1.

INTRODUCTION

Coastal zones serve as the interface between the land and marine environments. The length of the world’s coasts exceeds 1,6 million kilometers, and 84 percent of the world’s countries have a coastline.

Globally, coastal zones are more densely inhabited than inland regions, have faster rates of population increase and urbanization, and accumulate economic assets along with essential infrastructures¹. The low-elevation coastal zone, which is defined as the continuous and hydrologically linked zone of land along the coast with an elevation above sea level of less than 10 meters, comprises just 2% of the world’s total area, but is home to 10% of the global population². In response to population expansion and coastward migration linked with the worldwide trend of urbanization, it is expected that the coastal population will continue to increase in the future years. The global population is projected to reach 8.5 billion in 2030 and 9.7 billion in 2050³, and the majority of the world’s megacities are located in the coastal zone, the majority of them in river deltas⁴. In addition to unique economic, physical, and historical variables, the concentration of densely occupied agricultural regions in well-watered, productive deltas and coastal plains drives coastal migration⁵.

Coastal zones provide a considerable economic worth. For example, marine transport is vital to the global economy since more than 90 percent of global trade is handled by water, with a significant percentage of maritime routes in the coastal ocean. In 2010, the global ocean economy was worth 1.5 trillion dollars, with a significant contribution from oil and gas companies, ports and marine equipment, and ocean-based sectors dominated by fisheries and tourism⁶. In addition to these well-established activities, other ones, such as marine aquaculture, ocean renewable energy, and maritime safety and surveillance, are expected to see growth in the future decades⁶.

The ocean economy is expected to rise to more than three trillion dollars by the year 2030, according to conservative forecasts. A significant portion of this figure will come from coastal tourism, offshore oil and gas, and port operations. Between the years 2010 and 2030, it is anticipated that marine aquaculture would expand at a pace of 5.7 percent on a yearly basis.

Coastal environments are home to a diverse array of plant and animal life, both on land and in the water. Marine coastal habitats are among the most productive ecosystems on the planet, and they offer people a wide variety of social as well as economic benefits. They are responsible for producing ninety percent of the world’s fisheries and roughly eighty percent of the marine fish species that are known⁷. Reefs, mangroves, and sand dunes all play a part in controlling the environment and protecting the coastline⁸. One way in which they do this is by significantly reducing the force of wind-driven waves. Coral reefs and river estuaries both have a high biomass production, making them two of the most biodiverse and species-rich environments on the planet.

As a result, coastal zones have a significant amount of value from a social, economic, and ecological perspective⁹. They are contributing to human society in a variety of ways, including the provision of food, energy, and other resources; the protection of coastlines; the promotion of ocean recreation, tourism, and coastal livelihoods; the upkeep of water quality; the treatment of waste; the promotion of biogeochemical cycling and regulatory services; the support of both the green and blue economies; and, most importantly, the upkeep of the fundamental global life support systems.

However, coastal areas are vulnerable to a variety of threats, some of which originate naturally and others are the result of human activity. Extreme conditions in the natural environment can play a role in the development of natural hazards. For example, marine heat waves can lead to coral bleaching and fish mortality, large waves and extreme sea levels can cause coastal flooding, erosion can degrade ecosystems and habitats. Several of these problems are made worse by climate change and the resulting warming, as well as the rise in sea level and an increase in hazardous algal blooms¹⁰. On the other hand, coastal hazards that are caused by human activity include maritime pollution, unsafe maritime conditions, poor water quality, eutrophication, overfishing, degradation or loss of marine and coastal ecosystems¹¹. Maritime pollution is the discharge of pollutant substances into the ocean from ships. Therefore, the variables that cause maritime and coastal dangers may be found anywhere.

So, several steps must be made in order to maintain the health of coastal systems and rich ecosystems that can support the local species and meet human needs and services. The activities consist of better judgments about the regulation and protection of the use of ecosystem services, as well as enhanced efforts designed to minimize negative impacts and the magnitude of changes, such as the overexploitation of ecosystems. Information of the systems and the changes inside are crucial for managing coastal zones, as is the transfer of this knowledge to decision makers.

2.

REMOTE SENSING AND WATER QUALITY ASSESSMENT

Today, several directives linked to European Union are existing which work toward the goal of ensuring the ecological integrity of Europe’s waterways via effective management. However, most of statistical analysis and outputs have been based on in-situ measurements. The network from which these metrics derived have approximately 13000 sites across Europe, the number may seem high but it cannot cover all the different coastal environments of Europe. Although in-situ measurement may offer high accuracies, it is a time-consuming process, and hence it is not feasible to provide a simultaneous water quality metrics on a regional scale¹². Moreover, conventional sampling methods are not easily able to identify the spatial or temporal variations in water quality which is vital for comprehensive assessment and management of waterbodies¹³. For example, in situ sampling has the advantage of measuring with high accuracy Chlorophyll-a but is limited regarding the spatial and temporal coverage. Chlorophyll-a concentration is recorded as little as 3–20 times per year in coastal waterbodies, with sampling frequencies varying widely between member states (Figure 2). Such sparse information will likely fail to accurately represent the dynamic nature of many waterbodies.

