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The Belgian Coastal Zone (BCZ) in the Southern Bight of the North Sea is a highly dynamic and optically complex area. This is due to high non-algal particles (NAP) and coloured dissolved organic matter (CDOM) content which in spring adds together with undesirable blooms of the haptophyte Phaeocystis globosa. There is a need for improving the algorithms for chlorophyll a (chl a) retrieval in these highly turbid waters and for developing algorithms for species detection in order to attempt to create an early warning bloom system. This information will contribute to the knowledge of the extent and magnitude of the P. globosa bloom in Belgian waters. In this study, pure cultures of the main taxa present in the BCZ, diatoms and P. globosa, were combined with field measurements of light absorption of total particles, phytoplankton and dissolved material, pigment determination and phytoplankton counts to address the main objectives. Sampling was performed during 8 cruises covering winter, spring, summer and late summer, and along nearshore-offshore gradients from 2003 to 2006. The area is characterised by a high spatio-temporal variability of IOPs due to the high dynamics of the area in terms of currents, salinity gradients and biological production. During spring the presence of P. globosa modulates the IOPs across all the area, the particle absorption is significantly higher than summer and there is no significant coast-offshore variability for phytoplankton and CDOM. The design of chl a retrieval algorithms assumes negligible absorption of NAP and CDOM in the near infrared (NIR) and the use of a fixed value of specific phytoplankton absorption. It is shown that neglecting the NAP and CDOM absorption in the NIR will have a significant overestimation impact in retrieval of chl a. On the other hand, the specific phytoplankton absorption was found to be highly variable (0.015 „b 0.011 m2 mg chl a-1). Both results will affect directly the retrieved chl a. The spatial variability of CDOM was significant varying between 0.20-1.31 m-1 in the marine area and between 1.81-4.29 m-1 in the Scheldt estuary. CDOM was found to be related to salinity with conservative mixing within the Scheldt estuary and during some seasons in the BCZ, however deviations from conservative mixing suggest other inputs to the CDOM pool. Analyses of the spectral slope of the CDOM absorption curve revealed two main CDOM pools in the area, an allochthonous one delivered by the Scheldt estuary and an autochthonous one associated with the phytoplankton spring bloom decomposition. Algorithms for CDOM retrieval will be affected if the variability in the relation between CDOM and salinity is not taken into account. The optical characterisation of diatoms and P. globosa from the BCZ in pure cultures revealed that the main differences in the phytoplankton absorption spectra were found at 467 and 500 nm corresponding to the absorption of the pigments chlorophyll c3 (chl c3) characteristic of P. globosa and fucoxanthin, respectively. Accordingly, both the absorption at 467 nm and the ratio 500/467 nm were successfully used to discriminate the two taxa in cultures and field samples. This latter indicator was not preserved in the reflectance signal due to degradation of the signal when passing from absorption to reflectance, and thus could not be used for algorithm development. The spectral feature at 467 nm was later used as the basis for the development of a flag-type algorithm to detect chl c3 using either absorption or water-leaving reflectance data. Also, the correlation between the algorithm¡¦s retrieved chl c3 and P. globosa cell number allowed the quantification of the bloom. The main findings of this thesis highlight the importance of the IOPs characterisation for the improvement and development of ocean colour retrieval algorithms in these highly complex waters.

In-water biogeochemical constituents and bio-optical properties of the St-Lawrence Gulf and Estuary were monitored during 5 cruises conducted between 1997-2001 accross different seasons. Measured inherent optical properties (IOPs) included vertical profiles of the absorption and attenuation coefficients and the volume scattering function as well as absorption by particles, non-algal particles, phytoplankton and coloured dissolved organic matter (CDOM). Apparent Optical parameters (AOPs) included vertical profiles of the upwelling radiance and downwelling irradiance. The spectral shape of the major IOPs like absorption by phytoplankton, CDOM and non-algal particles as well as the particulate backscattering were parameterized using conventional models and adaptations of conventional models. Descriptive statistics of each variable in the collected dataset were analysed and compared with previous findings in the literature. The optical coherence of the measurements was verified using a radiative transfer closure approach. A complete set of IOP cross-sections for optically significant biogeochemical variables were generated. The magnitude and the spatial, temporal and spectral variation exhibited by the optically significant inwater biogeochemical constituents as well as the bio-optical parameters was consistent with our current knowledge of the ecosystem. The variation of the bio-optical parameters throughout the seasons was also coherent with our expectations. All the measured and derived parameters were found to vary within the ranges reported in the literature. Evidence was presented wherein the Gulf waters, which are usually considered as case I waters could also behave like case II waters. Moreover, spectral signatures exhibited by the IOPs and AOPs were coherent with the variation detected in the concentrations of the measured (optically significant) constituents. The extracted IOP cross-sections were consistent with the results of similar studies previously performed and could eventually be used in the estimation of the biogeochemical constituent concentrations given the related component IOPs. First-order radiative transfer closure was achieved; this underscored the validity of our experimental dataset based on considerations of higher level, integrative, physics. We argue that the current data collection campaign succeeded as a comprehensive framework for describing the behavior of the St-Lawrence bio-optical provinces within the context of remote sensing objectives. This bio-optical dataset should provide the basis for the development of a rigorous, satellite-based, remote sensing algorithm for the retrieval of near surface chlorophyll, fine-tuned to the local characteristics of the St-Lawrence system.

