PhD defense: Optical Methods, Uncertainties, and Algorithms for Remote Sensing of Coastal and Inland Water Quality


Organisation: Ghent University

Hydrologic optics and aquatic remote sensing are fields of applied optics that aim to retrieve information of state and process of aquatic systems from optical signals. Such an inference link depends on a rich set of paired optical and biogeochemical data, robust development and evaluation of algorithms and deep understanding of the underlying processes in order to support meaningful ecological interpretation. In this research we explore those different aspects by evaluating and proposing improvements for methods of field spectroscopy, describing a dataset with detailed compositional information covering a diverse range of environmental conditions in Belgian waters, and by providing algorithms to retrieve information of the phytoplankton assemblage for use with in situ optical instrumentation and remote sensors.

In this thesis are provided:

  1. a detailed evaluation of the uncertainty in downwelling irradiance estimations with diffuse reflectors, a topic overlooked despite large sets of data, particularly in inland waters, collected with this methodology;
  2. a detailed evaluation related to errors in water-leaving radiance measurements with the onwater approach, a relatively recent method for field spectroscopy;
  3. the description of the full set of paired optical and biogeochemical data collected in Belgian inland and coastal waters;
  4. the description and validation a method to extract additional spectral information from a system of overlapping wavebands and apply this concept to the OLI onboard Landsat 8;
  5. a reevaluation of the problem of optical detection of Phaeocystis globosa blooms in the BCZ by evaluating the origin of the signal detected by previously proposed hyperspectral algorithms and proposing a new algorithm to detect a reviously undocumented pigmentation pattern.

Alexandre Castagna (°1984, Rio de Janeiro, Brazil) graduated in Biological Sciences, with specialization in Ecology, at the Federal University of Rio de Janeiro (2006-2010). He acquired a Master of Science degree in Ecology and Evolution at the State University of Rio de Janeiro (2011-2013), studying the impact of the dust flux from Patagonia over the primary production of the Atlantic sector of the Southern Ocean. He started his training in hydrologic optics at the Oceanographic Department of the State University of São Paulo as a Ph.D. candidate (2015-2016, incomplete).

His studies involved the reconstruction of an uninterrupted series of chlorophyll a concentration in oceanic waters from the 70's to the present, based on the residual oceanic signal captured by sensors designed for atmospheric ozone monitoring. In 2016, moving to Belgium, he started a Ph.D. at the Protistology and Aquatic Ecology research group of the Ghent University, to study two extremes of application of remote sensing to inland and coastal waters: broad band very high spatial resolution sensors and hyperspectral sensors such as the upcoming PACE mission.

During his Ph.D., he had the opportunity to work with researchers from different institutions and specialization within optics. He was awarded with the Best Student Paper award, sponsored by The Oceanography Society (TOS), for his work on spectral enhancement of multispectral sensors, presented at the Ocean Optics XXIV conference. Since 2016, he has co-authored 15 studies published in peer-reviewed journals, four of which as first author. His current research encompasses methods of adjacency correction for remote sensing of small inland waters, developed at Ghent University and funded by the Belgian Science Policy Office (BELSPO). He is also the co-coordinator of the Nomenclature Focus Group, an encyclopedic effort in the scope of hydrologic optics and aquatic remote sensing, hosted by the GEO AquaWatch initiative and supported by International Ocean Color Coordinating Group (IOCCG). He has a broad range of scientific interests, from biology, statistics and physics and is an avid supporter of open information.