• Citer

Overview of ocean colour: theoretical background, sensors and applicability to detection and monitoring of harmful algal blooms (capabilities and limitations)

par Ruddick, Kevin;Lacroix, Geneviève;Park, Youngje;Rousseau, Véronique ;De Cauwer, Vera
Editeur scientifique Babin, Marcel;collin, S.;Cullen, John J.
Référence Real-time Coastal Observing Systems for Marine Ecosystem Dynamics and Harmful Algal Blooms, Theory, Instrumentation and Modelling, UNESCO Publishing, Vol. Oceanographic Methodology series, Oceanographic Methodology series, page (830)
Publication Publié, 2008-04-01

Partie d'ouvrage collectif

• Accès en ligne
• Détails
• Contenu
• Statistiques

Résumé :

Anecdotal and journalistic references to harmful algal blooms (HABs) often report the impact on water colour, using terms such as “red tide”. Indeed probably the earliest recording (Exodus 7:20-21) of a harmful algal bloom (HAB) remarks that the waters “turned to blood”. A few thousand years later photoelectric devices have replaced the human eye as detector, allowing a transition from subjective and descriptive observations to detailed and quantitative information on water constituents. The objective of this chapter is to summarise the status of optical remote sensing of HABs by reviewing ocean colour theory and giving an overview of available systems. Applications of optical remote sensing to the monitoring of HABs are reviewed, noting capabilities and limitations, and future perspectives are discussed. The scope of this chapter, as given by the title “ocean colour”, covers passive systems for optical remote sensing using the sun as direct light source (Mobley, 1994). While the term optical remote sensing could be considered to cover a wide variety of sensors, including the human eye, airborne photographic cameras, shipborne above-water radiometers, etc. the scope here will be limited to sensors providing digital image-based information, thus covering essentially satellite and airborne radiometers. The focus is on reflectance-based techniques because of their overwhelming importance in the available data stream. However, it is noted that some information on phytoplankton (without specific relevance to HABs) can be retrieved from remote sensing of sun-stimulated fluorescence (Babin, 2005; Gower et al., 1999). Active remote sensing systems based on, for example, laser-induced fluorescence (Hoge and Swift, 1981) are excluded. Applications are drawn mainly from marine waters because of the user interest and experience there though the validity of the theory and systems to estuarine and inland waters is also considered. The applications described in this chapter are drawn essentially from HAB studies. However, the theory described as well as many of the remarks made in this chapter are relevant both to HABs and to the much wider field of ecosystem dynamics. In fact, as will be discussed in detail later, ocean colour is used primarily for algae bloom (AB) detection and generally requires supplementary information from seaborne measurements or expert knowledge for determination of the harmfulness of the event. The question “Is the AB a HAB?” is a major challenge for the ocean colour technique. The definition of “harmful” in this context is left for others (Cullen, 2005; Geohab, 2001; Graneli et al., 1998). The present chapter thus covers a wide range of algal species with very different severities of impact. The content of this chapter has been selected according to the spirit of the workshop to provide readers with the theory relevant to understanding the basic principles of ocean colour observations and with an appreciation of the current capabilities and limits of these tools for HAB detection. The target readership is students and potential end-users of coastal observation systems who should thus acquire sufficient knowledge for the initiation and use of such observing tools. Thus, the focus is on general principles of ocean colour remote sensing and a broad understanding of processes with emphasis on their spectral reflectance, loosely termed as “colour”. Optical remote sensing specialists who require a more detailed description of radiative transfer theory and especially the complications of interacting processes, such as multiple scattering, and second order optical effects, should consult the references in the appropriate sections. Readers with no interest in the details of ocean colour theory and data processing may skip sections 2, 3.1 and 3.2 but are strongly advised to be aware of the variety of methods presented in sections 3.3, 3.4 and 3.5 and especially of their limitations. It is common for products from the main ocean colour sensors to give very different results in coastal waters even for data acquired almost simultaneously (De Cauwer et al., 2004). Key questions which are to be answered in this chapter include: • What is ocean colour? • What sensors are available and suitable for HAB detection? • What information can be obtained from such sensors? • What are the limitations of the ocean colour technique for HAB detection? • What developments can be expected in the future?