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Points to address here:
Use of EO data
Challenges in using EO data
What we try to achieve in here
In recent years, there has been a heightened interest in monitoring inland water quality, as rapid changes in human activity have been linked to rising eutrophication levels and increased silting. Decision makers have also noted the importance of monitoring networks, and water monitoring networks are starting to be deployed by governments, their growth occurring in parallel to legislation on water quality. While these networks are a positive development, they are not ubiquitous, limited both in space and in time. Earth Observation (EO) data in the optical domain has proven useful (e.g. (Carpenter 1983)) in this endeavour, as surface reflectance varies as a function of water composition. In principle, EO data has the ability to frequently monitor inland waters even in situations where no in situ observational networks are in place. Moreover, since sensor such as Landsat have been in operation for over three decades, it opens the possibility of assessing long term dynamics in water composition.
A number of complications need to be addressed in order to use EO data effectively to monitor lakes, however. For one, data acquisition is contingent on orbital selection, as well as cloudiness. In regions where clouds are prevalent, our ability to monitor lakes is thus severely reduced. Additionally, the sensors capture information on the state of the land surface, but are also affected by the optical properties of the atmosphere. Atmospheric properties are due to scattering and absorption by gases and particles, effects such as aerosol concentration and type, ozone concentration and water vapour. Atmospheric correction techniques are therefore required to compensate this important contribution. Finally, the interpretation of the data is complicated by the limited spectral sampling, and thus requires of models. Some of these models are empirical, and thus rely on local calibration, which might be sparse or directly unavailable. Mechanistic models are complex to develop, but some exist. If they are simple enough to require few data inputs to model most of the signal, they can be inverted to infer the state of the water.
Importance of water quality, both from an environmental and legislative viewpoint.
Also known as the Water Framework Directive (WFD), European Directive 2000/60/EC was born in order to blend up more locally focused legislation in each country of the EC about water management (just as most of our environmental legislation). It involves most of available water resources such as inland, transitional, coastal and groundwater; And it develops mechanisms not only to prevent further deterioration but also to enhance their ecological status.
It also enumerates a lot of specific elements (biological, chemical and hydromorphological) that can help us to quantify ecological alteration in a water body. And eutrophication is one the most important problem (and with widespread occurrence nowadays) that these elements can make us aware about. As defined by the Policy Summary of the guidance document on eutrophication assessment in the context of european water policies: Eutrophication is the accelerated production of organic matter, particularly algae and higher forms of plant life, in a water body usually caused by an increase in the amount of nutrients being discharged to the water body. This abnormal growth of organic matter promotes dissolved oxygen to decrease. And consequently, it can make submerged aquatic vegetation disappear, disturb the ecological balance of water organisms, and worsen water quality spreading the initial local effects to most of the nearby ecosystem. In some specific situations eutrophication can also promote toxic alga bloom and thus, cause preventive restriction to most of the regular uses of water. For inland waters, cyanobacterias are responsible of most of these blooms and they have been recognized as one important ecological elements used for the assessment of ecological status.
Cyanobacteria are photosynthetic prokaryotes found in most types of illuminated water environments. They are the Earth’s oldest (∼3.5 bya) oxygen evolving organisms. All synthesize chlorophyll a, and most of them produce the phycobilin pigment, phycocyanin, which gives the cells a bluish colour and is responsible for the popular name: blue-green algae (they may also develop red colour due to another pigment: phycoerythrin, but it is not so common).
Cyanobacteria can live in a wide range of environments. However there are some few variables that may determine their growing variability in a competitive environment:
Temperature, as they prefer warmer water conditions (Castenholz and Waterbury, 1989)
Radiation resistance: Terrestrial can tolerate harder desiccation as well as higher levels of ultra-violet irradiation than most eukaryotic algae.
Water stress conditions: They are one of the most successful organisms in highly saline environments and they can also adapt their photosynthesis process replacing water as the electron donor by H2S (Cohen et al. 1986)
Buoyancy (more common in freshwater plankton species): Due to their ability to form gas vacuoles. It can give them an important advantage in environments with poor vertical mixing.
Cyanobacterial blooms are frequently related to human influences in modifying the physical and chemical conditions of aquatic systems (Oliver and Ganf 2000, Carvalho 2013) and may affect the quality of water supplies; Not only by a typical eutrophication process but also because of the risk of the toxins that some of the bloom forming species produce (Posch et al. 2012). In fact, toxin-producing cyanobacteria and their presence in surface waters used for drinking water production has prompted the World Health Organization (WHO) to publish a provisional guideline value of 1.0 μg microcystin (MC)/l drinking water.
Stricter nutrient management will likely be the most feasible and practical approach to long-term cyanobacteria control in a warmer, stormier and more extreme world (Paerl et Al. 2012) related to both Climate Change and water pollution. Moreover, Nitrogen and Phosphorus inflows should be critically controlled for inland waters, as both nutrients cause substantially more algal growth than either added alone (Lewis et Al. 2011, Carvalho 2013).