Therefore, these difficulties of successive and integrated sampling become a significant obstacle to the monitoring and management of water quality. Remote sensing, for monitoring coastal waters, have given a new perspective and important understanding of the global systems and the dynamics within. For coastal observations high-resolution sensors are needed. Using them, it is feasible to analyze the impact of transports of dissolved and suspended matter from both terrestrial run-off and river inputs on water clarity and surface chlorophyll-a concentrations in coastal zones using remote sensing and bio-optical data.

With the evolution of space science and the increasing use of computer applications, earth observation techniques have become useful tools in achieving water monitoring. Earth observation derived metrics can help fill the gaps that were previously mentioned. It can also improve the understanding of temporal and spatial variability of several quality elements within water and above the sediment surface in intertidal and shallow areas help define environmental reference status of some quality elements using historical satellite data and also provide a harmonized approach for monitoring water quality across Member States with a cost efficiency approach. Generally, Remote Sensing is the gathering of data without direct physical interaction with the assessed objects, regions, or phenomena¹⁴. Sensors on satellites and airplanes collect data remotely by detecting and measuring electromagnetic energy reflected off the earth’s surface characteristics¹⁵. These features have unique spectral characteristics because the structure, physics, and chemistry of the observed surface affect the spectrum of the reflected radiant energy. Radiant energy’s qualities rely on parameters such as its intensity, wavelength, and angle of incidence¹⁶. Optical remote sensing systems are split into two categories: multispectral systems, which detect and record between one and approximately ten spectral channels, and hyperspectral systems, which detect and record between tens and hundreds of spectral channels.

Recent studies have reported that remote sensing methods may provide comprehensive views of vast aquatic habitats. In addition to sampling, remote sensing provides a cost-effective alternative for monitoring changes in aquatic habitats. On the basis of the link between water’s optical qualities and electromagnetic radiation, water quality may be determined using remote sensing¹⁴. Due to the fact that the backscattering properties of water are affected by the types and concentrations of components, electromagnetic radiation may be used to evaluate surface water quality¹⁷. Thus, a body of water’s radiative upwelling may be utilized to detect the existence of water elements and measure their concentration. One of the advantages of employing remote sensing in assessing water quality is the ability to cover vast areas of water bodies spatially. In addition, the repeated gathering of remotely sensed data permits the periodic monitoring of water quality and the identification of trends. In addition, remote sensing is typically a cost-effective alternative to in-situ measurements since it does not need extensive field sampling. Consequently, remote sensing techniques have been implemented in a variety of water quality applications¹⁸.

Specifically, Earth Observation has contributed massively to the monitoring of coastal zones and provided estimates of alteration in coastal ecosystems. Given the advances in the design of sensors and data analysis techniques, remote sensing has been capable to evaluate the quality of waters in coastal environments. Multispectral and hyperspectral sensors are used to monitor coastal water dissolved substances, and biotic/abiotic suspended particle concentrations. Remote sensing sensors can vary from aircraft of low and medium altitudes to satellites in space, depending on the requirements of resolution and cost limitations¹⁹. Many research studies have used satellite imagery in their methodology to obtain robust correlations between the waters’ spectral value and physical and biogeochemical constituents, such as transparency, chlorophyll concentration²⁰.

Recent advances in sensor technology have led to the development of hyperspectral sensors (also known as imaging spectrometers) capable of collecting imagery containing several hundred bands over the electromagnetic spectrum. The concept of hyperspectral remote sensing began in the mid-1980s and has been used most widely by geologists for identification and mapping of minerals. Spectroscopy can be used to detect individual absorption features due to specific chemical bonds in a solid, liquid, or gas. Today, these sensors greatly expand the potential of remote sensing to assess, map, and monitor the characteristics of all natural resources including marine coastal zones. Hyperspectral data are particularly useful in marine coastal zones because of the spectral complexity of suspended as well as benthic features found in these environments. The very high spectral resolution of hyperspectral sensors gives them the advantage over multispectral sensors in facilitating exceptional differentiation of objects based on their spectral response in the narrow bands. The application of hyperspectral remote sensing techniques to water resource problems is proving to be the most in-depth way of examining spatial, spectral, and temporal variations to derive more accurate estimates of information required for water resource applications²¹. The most commonly used hyperspectral sensors currently in space can be found in the following table:

Table 1.

