Hydrographic and bio-optical measurements were conducted along a south-north transect on the northwestern Barents Sea shelf from early spring to late summer in 2021. Strong climate change manifestations observed in this region are rapidly changing the marine environment. These rare observations cover the seasonal evolution from well-mixed and sea ice-covered winter conditions, through sea ice retreat in spring, to late summer where the sea ice had largely retreated and the water column was stratified due to sea ice melt. Phytoplankton drives the spatial and temporal variability in optical properties in most of the water column, but increased scattering and absorption could also be seen in the bottom boundary layer due to resuspended particles. The relationship between chlorophyll-a and particulate absorption deviates from the globally observed relationship, likely due to light adaptations in the ice-covered water masses. We recommend developing specific models for spring phytoplankton growth in ice-covered waters to accurately account for self-shading effects. The absorption of colored dissolved organic matter (CDOM) was relatively low, due to waters of Atlantic origin in the studied region, and varied considerably less than particulate absorption. CDOM was nevertheless the optically dominant ocean constituent in the very clear waters in late winter. Regional relationships for estimating particulate organic carbon (POC) and chlorophyll-a concentrations from in situ attenuation and fluorescence measurements were developed. POC may act as an alternative indicator to chlorophyll-a for optical properties in ice-covered open ocean, which is relevant for light availability parametrizations in biogeochemical ocean models.

Grâce au développement de nouveaux capteurs, la télédétection optique est devenue une alternative permettant d’observer la zone côtière à large échelle. Parmi les techniques développées pour déterminer ses propriétés bio-géophysiques figure le modèle de transfert radiatif (MTR). Ce chapitre livre des clés pour répondre à certaines questions relatives aux méthodes utilisées pour son inversion ainsi que des exemples d’applications côtières.

There have been considerable efforts to understand the hydrography of Storfjorden (Svalbard). A recurring winter polynya with large sea ice production makes it an important region of dense water formation at the scale of the Arctic Ocean. In addition, this fjord is seasonally influenced by freshwater inputs from sea ice melt and the surrounding islands of the Svalbard archipelago, which impacts the hydrography. However, the understanding of factors controlling the optical properties of the waters in Storfjorden are lacking and are crucial for the development of more accurate regional bio-optical models. Here, we present results from the first detailed optical field survey of Storfjorden conducted in early summer of 2020. Our observations are based on spectrometric analysis of water samples and in situ vertical profiles with an absorption and attenuation meter, a fluorometer, and a conductivity, temperature, and depth (CTD) sensor. In addition to the expected seasonal contribution from phytoplankton, we find that in early summer waters in Storfjorden are optically complex with a significant contribution from coloured dissolved organic matter (CDOM, 33 %–64 % of the non-water absorption at 443 nm) despite relatively low CDOM concentrations and in the nearshore or near the seabed from non-algal particles (up to 61 % of the non-water absorption at 550 nm). In surface waters, the spatial variability of light attenuation was mainly controlled by inorganic suspended matter originating from river runoff. A distinct subsurface maximum of light attenuation was largely driven by a subsurface phytoplankton bloom, controlled by stratification resulting from sea ice melt. Lastly, the cold dense bottom waters of Storfjorden from winter sea ice production periodically overflows the sill at the mouth of the fjord and can thus reach the Fram Strait. It contained elevated levels of both non-algal particles and dissolved organic matter, which are likely caused by the dense flows of the nepheloid layer interacting with the sea bed.

The spring season was the target for the Nansen Legacy cruise organized in late April and first half of May 2021 following the transect defined for this series of cruises to capture the variations of the year sampling physical, biological and chemical conditions in the ice and the sea. The transect went through both open water and ice. Seven process stations were visited (P1 through P7) together with smaller NLEG stations according to the program for the seasonal investigations. The first station (P1) was in open waters, while the remaining six main station had ice coverage of varying degree. Each of the process stations lasted 24 hours or more to allow a full diurnal cycle. Sampling included ice physics, ice samples, phytoplankton, zooplankton, marine chemistry and eco toxicology using acoustic, optical and robotics methods together with lab analyses of physical samples. Remote sensing data were also matched with in situ observations of both sea and ice conditions.

