Bofu Zheng & Ryan Saenger
Title: Nitrate deficit: a biogeochemical footprint of the 2020 red tide in the Southern California Bight
Abstract: Nitrate, as the most commonly available nitrogen source for phytoplankton in the general oceanic ecosystems, can be modulated by both physical processes and biological activities. During a red tide, much higher-than-normal nitrate demand is required to support the elevated phytoplankton biomass in a short amount of time, providing an excellent opportunity to study how the nitrate field responds. In this work, we reveal a subsurface nitrate deficit region featuring the biogeochemical footprint of the 2020 red tide in the Southern California Bight. With high-resolution physical, chemical (Nitrate), biological, and optical data collected from novel sensors onboard the Wirewalker profiler, and 70 years of climatological records from the CalCOFI program, we estimate that the nitrate deficit region is around 20 m deep with a nitrate loss of ~15 uM. Furthermore, Chl-a data exhibit coherent vertical paths from the surface down to the nitracline, suggesting downward migrations of L. polyedra (the dominant species) with a migration speed of around 400 um/s. Using empirical values (such as the Redfield ratio and Carbon:Chla ratio), depth-integrated nitrate loss is converted into biomass, which is nearly equivalent to observed Chl-a concentration, suggesting biological assimilation of nitrate. The total carbon mass of this bloom is estimated to be 7,000,000 kg, with the nitrate deficit data and bulk area of the bloom, providing a new constraint of the productivity of the red tide, other than satellite images.
Bio: Bofu is a 5th year PhD candidate working with Prof. Drew Lucas in the Multiscale Ocean Dynamics (MOD) group. Contact: boz080@ucsd.edu
Title: Acoustic localization of breaking waves
Abstract: During a recent experiment off the coast of southern California, a drifting hydrophone array listened to large wave breaking events from below while an airplane-mounted camera watched them from above. The aerial images of the sea surface directly above the hydrophone array made it possible to test the array's ability to resolve individual breakers. This talk will discuss what the hydrophone array "sees" at the surface from over 100 meters deep and how that compares to the images.
Bio: Ryan is a 3rd-year student in the Applied Ocean Sciences program at SIO, advised by Bill Hodgkiss and working with Luc Lenain. Contact: rsaenger@ucsd.edu