Specification of the most commonly used Hyperspectral sensors in water quality assessment.

The process of assessing the chemical, physical, and biological features of waterbodies and identifying potential pollution sources that decrease water quality is known as water quality study. It is possible for waste discharges, pesticides, heavy metals, minerals, bacteria, and sediments to degrade water quality. Diverse water quality standards have been devised to help in determining the level of water contamination and, subsequently, to uphold these requirements. For the present study, we have concentrated over most common parameters to be monitored for water quality assessment: Chlorophyll-a, Total Suspended Matter (TSM), Turbidity and Water transparency.

3.

MAIN DRIVERS AND CHALLENGES

Up to know, the missions mentioned previously have provided important benefit to scientific community to assess and model different parameters in order to derive water quality from satellite remote sensing data. However, for a commercial service adapted to the monitoring of man-linked activities such as aquaculture, fisheries, coastal tourism, harbors, there is a strong need of additional data in order to provide an efficient service. In particular the main drivers for complementary data are:

Additionally, it is important to take into account that most end-users are new to this kind of technics and therefore the final service will need to be user friendly, enabling both economical actors, regional and public stakeholders and potentially the general public to derive easily the main information needed. In addition, this service could also in a second time be extended to inland water monitoring, which would extend the benefit to more population and ecosystems.

At the moment, no such a service exists. The closest services existing are either the Copernicus Marine Environment Monitoring Service (CMES), Rheticus™ Marine²², developed by Planetek, or the WaterMonitor²³ application developed by VITO. However, none of them are able to provide the necessary information for the above-mentioned applications, either because of insufficient Ground resolution, strong dependance to 3^rd party microwave missions or in situ data, or limited zone coverage.

To fill this observational gap, the consortium composed of SAT4SPACE (France), Planetek Hellas (Greece), and Microelectronica SAS (Romania) has teamed together in order to propose a specific mission called ENTRUST. Its aim is to provide the necessary system to allow to monitor and control European coastal water quality and alert users of these areas with a local/regional scale with high revisit time. This mission, in its feasibility assessment stage, is complementary to the Copernicus satellite data, and would enable to take benefit from the 2 worlds: high end and very performing missions like the Sentinels, complemented by lower and smaller missions or systems, to provide very practical services to European end users. For this mission, two different kinds of systems were traded: drones on one hand, and nanosatellites on the other hand. The main first level specifications for the hyperspectral instrument are summarized in the table below (Table 2). The paradigm shift in this study is the switch from a very wide mission definition as it is performed usually for conventional scientific satellites, to a more narrow and focused mission, which allows the relaxation of a lot of constraints at instrument level, as well as a more agile and low-cost setup, which is highly desirable for the application.

Table 2.

Preliminary main parameters for the overall hyperspectral payload

4.

CAMERA AND MISSION CONSTRAINTS

At camera level, the main differences between the different hyperspectral cameras’ configuration relies on the following parameters:

At system level, being given the main requirements listed in table 2, one of the most constraintfull trade-off to be made is the type of platform to be used, the most adapted to the service needs. Indeed, the choice of the platform has important constraints on the type of Hardware (camera choice, detector choice etc.) to be selected, and needs to be chosen as early as possible.

Two types of platforms are considered for the ENTRUST project, because they answer to the need of regularity of measurements to be performed for the service: drones on one hand, cubesat on the other hand.

5.

CONCLUSION AND PERSPECTIVES

In conclusion, different system hypothesis were investigated in order to match the needs of water quality service feasibility. In summary, it was shown that:

From a market perspective, it is important to highlight that the use of nanosatellites today is primarily for large companies that may afford paying this service; to enable a wider spread of this technology three possibilities were identified, such as:

Concerning drones, their use can still be interesting for local/regional, small companies that need a random service, or a regulate one, fully decided by its own management. Nevertheless, there is still technical work to be endorsed to see how to maintain high performances in water quality retrieval, specially at data treatment level.

The next step for the ENTRUST project is to go towards practical demonstration, to validate not only the hardware configuration, but also to validate the data treatment scheme, that was not exposed within this paper. This demonstration would enable not only to be at the forefront of water quality management and monitoring for Europe, with innovative technologies and services, but could also be a first step to define better what type of information would be more relevant in the frame of the future upcoming missions such as the Copernicus future Sentinels.

The research and development leading to these results has received funding from the European Union Horizon 2020 Program “UFO project” under Grant Agreement 873411.

The authors would like to thanks the UFO consortium for its active support, as well as detector manufacturers and camera vendors for fruitful discussions on their available products.