Estimating Phytoplankton Food Quality Using a Hyperspectral Phytoplankton Functional Type Model for San Francisco Bay, California

The link between phytoplankton and energy transfer to higher trophic levels has been studied for more than 75 years. Light, nutrients, and food chain length strongly influence apex predators, fisheries, and overall environmental functioning. The community of practice routinely uses phytoplankton composition as an index of ecosystem function and health. For example, in San Francisco Bay, a shift from diatoms to cyanobacteria has been linked to a dramatic collapse of the food web. New methods and analyses have provided a rigorous quantitative basis for linking Phytoplankton Functional Types (PFTs) to carbon and energy transfer within aquatic systems, as the Phytoplankton Food Quality Index (FQI). In basic terms, FQI tracks long-chain essential fatty acids (LCEFA) available to consumers. There is strong and growing interest in using FQI to assess human and aquatic health, food security, and climate-related trends in the world’s oceans and coastal regions. FQI is robust across the freshwater to marine gradient, and not strongly influenced by short-term environmental fluctuations. FQI is instead influenced by phytoplankton community composition (i.e. PFTs). We demonstrate a proof-of-concept ability to measure hyperspectral remote-sensing reflectance (Rrs), convert that to PFTs using the PHYDOTax model, and then use the PFTs as input to FQI in San Francisco Bay, California. Requirements for PHYDOTax are that Rrs spectra must be available between 455-750 nm at 10 nm intervals. Advantages are that it was specifically designed to work in California coastal waters, and includes six PFTs, making it compatible with FQI. Results are statistically comparable to HPLC pigment methods and microscopy when using AVIRIS-Classic overflights and above-water radiometry from shipboard transects in San Francisco Bay. We show that FQI tracks climate-scale changes in phytoplankton composition in the estuary in response to the recent massive drought in California, but contrary to expectations, shifts in FQI were less related to an increase in cyanobacteria and more related to subtle changes in the rest of the community composition. Our results pave the way to routinely estimating PFTs and FQI in the coastal ocean when next-generation NASA satellites such as PACE and the Surface Biology and Geology sensor become available.