Physical controls on episodic nearshore phytoplankton blooms in Southern California

The nearshore and surfzone in Southern California are highly dynamic regions (< 20 m depth), and governed by different processes than water further offshore. As such, these are also unique environments for phytoplankton; affecting the spatial and temporal distributions of chlorophyll a (Chl a, a proxy for phytoplankton biomass) and nutrient delivery. This dissertation explores the challenges of making Chl a measurements in the turbid surfzone, and presents data from a novel sampling platform designed for nearshore and surfzone measurements. The underlying mechanisms driving Chl a variability over hourly, weekly and monthly scales are investigated. The data presented here was collected during a 4-month field experiment at Huntington Beach, CA during Summer and Fall 2006. Chapter one demonstrates the data errors induced by in situ optical fluorometers within the bubbly and sandy surfzone. A method for data correction is developed, and applied to surfzone field observations. Chapter two describes the sudden appearance at the surface of an alongshore-parallel band of a redtide dominated by the regionally common dinoflagellate Lingulodinium polyedrum (F. Stein). The red tide band was mapped in high spatial and temporal resolution using novel instrumentation including a global positioning system (GPS) -tracked jetski. Data from a series of moorings and CTD profiles provide insight into the sequence of events that culminated in the surface red tide appearance. And finally, Chapter three explores the drivers of episodic phytoplankton blooms over the entire four month record. Three pulses of estimated advective and turbulent vertical (NO₃) flux, are found to precede three phytoplankton blooms by approximately 8 days. Chl a predicted from a very simple NP model driven only by the estimated NO₃ fluxes captured the timing, width and approximate magnitude of each of the blooms (r² = 0.49), verifying that the vertical NO₃ flux was a important control on the bloom event