Space-time variability of bio-optical properties in the Southern California Bight
This dissertation examines the variability of physical and bio-optical properties in the Southern California Bight over various time and space scales. The research is divided into three chapters, and each chapter was written mostly as stand-alone pieces. An abstract is presented for each, and Tables, Figures and Appendices that support the text are shown at the end of each specific chapter. A list of all the references used in the text is shown at the end of the dissertation.
In Chapter One, “Assessing controls on cross-shelf phytoplankton and suspended particle distributions”, an underwater glider was used to “super-sample” the inner and mid-shelf Santa Barbara Channel (from 20 to 70m depth), providing data in detail never before observed. Highly resolved glider data in time and depth for a 4km long section perpendicular to the coast allowed answering questions such as: what is the space-time distribution of bio-optical properties (specifically phytoplankton and sediments) in the inner/mid-shelf SBC? And, what are the main controls on the variability of these properties? Six glider missions were completed, sampling a wide range of environmental conditions (e.g. upwelling, highly stratified, large blooms, small blooms, intense mixing). A storm event was also captured by the glider, providing a unique view of its evolution and effect on bio-optical properties. This confirmed the importance of instruments such as gliders in sampling events that are normally missed by ships and satellites. The data allowed characterizing the coastal zone in terms of surface, bottom and intermediate nepheloid layers of various compositions that evolve according to specific stratification conditions. Winds, precipitation, waves, and surface currents were used to infer some of the drivers of the variability. Waves were found as the main controls on sediment re-suspension during the storm mission, and advection processes were related to local changes in phytoplankton abundances. Unique examples of high-frequency events, such as the cross-shore propagation of a phytoplankton patch, the interaction between tides and sediment re-suspension and the co-evolution of phytoplankton and non-biogenic materials in similar portions of the water column – determining the complexity of the coastal ocean - are shown and discussed.
In Chapter Two, “Transport and Fate of heat, salt, oxygen and particles in the innershelf Santa Barbara Channel, CA”, the capabilities of the glider dataset are explored even further to ask questions that are in the very core of the studies in upwelling environments: How important is the cross-shelf exchange of materials to the health of nearshore ecosystems? Is the innershelf a source or a sink for particles/heat/salt? Is the nearshore zone net autotrophic or net heterotrophic? Do source/sink patterns change over time? Although the glider was not equipped with velocity meters, this study takes advantage of the Acoustic Doppler Current Profiler moored at the 10m isobath just North of the shallowest glider observations to provide depth-resolved velocity estimates for the calculations of biogeochemical fluxes across the innershelf. Despite large uncertainties, interesting patterns of the nearshore zone acting as a source or sink for phytoplankton, sediments and oxygen are observed for the different missions. Nonetheless, this study shows the feasibility of using depth-resolved instruments to assess cross-shore transport of biogeochemical properties, and provides insights into how an experiment should be designed to properly obtain those fluxes estimates in the future.
Chapter Three, “Satellite assessments of particulate matter and phytoplankton variations in the Santa Maria Basin and Southern California Bight”, takes a larger scale approach to determine what drives bio-optical variability in the Southern California Bight and Santa Maria Basin. This study took advantage of the now more than 13 years worth of quality optical imagery from SeaWiFS, MODIS and MERIS sensors, which were spectrally merged using a bio-optical algorithm to increase space-time coverage. 8-day composites and 2-km pixel sizes were used in the analysis, allowing the observation of weekly to seasonal and inter-annual changes in bio-optical properties, as well as long-term trends. Controls on chlorophyll distribution were different in different portions of the domain, indicating the importance of regional scale upwelling and larger scale changes in circulation patterns in determining productivity. Controls on backscatter far from shore mimicked the observed for chlorophyll. Near the coast, however, changes in backscatter were shown to be modulated by waves throughout the seasons and, episodically, by the large discharge events associated with El Niño conditions.