Optically Complex Waters
With more than 50% of the world’s population located along coastal water courses, there is increasing competition for water resources especially potable water, irrigation for agriculture, energy generation and indirectly through fisheries and aquaculture. Uncontrolled water use coupled with drought can have serious impacts on the ecosystem. Monitoring of water quality in coastal waters is an integral part of water resource management. It ensures the sustainable use of water and allows tracking the effects of anthropogenic influences.
In 1988 the European Union adopted new strategy for the Urban Waste Water Treatment Directive and the Nitrates Directive which resulted in improved legislation on water and bathing water quality. On 23 October 2000 the European Parliament and the Council adopted the Water Framework Directive establishing a framework for Community action in the field of water policy. The water framework directive includes the Bathing Water Directive which aims at achieving ‘good ecological status’ for all waters and for so-called ‘protected areas’ such as bathing waters. It resulted in a general definition of ecological quality status for coastal waters based on 19 key parameters which include microbiological, physico-chemical and ‘other’ parameters. Following this legislation much progress has been made in water protection in Europe and in individual Member States, and also in tackling significant problems at the European level. However Europe’s waters are still in need of increased efforts to get them clean or to keep them clean.
An important task in limnology and oceanography is the accurate estimation from spectral measurements of water leaving radiance of key water quality parameters and indicators. Remote sensing over coastal seas and inland waters has developed since the 1970’s from empirical approaches to produce qualitative water quality maps to analytical methods giving rise to quantitative maps. Today Earth Observation data has been successfully used to monitor optically-active substances in coastal and inland waters and offers great potential for integrated water quality monitoring by providing coverage of phytoplankton blooms and types, suspended sediment and coloured dissolved organic material at high spatial and temporal scales. A plethora of algorithms exist that can use multi and / or hyperspectral reflectance data. The following report reviews sources of hyper-spectral data and the current algorithms available that can use this data to retrieve a range of water quality products.
Extreme turbid waters
The extreme case 2 waters with very high TSM loads (>100 mg/l) will be termed here C2SX (Case 2 Scattering Extreme) waters, by extension of the common terminology of Case 2S waters. There is a significant user interest in such waters for management of coastal and estuarine sediment transport and associated dredging/dumping operations (Doxaran et al. 2009), for ecosystem modellers considering primary production and associated eutrophication and/or carbon fixation (Soetaert and Herman 1995) and for quantifying supply of organic carbon (Raymond and Bauer 2001) from the world’s major rivers (Amazon, Yangtze, La Plata, etc) to the coastal oceans.
Optical remote sensing of these waters has obvious potential, but has been hampered by the fact that ocean colour sensors and algorithms were historically designed for primarily Case 1 oceanic waters (Morel and Prieur 1977). The last decade or so has seen an enormous improvement in the design of ocean colour sensors and of the development of processing algorithms for turbid waters, stimulated by the user interest in coastal waters. These improvements have gradually pushed back the limiting turbidity for remote sensing, as can be seen in the progressively finer assumptions made as regards marine reflectance for atmospheric correction algorithms, and the development of algorithms for TSM retrieval in C2S and C2SX waters.
TSM is strongly correlated to inherent optical properties and the threshold of 100 mg/l corresponds approximately to a turbidity of 100 FNU, a backscatter coefficient at 555nm of 1 m-1 or a total scattering coefficient at 555nm of 50 m-1 (Ruddick 2012). TSM of 100 mg/l also correspond to a near infrared marine reflectance, rw(778nm), of about 0.06 or a SWIR marine reflectance, rw(1020nm) of about 0.004. These values are typical of thresholds where the near infrared similarity spectrum (Ruddick et al. 2006) starts to become invalid, requiring a non-linear reflectance model, and where SWIR marine reflectance becomes measurable (Knaeps et al. 2012), thus introducing new considerations for atmospheric correction and TSM algorithm development. These thresholds are somewhat arbitrary in nature and there may be debate within the community about whether to define a threshold for C2SX based on a user-requested gravimetric property (TSM), an inherent optical property (backscatter, scatter or turbidity) or an apparent optical property (reflectance).
Extreme absorbing waters
The extreme case 2 waters with high CDOM absorption coefficient, aCDOM(440nm)>1/m will be termed here C2AX (Case 2 Absorbing eXtreme) waters, by extension of the common terminology of Case 2A waters. Extreme absorption is mainly associated with the presence of coloured dissolved organic matter often accompanied by non-algae absorbing particles. As they are very hard to distinguish in remote sensing data because they both absorb strongly in the blue with exponentially decreasing absorption with wavelength, the dissolved (CDOM) and particulate (non-algae particle/detrital) components are often combined as total yellow substance (YS) by algorithms (Doerffer and Schiller 2007). In Europe, the Baltic Sea is a specific and large example case of C2AX waters with intense user interest. However, the potential C2AX user community for remote sensing data is much larger than just the Baltic Sea, e.g. the Black Sea is also characterised by high absorption. The importance of good data in C2AX waters will increase as ocean colour satellite data reaches the higher spatial resolutions (MERIS and OLCI: 300m) needed for large estuaries and inland waters, many of which are very dark because of high concentrations of the CDOM-rich degradation products of vegetation.
The user interest in C2AX is very similar to that of any coastal waters and includes: water quality monitoring, harmful algae bloom detection and the needs of ecosystem modellers for primary production and carbon cycle studies. EU member states are obliged to monitor the quality of their coastal waters (Ferreira et al. 2011) under the Water Framework Directive (WFD) and Marine Strategy Framework Directive (MSFD) and find satellite to be an efficient way to do so as regards chlorophyll a and associated eutrophication (Gohin et al. 2008), although some (Håkanson and Bryhn 2008) consider that satellite data in the Baltic Sea (C2AX waters) provides only qualitative and not quantitative information. Special blooms like cyanobacteria in the upper layers or on surface make a quantitative determination of chlorophyll extremely difficult (Kutser 2004) Harmful algae blooms in the Baltic Sea have long been studied using satellite remote sensing (Kahru et al. 1994). In these C2AX the CDOM absorption may or may not be a desired parameter in itself, but will nearly always be masking absorption from phytoplankton making the latter harder to accurately estimate.
Under low YS conditions, the reflectance has a distinct spectral shape in the visible range. Addition of YS lowers the broadband magnitude to a low level, from which it is very difficult to derive further information on water constituents, e.g. Chlorophyll a, out of the signal, quite apart from issues with the atmospheric correction in these cases. However, by means of appropriate methods as nonlinear regression with specially trained neural networks it should be feasible to further distinguish optical parameters in the water body.
In many coastal waters in the Baltic and also along the arctic coast high concentrations of absorbing material is accompanied by increased suspended material thus showing a composition of high absorbing with a moderate scattering situation. They indicate an increased erosion of terrestrial organic material to coastal waters, which is induced in arctic regions mainly by climatic changes.