Earth observation satellites cover large areas with frequent temporal repetition and provide us with new insight into ocean and coastal processes. Ocean colour measurements from satellite remote sensing are linked to the bio-optics, which refers to the light interactions with living organisms and dissolved and suspended constituents in the aquatic environment. Human pressures have changed the aquatic ecosystems, by, for example, the increased input of nutrient and organic matter leading to eutrophication. This thesis aims to study and develop the link between bio-optical data and the remote sensing method to the monitoring and management of the Baltic Sea. The results are applied to the European Union’s Water Directives, and the Baltic Sea Action Plan from the Helsinki commission. In paper I indicators for eutrophication, chlorophyll-a concentration and Secchi depth were evaluated as a link to remote sensing observations. Chlorophyll-a measurements from an operational satellite service (paper I) were compared to conventional ship-based monitoring in paper II and showed high correlations to the in situ data. The results in paper I, II and IV show that the use of remote sensing can improve both the spatial and temporal monitoring of water quality. The number of observations increased when also using satellite data, thus facilitating the assessment of the ecological and environmental status within the European Union’s water directives. The spatial patterns make it possible to study the changes of e.g. algae blooms and terrestrial input on larger scales. Furthermore, the water quality products from satellites can offer a more holistic and easily accessible view of the information to decision makers and end-users. In paper III variable relationships between in situ bio-optical parameters, such as coloured dissolved organic matter (CDOM), dissolved organic carbon, salinity and Secchi depth, were found in different parts of the Baltic Sea. In paper IV an in situ empirical model to retrieve suspended particulate matter (SPM) from turbidity was developed and applied to remote sensing data. The use of Secchi depth as an indicator for eutrophication linked to the concentrations of chlorophyll-a and SPM and CDOM absorption was investigated in paper V. The variations in Secchi depth were affected differently by the mentioned parameters in the different regions. Therefore, one must also consider those when evaluating changes in Secchi depth and for setting target levels for water bodies. This thesis shows good examples on the benefits of incorporating bio-optical and remote sensing data to a higher extent within monitoring and management of the Baltic Sea. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript.</p>

The Belgian Coastal Zone (BCZ) in the Southern Bight of the North Sea is a highly dynamic and optically complex area. This is due to high non-algal particles (NAP) and coloured dissolved organic matter (CDOM) content which in spring adds together with undesirable blooms of the haptophyte Phaeocystis globosa. There is a need for improving the algorithms for chlorophyll a (chl a) retrieval in these highly turbid waters and for developing algorithms for species detection in order to attempt to create an early warning bloom system. This information will contribute to the knowledge of the extent and magnitude of the P. globosa bloom in Belgian waters. In this study, pure cultures of the main taxa present in the BCZ, diatoms and P. globosa, were combined with field measurements of light absorption of total particles, phytoplankton and dissolved material, pigment determination and phytoplankton counts to address the main objectives. Sampling was performed during 8 cruises covering winter, spring, summer and late summer, and along nearshore-offshore gradients from 2003 to 2006. <p>The area is characterised by a high spatio-temporal variability of IOPs due to the high dynamics of the area in terms of currents, salinity gradients and biological production. During spring the presence of P. globosa modulates the IOPs across all the area, the particle absorption is significantly higher than summer and there is no significant coast-offshore variability for phytoplankton and CDOM. <p>The design of chl a retrieval algorithms assumes negligible absorption of NAP and CDOM in the near infrared (NIR) and the use of a fixed value of specific phytoplankton absorption. It is shown that neglecting the NAP and CDOM absorption in the NIR will have a significant overestimation impact in retrieval of chl a. On the other hand, the specific phytoplankton absorption was found to be highly variable (0.015 „b 0.011 m2 mg chl a-1). Both results will affect directly the retrieved chl a. The spatial variability of CDOM was significant varying between 0.20-1.31 m-1 in the marine area and between 1.81-4.29 m-1 in the Scheldt estuary. CDOM was found to be related to salinity with conservative mixing within the Scheldt estuary and during some seasons in the BCZ, however deviations from conservative mixing suggest other inputs to the CDOM pool. Analyses of the spectral slope of the CDOM absorption curve revealed two main CDOM pools in the area, an allochthonous one delivered by the Scheldt estuary and an autochthonous one associated with the phytoplankton spring bloom decomposition. Algorithms for CDOM retrieval will be affected if the variability in the relation between CDOM and salinity is not taken into account. <p>The optical characterisation of diatoms and P. globosa from the BCZ in pure cultures revealed that the main differences in the phytoplankton absorption spectra were found at 467 and 500 nm corresponding to the absorption of the pigments chlorophyll c3 (chl c3) characteristic of P. globosa and fucoxanthin, respectively. Accordingly, both the absorption at 467 nm and the ratio 500/467 nm were successfully used to discriminate the two taxa in cultures and field samples. This latter indicator was not preserved in the reflectance signal due to degradation of the signal when passing from absorption to reflectance, and thus could not be used for algorithm development. The spectral feature at 467 nm was later used as the basis for the development of a flag-type algorithm to detect chl c3 using either absorption or water-leaving reflectance data. Also, the correlation between the algorithm¡¦s retrieved chl c3 and P. globosa cell number allowed the quantification of the bloom. The main findings of this thesis highlight the importance of the IOPs characterisation for the improvement and development of ocean colour retrieval algorithms in these highly complex waters.<p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished