Calibration and validation of a new instrument for measuring the angular light scattering properties of seawater
The inherent optical properties (IOPs) of seawater describe the interactions of seawater constituents with light and dictate its propagation throughout the water column. The IOPs are directly linked to the concentration, size and composition of suspended particles, and thus are used to estimate important biogeochemical parameters from satellite or in situ optical measurements. One of the most fundamental IOPs describes the angular distribution of unpolarized light scattering; the volume scattering function (VSF). Despite the relative importance of the VSF, the ocean optics community has historically relied on theoretical models and a limited dataset of measurements made using custom-built instruments. A new instrument capable of in situ operation has recently become commercially available (LISST-VSF, Sequoia Scientific) to provide measurements of the VSF at a single wavelength with high angular resolution, as well as characterizing the degree of linear polarization of the scattered light field. The availability of such instrumentation has the potential to significantly enhance our understanding of light scattering in the ocean. Before this can be realized, however, a thorough and independent characterization, calibration, and validation of the instrument must be undertaken. I describe a series of experiments using NIST certified polystyrene bead standards coupled with theoretical light scattering calculations to both evaluate instrument performance and enhance raw data processing including the development of an angle-dependent calibration correction function. Corrections on the order of 150 – 200 % were required for nearly all scattering angles. The application of these results to independent laboratory suspensions and natural seawater samples will be presented.
Depth-dependent thermal stress around corals in the tropical Pacific Ocean.
Thermally driven bleaching events are a growing concern for reef ecosystems across the tropics. To assess and predict thermal stress impacts on reefs, remotely observed sea surface temperature (SST) commonly is used, however, reef communities typically extend to depths where SST alone may not be an accurate measure of in situ variability. We will present nearly two decades of temperature observations (2-90m depth) at three stations around Palau which were used to develop an empirical model of temperature variability versus depth based on SST and sea level anomaly (SLA). The technique yields depth-averaged R2 values > 0.8, with SLA predicting fore-reef temperatures near the thermocline and SST capturing upper mixed layer temperatures. SLA complements SST by providing a proxy for vertical isotherm displacements driven by local and remote winds on intraseasonal to interannual time scales. Utilizing this concept, thermal stress can be predicted into the mesophotic zone, an ecosystem shown susceptible to bleaching.