is equipped with a Sequoia LISST-200x and an YSI EXO Turbidity meter. Suspended sediment characteristics are determined every hour at three depths, next to water temperature, salinity and turbidity. Water samples are taken simultaneously to determine suspended sediment concentration, and flow is monitored continuously using a vessel-mounted ADCP. The full campaign includes two 13-hour measurements and covers two locations in the New Waterway.

The flow in the upper layer of the water column shows to be decoupled from the saline layer below. Before the flood acceleration phase, the upper and lower layer show an opposite flow direction, corresponding to the findings of De Nijs et al. (2010). The LISST-measurements confirm that suspended sediment in the upper water layer contains a high amount of clay and silt, while the material close to the bed is predominantly sand. This suggests a correlation between grain size and net transport direction. It should be noted that a major part of suspended sediment seems to be transported in the saline bottom layer, and that near-bed processes and local sediment availability could play an important role in the net sediment transport. Continued measurements and the modelling study will further reveal the sensitivity of the net sediment transport to sediment type, and provide insight in the effect of channel deepening.

Cox, J.R., Y. Huismans, J.F.R.W. Leuven, N.E. Vellinga, M. Van der Vegt, A.J.F. Hoitink, and M.G. Kleinhans (2020). “Anthropogenic effects on the Contemporary Sediment Budget of the Lower Rhine-Meuse Delta Channel Network.” Manuscript submitted to Earths Future.

Nijs, Michel A. J. de, Johan C. Winterwerp, and Julie D. Pietrzak (2010). “The Effects of the Internal Flow Structure on SPM Entrapment in the Rotterdam Waterway.” Journal of Physical Oceanography 40, no. 11: 2357–80.

How to cite: Niesten, I., Hoitink, T., Vermeulen, B., and Huismans, Y.: Mixed sediment transport in a stratified estuary: first insights from a field study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14362, https://doi.org/10.5194/egusphere-egu21-14362, 2021.

Discharges from the largest rivers of the World to coastal sea form sea-wide freshened surface layers which areas have order of hundred thousands of square kilometers. Large freshened surface layers (which are among the largest in the World Ocean) are located in the Kara, Laptev, and East-Siberian seas in the Eastern Arctic. This work is focused on the structure and inter-annual variability of these freshened water masses during ice-free periods. The freshened surface layer in the Laptev and East-Siberian seas is formed mainly by deltaic rives among which the Lena River contributes about two thirds of the inflowing freshwater volume. Based on in situ measurements, we show that the area of this freshened surface layer is much greater than the area of the freshened surface layer in the neighboring Kara Sea, while the total annual freshwater discharge to the Laptev and East-Siberian seas is 1.5 times less than to the Kara Sea (mainly from the estuaries of the Ob and Yenisei rivers). This feature is caused by differences in morphology of the estuaries and deltas. Shallow and narrow channels of the Lena Delta are limitedly affected by sea water. As a result, undiluted Lena discharge inflows to sea from multiple channels and forms relatively shallow plume, as compared to the Ob-Yenisei plumes which mix with subjacent saline sea water in deep and wide estuaries. The shallow Lena plume spreads over wide area (up to 500 000 km²) in the Laptev and East-Siberian seas during and shortly after freshet period in summer and then transforms to the Laptev/East-Siberian ROFI in autumn. Area and position of the relatively shallow freshened surface layer in the Laptev and East-Siberian seas have large inter-annual variability governed by local wind forcing conditions, however, do not show any dependence on significant variability of the annual volume of discharge rate from the Lena River. The deep freshened surface layer in the Kara Sea also has distinct seasonal varability of area and position, however, is stable on inter-annual time scale.

How to cite: Osadchiev, A. and Frey, D.: Structure of the freshened surface layer in the Eastern Arctic during ice-free periods, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-255, https://doi.org/10.5194/egusphere-egu21-255, 2021.

