Optical Phytoplankton Discriminators

Because of their complexity, understanding and predicting K. brevis (Florida red tide) blooms require holistic chemical and physical information together with cell concentrations over relevant spatial and temporal scales. However, traditional methods of measuring K. brevis abundance (e.g. microscopic examination) are costly, tedious, and intermittent. Mote has maintained a HAB observatory network for 12 years consisting of fixed-location in situ Optical Phytoplankton Discriminators (OPD) capable of extended unattended detection of K. brevis blooms. 

        Presently, two fixed-location OPDs are located near human population centers and bracket frequent initiation sites of K. brevis blooms (Heil et al., 2014). The configurable OPDs currently measure and stream K. brevis “similarity indices” (SIM index) to the SO-COOL monitoring network every two hours. The OPDs evaluate the 4^th derivative of a sample’s particulate absorbance spectrum and compare that to a library of standard spectrums (of K. brevis and other species), generating SIM indices that signify the presence of K. brevis if larger than a threshold value (Figure 1) (Kirkpatrick et al., 2000; Robbins et al., 2006). The OPD is the only instrument that is presently capable of providing real time taxonomic data while deployed robotically and autonomously (on AUVs) and has been used by researchers both nationally and internationally. 
        The OPDs also provide a full visible spectrum of CDOM absorption. Unlike fluorescence proxies, spectrophotometric CDOM can quantify the spectral slopes of coastal and oceanic waters, providing insight into CDOM sources and photodegradation history (Helms et al., 2008). Combined with ancillary sensor suites (i.e. CTD, nutrients, and/or chlorophyll) the installations provide not only K. brevis abundance, but also comprehensive insight into the range of physical and biogeochemical conditions that promote HAB blooms.  More information regarding the ability of the OPD to provide information on the inherent optical properties of water is found under that project page.

Figure 1 – Clockwise from lower left: 1) Picture of OPD with housing removed; 2)  The San Carlos Bay OPD is mounted on a channel marker, while the previous Sanibel location, Tarpon Bay, was mounted on a dock; 3) OPDs are currently in service at the San Carlos Bay, Sanibel site and the New Pass, Sarasota Mote dock. 4) Screenshot from the SO-COOL website (coolcloud.mote.org) showing the real-time data from the two sites, as well as a time series of Karenia brevis similarity indices demonstrating a short-lived Florida red tide bloom.