Bio-optics

Understanding primary production in the Gulf of Alaska is of vital importance to understanding climate change and fisheries production. However, directly measuring phytoplankton biomass and growth rates is extremely laborious, thus measurements during a multi-week regional survey are often limited to a handful of key stations. In order to understand the patterns and controls of phytoplankton growth across an entire region (and one that is undergoing rapid change), we need to be able to measure the phytoplankton community more efficiently. The good news is the ocean is blue (i.e. the water absorbs more green and red light while reflecting more blue light), but phytoplankton are green (i.e. they absorb more blue and red light while reflecting more green light), and this means that by measuring the color of the surface ocean, we can estimate, with pretty remarkable accuracy, the amount of phytoplankton present. What’s more, light measurements can be made extremely fast, so by utilizing ‘ocean color’ we can drastically increase our ship-board sampling capacity! This approach is called bio-optics, and we have built an optical system for use on ships that measures various light characteristics of the surface ocean almost entirely on its own (we call this an autonomous platform). When the ship is steaming normally (around 8-10 knots), we end up with a measurement every 200 meters, which is a drastic improvement in resolution compared to direct measurement techniques (~50km) or even those made by satellites (~1-4km).

The Underway Optics System (May 2021) instaled on the R/V Sikuliaq

The first step in this research is working with the Northern Gulf of Alaska Long Term Ecological Research program to calibrate our light measurements to the direct measurements in the Gulf of Alaska. This needs to be done in tandem with a rigorous analysis of accuracy compared to the classical methods.

Figure 1: Preliminary data from the summer 2020 research cruise. Black dots are the optical measurements and red circles are the classical discreet samples.

The next steps are to make use of this data. With this information, we can begin comparing estimates made by NASA satellites to our optical measurements, correlating primary production with physical and chemical properties, and examining how frontal systems in the GoA affect primary production. All of this and much more are all on the docket for future research.