The Kara Sea receives about 55 % of the total continental runoff to the Siberian Arctic. Water of the Yenisei and Ob Rivers with low salinity (mineralization), flowing into the sea, forms a surface desalinated layer. The desalinated layer spreads over the sea area under the influence of hydrological and meteorological factors. Meltwater generated by the melting of marine and riverine ice and precipitation contribute to the formation of a surface desalinated layer along with continental runoff.

Determining the amount of fresh water is not accurate enough if only the salinity of surface water is considered. It is possible to identify riverine water and meltwater using hydrochemical proxies. The ratio of the major ions in seawater differs from that in riverine and meltwater. River waters are characterized by an increased content of silicate and reduced values of total alkalinity. At the same time, it is possible to identify the waters of the Ob and Yenisei Rivers by the estimated values of the total alkalinity and dissolved inorganic carbon obtained during the research expeditions to the Kara sea from 1993 to 2020.

The calculation of the parts of waters of different origin is done as a result of solving a system of equations. It includes the salinity and alkalinity values of the observed surface waters and those presumably involved in the mixing process. The salinity and alkalinity values of meltwater are taken as 0 and 134 µM respectively.

The total contribution of the Ob and Yenisei runoff ranges from 20 to 90% as it approaches the estuarine areas. The correlation coefficient between the proportion of river water and the salinity of the surface layer is quite high, it is equal to -0.9. This characterizes the inverse linear relationship. The separate contribution of the waters of the Yenisei differs from the contribution of the waters of the Ob, which is related to the hydrological conditions of the rivers.

The contribution of meltwater to the formation of the surface layer of the Kara Sea did not exceed 20%, with the exception of the coastal zone of the Novaya Zemlya. In this coastal zone, meltwater provides the greatest contribution compared to the other sources, which is associated with glacial runoff.

The work is implemented in the framework of the state assignment of the Shirshov Institute of Oceanology RAS (theme No. 0149-2019-0008), with the support of the Russian Scientific Foundation (project № 19-17-00196) and the grant of President of Russian Federation № MK-860.2020.5.

How to cite: Kazakova, U. and Polukhin, A.: Estimation of the seasonal input of freshwater in the Kara sea surface layer using hydrochemical proxies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14260, https://doi.org/10.5194/egusphere-egu21-14260, 2021.

The main objectives of this work was the acquisition of new data on floating marine macro litter (FMML) and natural floating objects in the Arctic seas, an initial assessment of the level of pollution by FMML and an analysis of potential sources. The results of this study present the first data on FMML distribution in Russian Arctic shelf seas in relation to oceanographic conditions (i.e. position of water masses of different origin as described by temperature, salinity, dissolved oxygen and pH). The main finding of this study is that FMML was found only in the water of Atlantic origin, inflowing from the Barents Sea, where FMML average density on the observed transects was 0.92 items/ km2. Eastern parts of the study, Kara Sea, Laptev Sea and East Siberian Sea were practically free from FMML. The input from rivers appears to be negligible, at least in autumn.

How to cite: Pogojeva, M. and Yakushev, E.: Floating marine macro-litter distribution in the Russian Arctic Seas in relation to oceanographic characteristics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2028, https://doi.org/10.5194/egusphere-egu21-2028, 2021.

Being an important source of renewable energy, offshore wind farms (OWFs) are currently flourishing in European coastal seas, with a largely unknown long-term impact on the environment. By providing hard substrate habitat to fouling species (such as the blue mussel), who filter water and excrete rapidly sinking fecal pellets, OWFs change the sediment composition and its carbon balance through biodeposition. 

Here we coupled a hydrodynamic model (including tides), a wave model and a sediment transport model with a description of organic carbon dynamics. The coupled model was run for the Southern Bight of the North Sea under different scenarios: i) no OWFs; ii)  current OWF placement; and iii) several scenarios for future OWF placement in a new concession area, that differ in the number of installed monopiles and their placements.

