Remote sensing of the coastal zone of the European seas

Satellite remote sensing of the ocean has become a very important component of the global ocean observing system, contributing to operational and research oceanography, climate studies and maritime activities since at least the beginning of the eighties. During past years most satellite oceanography research has focused on open-ocean remote sensing. Recent increased attention on the coastal zone has produced further important advances, especially in ocean colour remote sensing and maritime applications. These advances are not only due to increased spatial and spectral resolution of satellite data, but also, and we would suggest mainly, due to advances in understanding of the bio-optical properties of the complex coastal waters, faster and more automated processing of the enormous amount of satellite data, and increased understanding of the importance of using complementary remote sensing techniques (e.g. optical and synthetic aperture radar or altimeter and thermal infrared).

The EARSeL Special Interest Group, Remote Sensing of the Coastal Zone, promotes information exchange among people interested in remote sensing applications on coastal and regional oceanography, internal water (lakes, lagoons and rivers), and morphodynamical coastal processes, as well as their relevance for coastal engineering and maritime applications.

The 8^th EARSeL Workshop on Remote Sensing of the Coastal Zone was held in 2017 at the Museum of the World Ocean, Kaliningrad, Russia. Thirty five participants from 10 countries attended the workshop, and made 42 contributions, including 19 oral and 23 poster presentations. The presentations covered a variety of topics including satellite and in situ remote sensing, new technologies and ocean management.

This special issue presents extended contributions from selected papers presented at the workshop. The special issue comprises eight papers focused on remote sensing of the coastal zone authored by 30 scientists from 9 universities and academic institutions. The papers cover several different topics in remote sensing applications including optical properties of coastal waters, new technologies for in situ remote sensing and assimilation of remote sensing data in numerical ocean models. The common thread that connects all these papers is an effort to improve remote-sensing techniques in the complex coastal zone environment.

The spatial distribution and seasonal interannual variability of total suspended matter concentration in coastal areas is an interesting indicator of organic matter production, terrestrial runoff and wind-driven re-suspension of particles and can be monitored from space using recently developed empirical algorithms as demonstrated by Bukanova et al. (2018) for the particular case of runoff areas of Vistula River and Baltiysk and Klaipeda straits in the south-eastern Baltic Sea. However, the major limitation to the remote sensing of relevant environmental parameters of the coastal regions is still insufficient knowledge of the relations between remote sensing estimates of apparent optical properties and Inherent Optical Properties (IOPs) of the sea water. The paper by Drozdova et al. (2018) contributes to the characterization of the spectral absorption and backscattering coefficients for marine water column constituents, focusing on coloured dissolved organic matter (CDOM) in the Arctic shelf waters affected by freshwater runoff, using optical indices calculated from absorption and fluorescence spectra, and by applying CDOM fluorescence quantum yield as a function of excitation wavelength. Optical properties of the ocean coastal waters can also be used to infer information on sea bottom vegetation. Traganos and Reinartz (2018) propose the use of a semi-analytical inversion model which employs reflectance data obtained from Sentinel-2 in five bands, from visible to near infrared, to derive water column and bottom properties, and apply the approach to selected regions of the Aegean Sea. The Posidonia oceanica seagrass meadows were mapped with Sentinel-2 imagery using an analytically described methodological workflow which includes atmospheric correction, super-resolution of the coastal aerosol band, air-water interface correction to estimate sub-surface remote sensing reflectance, and machine-learning based classification using Support Vector Machines.

Active microwave sensors offer the advantage of working in almost all weather conditions and is not dependent on daylight, which usually is a problem for high latitude regions characterized by a high occurrence of clouds and reduced solar irradiance during winter. Zhelezova, Krek, and Chubarenko (2018) propose a microwave remote sensing application to study the dynamics of the polynya in the Vistula Lagoon of the Baltic for the period 2011–2017. The use of a suite of microwave space sensor and platforms, ENVISAT (European Space Agency, ESA), RADARSAT-1 (Canadian Space Agency, CSA), RADARSAT-2 (McDonald, Dettwiler & Associates, MDA, Canada), TerraSAR-X (German Aerospace Center, DLR, Germany), Cosmo-SkyMED-1, -2, -3, -4 (Italian Space Agency, ASI), and Sentinel-1A/B (ESA) satellites, facilitated an interesting analysis of the forces driving the regional dynamics of the polynya.

New technology developments for in situ remote sensing of coastal waters are also an important component of the remote sensing of the coastal zone, either because they complement space observations or because they anticipate new space technological innovations. The Ecological Monitoring of Marine Aquatoria (EMMA) instrument (Rostovtseva, Goncharenko, and Konovalov 2018) is an example of such developments. This instrument is a passive radiometer that measures the radiance spectra of the seawater from on board a moving vessel and estimates the concentration of water constituents such as suspended matter, CDOM and phytoplankton pigments.

One of the most promising remote sensing techniques for space and in situ observations of the coastal zone is LiDAR (light detection and ranging), a technology that has recently received increasing interest and consideration for future space ocean missions. A LiDAR will provide vertical profiles of aerosols and thin clouds on the next EarthCARE ESA mission (launch date: August 2019) and will be considered as payload for the next ocean space missions. In this context, the development of new in situ LiDAR systems can contribute to the design of future space missions by providing useful data for pre launching tests. In this issue a LiDAR system, designed for detection and investigation of aerosol-gas formations in the atmosphere, as presented by Gritsuta et al. (2018), is a contribution to a better definition of information needed for atmospheric corrections of ocean colour data in coastal regions.

The exploitation of remote sensing data for regional and coastal application and services has significantly grown during recent years. Operational satellite data are provided by the Copernicus Marine Service (http://marine.copernicus.eu) and assimilated in Ocean General Circulation Models (OGCM). While satellite sea surface temperatures and altimetry data have been assimilated in operational models for many years, the assimilation of ocean colour data is a quite novel approach to the modelling of the marine ecosystem. Nevertheless this modelling technique has already proved it can efficiently contribute to reduce some well-known disadvantages of ocean colour satellite measurements. Satellite measurements can, in fact, provide information on the processes in the near-surface layer only and are limited by the presence of clouds that significantly restrict the amount of useful data. Assimilation of ocean colour data to a three-dimensional bio-geochemical model makes it possible to extrapolate these data from the sea surface to the deeper layers and to fill the cloud-caused gaps. An example of this, is Dorofeyev and Sukhikh (2018), which demonstrates the advantages of using an assimilation approach to reconstruct the 3-dimensional evolution of the Black Sea ecosystem during the SeaWiFS and MODIS operations period.

The management of the coastal zone requires regular monitoring that should be based on a variety of instruments and methodological approaches to optimize results and cost-benefit ratios. An example of such an approach has been proposed by Gonçalves et al. (2018). They propose a multi-instrument monitoring system that includes a stereoscopic video-based terrestrial mobile mapping system, three airborne digital photography systems (mounted on a small manned airplane, a fixed-wing UAV and a multi-rotor UAV, respectively) and an airborne LiDAR. Such a system has the potential to aid surveyors in their decision regarding which equipment to acquire and which method to use in a coastal survey context. They conclude that the different systems complement each other in providing a comprehensive picture of coastal morphology and dynamics, and the choice requires a consideration of the applicability and cost-benefit ratios associated with the specific survey aims, areas and local conditions.

Acknowledgments

We would like to take this opportunity to thank all authors for submitting their papers, the reviewers for their constructive comments, and the Editor-in-Chief, Professor Timothy Warner for his constant support to this special issue.