New observational approaches for optically complex waters based on high-resolution transmissometry

There is a growing concern for protection and conservation of coastal ecosystems, which require a proper understanding of ecosystem dynamics and the ability to detect and predict changes in the ecosystem state. For this purpose, there is general agreement on the requirement of multidisciplinary and multiscale observing systems, due to the complex physical and biogeochemical processes occurring in these environments at a wide range of time-space scales. In this sense, optical-based approaches arise as powerful tools since they allow to assess biogeochemical constituents at high temporal and spatial resolution. Moreover, the recent development of cost-effective, hyperspectral, compact and low power optical sensors has broadened their observational capabilities by allowing the assembly on a variety of platforms and by reducing the operational costs. Among the different optical properties, the beam attenuation coefficient presents numerous advantages due to the general availability and simplicity of operation and data processing of the required instrumentation (i.e. transmissometers). In this context, this thesis seeks to improve the capabilities for coastal waters monitoring and assessment by developing new observational strategies based on high resolution beam attenuation measurements. With this aim, the reliability of a recent commercially available advanced-technology transmissometer (i.e. VIPER, TriOS GmbH) has been determined by analyzing the instrument uncertainties and performance. Despite some issues related to the thermal management of the LEDs and the contamination of ambient light, VIPER measurements have been validated in the field by comparison with other transmissometers. Then, to evaluate to what extent is possible to retrieve environmental information from these measurements, attenuation data collected in the Mediterranean estuary of Alfacs Bay have been analyzed against laboratory-derived biogeochemical variables. The analysis has successfully used three major spectral features of the beam attenuation coefficient as qualitative proxies for the biogeochemical variables: coloured dissolved organic matter (CDOM), Chlorophyll a (Chl-a) and total suspended matter (TSM), allowing the detection of changes and patterns in these variables at high temporal and spatial resolution. Thereby, high-frequency in situ beam attenuation measurements are proposed as cost-effective, rapid and simple method to better understand the complex physical and biogeochemical interactions in coastal environments. Nevertheless, there are numerous factors affecting the attenuation signal, which restrict the capability of these measurements to quantitatively assess the biogeochemical constituents and involve that complementary optical data are required. For this reason, an additional approach has been investigated by including spectral absorption measurements collected in situ with a dual spectrophotometer widely used by the scientific community. An inversion model of attenuation and absorption data has been developed based on local material-specific inherent optical properties to quantitatively estimate CDOM, Chl-a and mineral suspended solids in optically complex waters. The advantages of this approach lie in the simple input requirements, the avoidance of error amplification, full exploitation of the available spectral information and the reasonably successful retrieval of constituent concentrations. In summary, this thesis has proposed two complementary beam attenuation-based approaches for complex coastal waters. On one hand, cost-effective and compact transmissometers allow to determine the scales of variability in the in-water constituents, providing a comprehensive overview of the biogeochemical processes. On the other hand, this information is highly valuable to define the optimal sampling strategy to be adopted with more sophisticated optical sensors required to estimate the concentration of the biogeochemical variables in a water body.