Inherent Optical Property (IOP) determination

            While technologies to measure water clarity and CDOM in the field and from remote sensing have revolutionized in the past decades, coastal zones are under significant influences of tides, seasonal inflows, and sediment resuspension events.  As a result, optical properties in these Case II regions have high frequency and spatial variations not typically captured with daily remote sensing products (IOCCG, 2000).  There are few installations providing continuous IOP data, and even fewer providing a full suite of explicitly partitioned absorption and scattering properties, thus making it difficult to evaluate spatial and temporal heterogeneity, to determine long-term climatically-driven trends, and to evaluate the uncertainties due to assumptions inherent in standard remote sensing products when applied to these complex waters.
            Mote Marine Laboratory has designed a submersible, highly sensitive absorption instrument, the Optical Phytoplankton Discriminator (OPD; Figure 1) which has been in operation since 2005 to measure high-frequency CDOM (a_g) and particulate (phytoplankton and detrital; a_ph + a_d) absorption spectra 

Figure 1: Mote’s Optical Phytoplankton Discriminator allows for the autonomous collection of both raw and filtered absorption spectra in a long path-length liquid waveguide cell.

  at several coastal sites in Florida (Kirkpatrick and Hillier, 2007; Shapiro et al., 2014). Unattended OPD measure absorption spectra of raw (with the ability to concentrate particulates) and filtered (0.2 μm) samples using extended pathlengths and at user-selected frequencies.  The instrument provides a) full spectrum results (300-800nm); b) has an onboard calibration system; and c) has the ability to partition the particulate absorbance spectra (a_ph + a_d)  from the CDOM spectra (a_g) via a unique hollow fiber filtration scheme;  and d) using a 4^th derivative approach, separate the phytoplankton (a_ph) and the detrital (a_d) components (Figure 2).

Figure 2: Example spectra measured or derived from in situ OPD measurements of a coastal water. The total particulate spectrum was obtained by difference of the filtered measurements from the unfiltered measurements.  Then, the particulate spectrum was de-convoluted to obtain the particle backscattering (b_b) loss, and the detrital (a_d) and phytoplankton pigment (a_ph) absorption spectra using a novel 4^th order derivative and least squares regression technique.

These data have recently been statistically related to co-located measurements of relevant physical and biogeochemical parameters (e.g. salinity, temperature, Chl. a, FDOM, flow, and tide height from Sanibel Captiva Foundation “RECON” sensor suites), have the ability to detect high frequency events in both absorption and spectral slope (Figure 3), and are yielding novel insights into variation of and environmental controls on CDOM spectral absorbance. Covariate plots constructed from the time-series data in (Figure 4) demonstrate relationships of remarkable predictive utility permitting variable spectral slopes to be predicted given salinity and a single measurement of base spectral absorption.

Figure 3: Example time series of OPD CDOM absorption and spectral slope measurements (Mote) compared to salinity and FDOM measurements (SCCF) at the co-located OPD/RECON Sanibel site.  The lower time series is a zoomed section to demonstrate the ability for the instrument package to resolve high-frequency intertidal variations.

Figure 4: Covariate plots constructed from the time series in Figure 3 demonstrate that CDOM spectral slope evaluated between 350 and 400 nm can be better predicted from both a₄₄₀ and salinity than either measurement alone. This result suggests that for this particular site, remote sensing efforts can be improved by modeling CDOM spectral slope using the relationship derived from the bottom plot: S_350-440 = 0.779 / (salinity x a₄₄₀)

Remote estimation of CDOM and water clarity have evolved to employ sophisticated algorithms based on radiative transfer theory (Lee et al., 2002, 2005, 2009; Le and Hu, 2013). These algorithms generally work well in the open ocean where relationships of the optical properties of the in-water constituents are well understood (e.g., CDOM and phytoplankton co-vary, or CDOM spectral shapes and phytoplankton absorption shapes are well defined). Such relationships are assumed implicitly in the empirical algorithms and explicitly in the analytical or semi-analytical algorithms. However, such spectral relationships for CDOM (defined by the spectral slope) are highly variable in coastal environments in both space and time where CDOM is dominated by terrestrial sources, and can lead to significant errors when a fixed CDOM spectral slope is used. This situation is exacerbated across regions by the influence of land cover on CDOM spectral slope as demonstrated by Le et al. (2015) where several Gulf of Mexico coastal estuaries were compared. Adding to this complexity is the variable proportions between CDOM and particulate absorption and variable spectral slopes of particulate absorption, which all impact algorithm performance. Because water clarity is usually expressed by the diffuse light attenuation (K_d, m^-1; Lee et al., 2015) and K_d is dominated by total absorption (in coastal waters, total absorption is often dominated by CDOM), the accurate full spectrum estimation of CDOM absorption is critical in deriving overall water clarity from remote sensing.

Given the need for accurate CDOM and water clarity data for estuaries and coastal waters and difficulties in measuring these properties continuously using either typical field techniques or remote sensing, this project seeks to leverage existing infrastructure to enhance and automate high frequency IOP measurements, link in situ full spectrum CDOM and particulate characterization to more routine proxy measurements in Case 2 waters (e.g. salinity, FDOM, tide height, river flow), and to refine existing remote sensing algorithms incorporating the enhanced measures of particulate and dissolved absorption and spectral slopes to derive and provide CDOM and water clarity data products of enhanced accuracy.

Figure 5: Continuous measurements of filtered a₄₄₀ (left) and S_350-400 (right) obtained from an OPD integrated into a flow-through system of a ship demonstrate the ability of the instrument to spatially map variations of the inherent optical properties of water.