Detection and Quantification of Algal Bloom Dynamics in the Great Barrier Reef Lagoonal Waters Using Remote Sensing and Bio-Optics

David Blondeau-patissier (2011). Detection and Quantification of Algal Bloom Dynamics in the Great Barrier Reef Lagoonal Waters Using Remote Sensing and Bio-Optics PhD Thesis, School of Geography, Planning and Environmental Management, The University of Queensland.

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Author David Blondeau-patissier
Thesis Title Detection and Quantification of Algal Bloom Dynamics in the Great Barrier Reef Lagoonal Waters Using Remote Sensing and Bio-Optics
School, Centre or Institute School of Geography, Planning and Environmental Management
Institution The University of Queensland
Publication date 2011-05
Thesis type PhD Thesis
Supervisor Professor Stuart Phinn
Professor Arnold Dekker
Dr Vittorio Brando
Dr Scarla Weeks
Total pages 334
Total colour pages 108
Total black and white pages 226
Language eng
Subjects 04 Earth Sciences
Abstract/Summary Ocean surface processes occur at different time and spatial scales. Algal blooms in particular are transient, naturally occurring phenomena that are central to the structure and functioning of the marine food web. The monitoring of algal bloom events in coastal waters has become increasingly important because of the growing environmental pressure on coastal ecosystems. Ocean colour remote sensing is the only technique capable of monitoring the cyclicity and spread of algal blooms over large scales of space and time. The high temporal frequency at which satellite images are acquired allow for the integration of climatology and time series of satellite-derived surface water properties for detecting annual and seasonal trends, as well as discrete algal bloom events. The primary aim of this thesis was to investigate the dynamics of algal blooms in the tropical coastal, lagoonal and reef waters of the Great Barrier Reef (GBR), Australia, using satellite data from the European Space Agency Medium Resolution Imaging Spectrometer (MERIS) ocean colour and National Aeronautical and Space Administration Moderate Resolution Imaging Spectroradiometer (MODIS) Sea Surface Temperature (SST) sensors. To achieve this research aim, it was necessary to first characterise the inherent optical properties (IOPs) and concentrations of chlorophyll (Chl), coloured dissolved organic matter (CDOM) and total suspended solids (TSS) in the GBR surface waters. A total of 129 in situ samples from various campaigns carried out in the GBR between 2002 and 2005 was used to optically characterise six regions. Most of the stations sampled during these field campaigns were in coastal waters. The results of this first objective illustrated the optical complexity of the GBR waters and emphasized the need to regionalize the GBR system based on water quality parameters. Spatially, the surface waters of the Fitzroy, a shallow, macrotidal estuary located in the southern GBR, exhibited the strongest variability in all the inherent optical properties and biogeochemical concentrations sampled. CDOM slopes were found to increase from the reef to the coastal waters of the GBR, and the CDOM properties of the Fitzroy were of autochthonous and terrestrial origins. The relationship between the phytoplankton absorption at 440 nm and Chl was found to differ from reference models. Picophytoplankton (< 2 µm) was the dominant fraction of the phytoplankton community in most regions, with a relative contribution of 65% in the reef waters. Temporally, optical properties and concentrations were higher in most regions during wet season months (December to April). However, additional field measurements are required to supplement this limited (N=8) wet season dataset and to help better characterise the effects of the wet season on the GBR waters. A second objective was to determine the most suitable ocean colour sensor for detecting and monitoring algal blooms in the GBR waters. Despite a less frequent (two- to three-day) revisit cycle than MODIS, MERIS was found to be better suited for this task. MERIS has more (15) and better positioned spectral bands than MODIS that allow for the retrieval of water quality parameters as well as the detection of algal surface expressions. This second objective also demonstrated that the MERIS Freie Universität Berlin (FUB) Algal 2 was the most robust and accurate algorithm for the retrieval of Chl (the main proxy for phytoplankton biomass) in the GBR waters. The FUB Chl algorithm was empirically fine-tuned to improve its performance at retrieving Chl in the GBR waters using in situ Chl and phytoplankton absorption (440 nm) data. However, FUB underestimated TSS and CDOM in the GBR waters. The MERIS Fluorescence Line Height (FLH), the Spectral Shape (SS) and the Maximum Chl Index (MCI) were also tested for the retrieval of fluorescence and algal surface expressions. The MERIS FLH was used to provide additional information on algal status