The Nansen Legacy cruise Q1 (Q1: 1st quarter of the year) was part of the seasonal investigation of the northern Barents Sea and adjacent Arctic Basin. The cruise was conducted in March 2021, and focused on comparing the physical, chemical and biological conditions along the Nansen Legacy main transect in open waters and within the sea ice. The cruise addressed objectives of the Nansen Legacy work packages ‘Physical drivers’ (Research Focus 1), ‘Human impact’ (Research Focus 2) and ‘The living Barents Sea’ (Research Focus 3). In total, seven process stations (P1, P2, P3, P4, P5, P6 and P7) were conducted, with some more additional (NLEG) stations in-between.

Nearshore areas around the world contain a wide variety of archeological structures, including prehistoric remains submerged by sea level rise during the Holocene glacial retreat. While natural processes, such as erosion, rising sea level, and exceptional climatic events have always threatened the integrity of this submerged cultural heritage, the importance of protecting them is becoming increasingly critical with the expanding effects of global climate change and human activities. Aerial archaeology, as a non-invasive technique, contributes greatly to documentation of archaeological remains. In an underwater context, the difficulty of crossing the water column to reach the bottom and its potential archaeological information usually requires active remote-sensing technologies such as airborne LiDAR bathymetry or ship-borne acoustic soundings. More recently, airborne hyperspectral passive sensors have shown potential for accessing water-bottom information in shallow water environments. While hyperspectral imagery has been assessed in terrestrial continental archaeological contexts, this study brings new perspectives for documenting submerged archaeological structures using airborne hyperspectral remote sensing. Airborne hyperspectral data were recorded in the Visible Near Infra-Red (VNIR) spectral range (400–1000 nm) over the submerged megalithic site of Er Lannic (Morbihan, France). The method used to process these data included (i) visualization of submerged anomalous features using a minimum noise fraction transform, (ii) automatic detection of these features using Isolation Forest and the Reed–Xiaoli detector and (iii) morphological and spectral analysis of archaeological structures from water-depth and water-bottom reflectance derived from the inversion of a radiative transfer model of the water column. The results, compared to archaeological reference data collected from in-situ archaeological surveys, showed for the first time the potential of airborne hyperspectral imagery for archaeological mapping in complex shallow water environments.

Due to its high spectral and spatial resolutions, airborne hyperspectral imaging has great potential for becoming a powerful large-scale monitoring tool for coral reef communities. In recent years, methods based on radiative transfer model inversion have shown promising results for extracting information about seabed type, bottom depth and water constituents from hyperspectral imagery. However, low signal-to-noise ratios (SNR) due to low water-leaving radiance combined with environmental variability make it very difficult to design an optimal processing algorithm. Here, we selected a state-of-the-art, forward semi-analytical model in which we included a mixing model of four seabed albedo, namely sand, corals, algae and seagrass. The purpose of this paper was then to compare different setups of the inversion scheme, each one having its own theoretical strengths and weaknesses regarding the different confounding factors. Six inversion setups were implemented, corresponding to the combinations between (i) three cost functions: least square (LS), spectral angle mapper (SAM) and least square on spectral derivative (LSD), and (ii) two physical constraints imposed on the seabed type retrieval: abundance sum-to-one constraint (ASC) and a relaxed version (RASC). Performances of bathymetry and seabed type retrieval were evaluated on hyperspectral data acquired in a coral reef environment in Réunion Island. Our results showed that the accuracy and robustness of the bathymetric estimation were greatly influenced by the choice of the inversion setup. RASC-LSD produced the overall best performances even if SAM-based inversion setups showed particularly low error dispersion with respect to Lidar derived bathymetry. RASC-LSD also produced the most accurate results in terms of spatial coverage of benthic components on a very shallow area (inner part of a fringing reef). The results on our study areas clearly highlighted the interest of relaxing the ASC when the bottom depth is shallow. In deeper areas, ASC versions of LS- and LSD-based inversion setups produced the best seabed type mapping results. Only broad seabed types could be retrieved in areas deeper than 10m.