UC Santa Barbara

Coastal ocean ecosystems are invaluable resources through their contributions to the global ocean carbon cycle, local fishery economies, and shoreline protection. The patterns they display are determined by physical forcings which operate on a variety of scales in space and time. Therefore, identifying and quantifying the physical forcings that determine spatiotemporal variability in coastal ocean ecosystems is essential in understanding and predicting their overall health. In this dissertation, I utilize remote sensing and in situ observations alongside model output to investigate physical oceanographic mechanisms and ecosystem response in the Santa Barbara Channel (SBC), California, USA. The SBC is home to very productive pelagic phytoplankton and benthic kelp forest communities. Additionally, the geographic setting, topography, and bathymetry of the SBC complicate physical processes and create a mosaic of conditions which affect the ecosystems therein. The goals of this research were to investigate the dominant forcings of local surface flow patterns, quantify the impact of physical nutrient delivery mechanisms on phytoplankton biomass, and explore phytoplankton and kelp biomass variability on sub-SBC scales. First, long term remote sensing surface current measurements from high-frequency radar (HFR) are utilized to force particle simulations and quantify local surface water retention. Mean retention times, calculated as the amount of time a particle stays within 20km of its starting location, are ~4 days but are spatially variable. Persistent cyclonic eddies drive high retention in the central SBC while weak, oscillatory currents drive high retention in the eastern SBC. Second, HFR surface currents and vertical velocities derived from a fine scale local wind model are combined with nutrient concentration estimates to quantify advective nutrient fluxes. Time series of these fluxes are analyzed alongside other nutrient supply mechanism metrics and remote sensing estimates of phytoplankton biomass to investigate phytoplankton bloom response. Interannual variability in seasonal bloom magnitude and infrequent minor fall blooms are more accurately reflected in the fluxes quantified here than in commonly used nutrient supply metrics, which only capture seasonal variations in regional phytoplankton biomass. Third, the advective nutrient flux metrics and satellite estimates of phytoplankton biomass from chapter 2 are analyzed on sub-SBC scales to investigate spatial coherence in biomass response to physical forcings. Satellite estimates of kelp canopy biomass and a high-resolution wave model are also incorporated to explore differences in coastal ecosystem response. Phytoplankton biomass is found to be more spatially coherent than kelp canopy biomass across regions of the SBC. These spatial differences in kelp canopy biomass correspond to spatial inconsistencies in environmental condition timing and magnitude. Additionally, complexities induced by the offshore Channel Islands significantly complicate physical forcings and biomass patterns. Overall, this research elucidates the physical processes driving observed local biological and ecological patterns and highlights their spatiotemporal variability on a variety of scales.