DOI: 10.25549/usctheses-c127-654517
title: Underwater hyperspectral optical measurements as a tool for characterizing the spatial-temporal distribution of water column constituents contributing to ocean color

description: In coastal areas the spectral composition and intensity of light can change significantly in response to the optical properties of the water across horizontal distances and over different periods of time. Resuspension of particulate matter, transformations of the chemical and physical characteristics of the suspended matter, release of organic carbon from suspended particles, changes in phytoplankton biomass and species composition, and alteration of natural environments by human activities, all contribute to the optical complexity of the water in coastal areas. Due to the high productivity of this area and its importance in the global ocean there is a need to better characterize water optical properties in coastal areas and, hence improve our understanding of how the coastal oceans respond to short-term events, a variety of natural processes and human activities, over different spatial ranges.  This will lead to better monitoring of long-term changes due to climate and sea level change. ❧ In order to understand the distribution of hyperspectral optical properties in coastal areas in space and time, a better understanding of their relationship to processes occurring in the water column is needed but the bio-optical complexity and physical dynamics of coastal areas impose a limitation. In this study new tools and approaches were tested taking into account the complex distribution of optical properties associated with the physical and biological environment. In particular, the spatial and temporal variations of in situ hyperspectral optical properties and hydrographic observations from two contrasting island environments were studied: a dynamic strait in the Philippine Islands and a shallow coral reef area from Bermuda Islands. In the Philippines remote sensing validation was also studied to provide a tool for monitoring straits at a larger spatial and temporal scale. Specifically this study focused on optical proxies for biogeochemical constituents including the concentration of total suspended particulates, concentration of colored dissolved organic matter (CDOM), chlorophyll a concentration, and particle size distribution. ❧ In the Philippines, we tested the use of quasi-synoptic observations achieved with an undulating towed vehicle to characterize the water constituents and distribution of water masses in response to strong tidal forcing in a highly dynamic strait. The hyperspectral optical properties and their derived products were used to successfully characterize the transport and exchange of dissolve and particulate matter throughout semidiurnal and diurnal tidal fluctuations. Statistical analyses of the hyperspectral data set provided discrimination of water masses within the strait. These differences allowed delineating particular water masses coming in and out the strait with the tidal cycle, carrying particulate materials with characteristic spectral properties. The general pattern observed was a high oxygen level, small suspended particles, and low chlorophyll concentration in the oceanic waters entering the strait during flood tide. ❧ Also in the Philippines, we evaluate the quality of remotely sensed ocean color using the hyperspectral in situ inherent optical properties. The use of MODIS images at 250 m resolution was validated for this area using the Carder, QAA, GSM, Gould and OC3 standard ocean color models previously developed mainly for open ocean areas. We evaluated the validity of using MODIS imagery to study a region at high spatial and temporal resolution to measure the distributions of dissolved and suspended particulate matter, and chlorophyll-a concentration. Ocean color products derived from MODIS data positively correlated with the tridimensional in situ observations gathered in the San Bernardino strait. Satellite products were able to resolve the intrusion of oceanic water into the strait during flood tide, and the seaward transport of interior water during the ebbing tide. This study demonstrated for the first time the utility of using 250 m resolution MODIS images to resolve inherent optical properties and biogeochemical variables in a strongly forced coastal archipelago strait. Giving the limited in situ observations for the Philippines this validation study provides promises for monitoring at large spatial and temporal scales hyperspectral optical properties of straits. ❧ In the Bermuda Islands, we characterize spatial gradients of hyperspectral optical properties to estimate suspended materials, dissolved matter, particle size distribution and phytoplanktonic pigments in the water column. This enables the identification of the processes that primarily cause spatial variation in the optical characteristics from inland to the open ocean in this poorly characterized coral reef lagoon. A well defined gradient from inshore to the open ocean was observed, with the highest chlorophyll concentrations, dissolved absorption and high concentration of small particles close to land. Hyperspectral absorption and scattering magnitudes and shapes also discriminated between inside and outside areas. Although other processes may influence the observed optical gradient and characteristics, phytoplankton and suspended sediments are the dominant components of the spectral signatures in the water column. The hyperspectral optical measurements were able to distinguish sources with different biogeochemical signatures in this large island lagoon, indicating that ocean color observations can aid in better understanding how natural and anthropogenic disturbances may disrupt shallow coastal ecosystems, and help develop and tune remote sensing algorithms for investigating spatial-temporal variations of suspended materials in shallow coastal waters.