Simulations showed that the tidal remobilization of mineral particles by the dominant current is orders of magnitude higher than their biodeposition from the OWFs. The total organic carbon (TOC) flux, however, appeared to be highly altered (up to 50%) by OWF biodeposition, especially in 5 km vicinity of the monopiles. At a greater distance (5 - 30 km away from the monopiles), the TOC biodeposition flux decreases. The majors alteration in the TOC flux is aligned with the major axis of the regional tidal current and the main direction of the residual current, with local residual gyres acting as TOC traps.

A future OWF, whose current concession zone overlaps a protected Natura 2000 area with its gravel beds acting as biodiversity hotspots, is expected to affect them through TOC biodeposition flux alteration. However, the magnitude of the impact appeared to be strongly dependent on the monopile placement, and very little on the number of monopiles. The gravel beds will experience a 50% TOC influx increase, if the monopiles are placed over them or just next to them, but already at 3 km distance this increase would be less than 10 %.

How to cite: Ivanov, E., Capet, A., De Borger, E., Degraer, S., Delhez, E., Soetaert, K., Vanaverbeke, J., and Grégoire, M.: Modelling of the offshore wind farm footprint on organic and mineral particle deposition flux, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2236, https://doi.org/10.5194/egusphere-egu21-2236, 2021.

Sediment resuspension and transport on continental shelves are primarily driven by episodic energetic events, such as storm. Unfortunately, resuspension processes remain poorly quantified using traditional sampling techniques due to the intermittency and the intensity of these events. The recent integration of Acoustic Doppler Current Profilers (ADCPs) onto underwater gliders changes the way current and sediment dynamics in the coastal zone can be monitored. Their endurance and ability to measure in all weather conditions increase the probability of capturing sporadic meteorological events. We used a Slocum glider equipped with a CTD (Conductivity, Temperature, Depth), an optical payload and a RDI 600 kHz phased array ADCP to examine storm-induced sediment resuspension in the Gulf of Lion’s shelf (NW Mediterranean). Observations show that early in the storm, when the waves are highest, resuspension is limited by stratification. During the storm, erosion of the pycnocline through thickening of the bottom and surface mixed layers lead to resuspension in the full water column. Coincident optical and acoustic backscatter measurements indicate that the resuspended particulate assemblage is homogeneous and composed of large particles. Glider-ADCP observations showed for the first time that waves may be the predominant forcing which drive the resuspension on the outer shelf (> 80 m) during the winter storm. While, in the Gulf of Lions, which is considered as a relatively low energy continental shelf, modeling studies consider that only current drive resuspension in the outer shelf. This study highlights the usefulness of glider-ADCP to describe episodic processes and to support validation and improvement of regional hydrodynamic models.

How to cite: Gentil, M., Bourrin, F., Durrieu de Madron, X., and Estournel, C.: Glider observations of sediment resuspension during storm conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7239, https://doi.org/10.5194/egusphere-egu21-7239, 2021.

Vertical velocities knowledge is essential to study fine-scale dynamics in the surface layers of the ocean and to understand their impact on biological production mechanisms, in both coastal and offshore environments. Indeed, the general interest in fine-scale and, more precisely, in the determination of vertical velocities, is explained by their key role in global oceanic balance and their impact on the vertical transfer of nutrients and carbon budget despite their low intensity. With the increasing global warming issues linked to the forcing of the carbon cycle by anthropogenic activities, the estimation of vertical velocities becomes an essential information for a better representation of biogeochemical budgets. However, these vertical velocities have long been neglected, simply parameterized, or considered as not measurable, due mainly to their order of magnitude (mm s^-1), generally much lower than the one of the horizontal velocities (cm s^-1). Consequently, direct in situ measurement of vertical velocities is still currently one of the biggest challenges in physical oceanography.