from the FUB Chl, and the MCI was selected as the best performing algorithm for the retrieval of surface algal expressions. The MCI was subsequently used for the mapping and monitoring of Trichodesmium sp. blooms, a phytoplankton taxa that regularly occurs in the GBR. The third objective focused on the four-year statistical analysis of five MERIS ocean colour products (Chl, CDOM, TSS, FLH, MCI) and MODIS SST to extract spatial and temporal (annual and seasonal) patterns of phytoplankton bloom variability in the GBR between 2006 and 2010. Monthly climatology maps, latitude-time plots (Hovmöller plots) and time-series helped describe the dynamics of algal blooms in the coastal, midshelf and oligotrophic lagoonal waters of the GBR. Although the period of time explored was limited to four years, these results provided complementary information to the previous findings: the GBR is a complex system in both optical properties and bloom dynamics. Monthly climatology maps and Hovmöller plots of Chl demonstrated that a seasonal, cross-shelf increase in phytoplankton growth occurred during the wet season months. They also revealed that the southern GBR (19-26S) was characterized by a higher phytoplankton biomass in comparison to the northern GBR (10-19S). This spatial difference is due to the southern GBR hosting larger river catchments, therefore influencing the quantity of nutrients delivered to the GBR nearshore waters during freshwater runoffs. The waters of the northern GBR however showed pulses of Chl of short durations likely associated with intense, but short-lived, small tropical river flows. Time-series of Chl for the coastal, midshelf and lagoonal waters, extracted for both the southern and northern GBR, exhibited significantly different patterns. These six time-series confirmed that phytoplankton blooms were spatially heterogeneous and that their timing and magnitude varied from year to year. The frequency of these Chl cycles was found to decrease from the coastal to the oligotrophic lagoonal waters, the latter being characterised mostly by a strong seasonal cycle during the wet season. The year 2009 was higher in Chl, in particular during the wet season months. This was likely the result of an enhanced nutrient supply from the ecological disturbance caused by tropical cyclone Hamish in March 2009. The north-to-south track of cyclone Hamish was unusual because it followed the whole GBR matrix, whereas tropical cyclones occurring on the east coast of Australia are usually confined to the northern GBR (north of Cairns). Finally, algal surface expressions of what was believed to be Trichodesmium sp. blooms were found to occur mainly between August and December in the southern GBR, and predominantly in the oligotrophic lagoonal waters. The frequency and intensity of these blooms of cyanobacteria were found to increase over the four years of this study. It is possible that stronger water column mixing in the northern GBR hampers the development of regular Trichodesmium sp. blooms, while predation or competition limit their occurrence in the coastal and midshelf waters. Phytoplankton dynamics in tropical marine ecosystems have been largely under-studied and this research partly addressed this knowledge gap. The findings of this work are of direct benefit to Australia’s capacity for environmental detection, monitoring and prediction of algal bloom events in the GBR, providing trends in phytoplankton biomass across the entire shelf during a period of four years. Despite the limited study period and some possible artefacts in the data, the results of this PhD research provided an assessment of algal bloom dynamics in the GBR waters by the combination of six satellite products. Future research would include the merging of MERIS and MODIS ocean colour products to complement the limited temporal sampling frequency of MERIS. Additional satellite datasets on wind speed and direction, as well as sea surface height would benefit to this study because it would allow the analysis of their effects on algal bloom occurrences in the GBR. For instance, it has been commonly reported in the literature that wind speed and the occurrence and distribution of Trichodesmium sp. bloom events are related.
Keyword Great Barrier Reef (Qld.)
algal blooms
marine optics
ocean colour
Satellite oceanography
statistical analysis
Additional Notes Page numbers that should be printed in colour: 1 35 40 41 43 44 46 47 48 50 51 52 53 56 75 77 79 80 82 83 84 87 88 102 106 127 128 129 130 131 132 134 136 138 140 141 142 143 145 147 148 149 150 152 154 155 156 159 160 161 171 173 176 183 185 186 187 189 190 191 193 194 196 197 198 199 200 201 203 204 205 207 209 210 212 213 214 215 219 220 224 225 226 227 228 230 258 259 260 264 266 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284

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Created: Mon, 14 Nov 2011, 21:35:43 EST by Mr David Blondeau-patissier on behalf of Library - Information Access Service