We have been working to develop a new method for direct in situ measurement of vertical velocities using data from different Acoustic Doppler Current Profilers (ADCPs) associated with CTD probes, and we performed a comparative analysis of the results obtained by this method. The analyzed data were collected during the FUMSECK cruise (2019, Ligurian Sea), from three ADCPs: two Workhorse (conventional ADCPs), one lowered on a carousel and the other deployed in free-fall mode, and one Sentinel V (a new generation ADCP with four classical beams and a fifth vertical beam), also lowered on a carousel. Our analyses provided profiles of vertical velocities of the order of mm s^-1, as expected, with standard deviations of a few mm s^-1. While the fifth beam of the Sentinel V has shown a better accuracy than conventional ADCPs, the free-fall technique has provided a more accurate measurement compared to the carousel technique. Some of these measurements were gathered along the edge of the Northern Current and this new information on coastal edge currents represents a key point for the future improvement of coastal altimetry in particular.

Finally, this innovative study opens up the possibility to perform simple and direct in situ measurements of vertical velocities, coupling the free-fall technique with a five-beam ADCP. Hence, we plan to deploy a free-falling Sentinel V in offshore areas characterized by intense fine-scale ocean dynamics, but also and above all, in coastal areas, where topographic forcings are typically the source of high amplitude vertical velocities.

How to cite: Comby, C., Barrillon, S., Fuda, J.-L., Doglioli, A., Tzortzis, R., Gregori, G., Thyssen, M., and Petrenko, A.: New insights for direct in situ measurement of oceanic vertical velocities in fine-scale studies., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4632, https://doi.org/10.5194/egusphere-egu21-4632, 2021.

FUMSECK (Facilities for Updating the Mediterranean Submesoscale - Ecosystem Coupling Knowledge) is a one-week cruise, which took place in spring 2019, in the gulf of Genoa (NW Mediterranean Sea), onboard the R/V Téthys II. It was conducted in preparation of the BioSWOT-Med cruise in the SW Mediterranean Sea in 2022, planned as part of the ``Adopt a Cross Over'' initiative organising simultaneous oceanographic cruises around the world during the SWOT fast sampling phase. During FUMSECK we tested various technological innovations for the study of fine-scale dynamics and their coupling with biogeochemistry.

By their interactions, the fine scales could induce some ageostrophic and tridimensional dynamics, which are a critical point for the understanding of the vertical exchanges and their effect on biogeochemistry. Therefore, the fine scales play a key role in the oceans global balance and, despite their low intensity, clearly impact processes such as nutriment vertical transfer and carbon export. However, their ephemeral nature complicates their in situ measurements, which are nevertheless essential for their understanding and for the confirmation of the models’ prediction and the satellite observations. Furthermore, measuring vertical velocities in situ represents a real challenge since they are several orders of magnitude below the horizontal ones.

The FUMSECK cruise benefited from the automatic Lagrangian SPASSO treatment of the satellite data with an onshore team providing a daily bulletin of analysis and guidance on the fine-scale structures in the studied area. The distribution of phytoplankton functional groups at a small spatio-temporal scale was measured by automated flow cytometry with imaging. This technology allows to address the distribution of phytoplankton at fine scales within its hydrodynamic context. Several methods of measuring vertical velocities have been deployed, using different ADCP at fixed depth and in profile, FF-ADCP (Free Fall ADCP), the VVP (Vertical Velocities Profiler) prototype developed at MIO, and a SeaExplorer glider. These methods have shown promising results for in situ measurement of vertical velocities. Overall results show an abrupt change of population associated with a fine-scale structure appearance in relation with a storm event.

In addition, in order to study the physical part of the biological carbon pump, we experienced the release, following, pumping and detection by cytometry of a sample of biodegradable micro-particles that mimic the phytoplankton, and established a proof-of-concept for this method. Finally, we studied the MVP (Moving Vessel Profiler) instruments behaviour and reduced significantly a rotative effect.

We will describe the instrumental and analysis methodology deployed during FUMSECK in the study area of the Ligurian Sea, including the Northern Current, and present the results on the fine-scale dynamics and their impact on biology.

How to cite: Barrillon, S., Comby, C., Fuda, J.-L., Petrenko, A., Thyssen, M., Grégori, G., Bosse, A., Tzortzis, R., Bhairy, N., Cyr, F., Bataille, H., d'Ovidio, F., and Doglioli, A.: Study of fine-scale dynamics and their coupling with biogeochemistry - FUMSECK cruise, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7199, https://doi.org/10.5194/egusphere-egu21-7199, 2021.

The Baltic Sea is characterised as a semi-enclosed brackish Sea that has experienced increased eutrophication, hypoxia, and increased temperature over the last ~100 years making Baltic Sea one of the most severely impacted oceanic environment by climate change. Biological fixation of dinitrogen gas (N₂) is an essential process to make atmospheric N₂ available for marine life. This process is carried out by specialised organisms called diazotrophs and is catalysed by the energetic-consuming enzyme nitrogenase. Nitrogenases exist in three subtypes depending on their metal cofactors, (1) the most common molybdenum-dependent (Nif), (2) the vanadium-dependent (Vnf) and (3) the Iron-Iron-dependent nitrogenase (Anf). To date, the effect of climate change on those three enzyme subtypes and their potential role a future ocean is yet to be explored. The predicted ongoing oxygen loss in the ocean may limit Mo's availability and trigger a shift from the abundant Nif-type nitrogenase to Vnf or Anf and, therefore, a potential shift in the diazotrophic community. This study explored the climate change-related pressures on N₂ fixation and the diazotrophic community based on nifH and vnf/anfD amplicons. At the time of sampling, we found a post-bloom high-nutrient low-chlorophyll situation. Cyanobacterial groups, Nodularia and UCYN-A, dominated the diazotrophic community and showed a horizontal where UCYN-A were the dominant fixers at 20 m. Based on alternative nitrogenases amplicons, Rhodopseudomonas was the dominating microbe in the surface water. This paper presents the first hint of active nitrogenases in surface water and further establish UCYN-A as a significant player in Baltic Sea primary production.

How to cite: Reeder, C. and Löscher, C.: Nitrogen fixation in a diazotrophic post-bloom situation in the Baltic Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-911, https://doi.org/10.5194/egusphere-egu21-911, 2021.

Coastal regions by their very nature are dynamically diverse.  Within one geographical region there are often multiple areas dominated by substantially different dynamics that shape not only the physical characteristics but also the ecosystem.  The Salish Sea, in the northeast Pacific, is an excellent example with strongly tidally mixed regions, freshwater-dominated regions, and regions directly influenced by the open ocean.  These regions are generally well known and multiple disciplines refer to them with various boundaries and under various names.  Here we use unsupervised clustering on numerical model results to formalize these regional provinces.  The model is SalishSeaCast,  a three-dimensional real-time coupled bio-chem-physical model based on the NEMO framework.  We find that the regions clustered on ecosystem variables (phytoplankton biomass) spatially coincide with those clustered on physical variables, particularly the stratification as diagnosed by the halocline depth.  The clusters are robust across years with interannual variability manifesting mostly in changes in the size of the clusters.  As the clusters are dynamically distinct, they provide a natural framework on which to evaluate the model against observations.  We find that the model accurately simulates each of the major clusters.  The spatial and temporal resolution of the model can then characterize these different clusters more systematically than the observations, revealing biases associated with sparse sampling in the observations. Two examples will be given, one addressing a long-standing issue of the productivity gradient in the stratified main basin, the Strait of Georgia, and another concerning the seasonal cycle of productivity in the ocean-influenced Juan de Fuca Strait.

How to cite: Allen, S., Jarnikova, T., Olson, E., and Ianson, D.: Clustering coupled biochemical-physical model results formalizes regional provinces in a coastal region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13765, https://doi.org/10.5194/egusphere-egu21-13765, 2021.