Physical Processes and their Influence on Zooplankton Composition and Foraging Opportunities for Seabirds in a Strong, Persistent Upwelling Region: Gulf of the Farallones-Cordell Bank (California

I present three studies conducted within shelf waters downstream of a persistent upwelling region. In these studies, I analyzed small-scale oceanographic features, zooplankton composition, and seabird dynamics. These studies help elucidate the physical and biological oceanography occurring in persistent upwelling zones.;In Chapter 1, I identified and analyzed distinct small-scale ocean front features, thus addressing the characteristics, location, and frequency of occurrence of surface fronts in a strong, persistent upwelling area. I explored this through in situ field observations in the GOF-CB region primarily using ship underway data that can resolve small-scale structures. A variety of surface fronts were found in the GOF-CB region from 2004 to 2010. Many of these fronts had strong density gradients, causing frontogenesis as well as the accumulation of plankton and buoyant materials. The abundance of fronts, however, varied significantly between seasons and years. An increase in front abundance was observed during the upwelling season and in 2005, in comparison to the latter years of this study, particularly 2008. Additionally, in contrast to studies conducted in intermittent upwelling regions, I failed to locate any fronts in this region that are classic upwelling fronts. This study demonstrates that fronts in the GOF-CB region are typically short-lived (time scales shorter than synoptic variability, e.g. upwelling-relaxation cycles) and spatially limited. In general, these small-scale fronts will only be important ecologically if planktonic taxa, and subsequent predators can take advantage of accumulation on these short time scales.;In Chapter 2, I examined a zooplankton time series collected within a strong, persistent upwelling region. I analyzed variability in zooplankton abundance and species composition with environmental conditions during the upwelling seasons (April to July) within the GOF-CB region from 2004 to 2009. I found zooplankton abundance and species composition differed significantly, particularly between the first three years (2004 to 2006) and the latter three years (2007 to 2009) of the study. From 2004 to 2006, corresponding environmental measurements showed lower alongshore wind stress, decreased upwelling, minimal alongshore flow, and warmer ocean temperatures throughout the region, whereas 2007 to 2009 demonstrated the opposite patterns at all temporal and spatial scales of environmental measurements. Zooplankton abundance and species composition correlated with several of these environmental variables, including surface flow past Point Reyes and CTD salinity. This signifies a close linkage between this zooplankton time series and basin-wide, regional, and in situ environmental forcing within this strong, persistent upwelling region. Understanding zooplankton abundance and species composition in the GOF-CB region will elucidate population dynamics of upper trophic levels and may assist with future year-to-year predictions of general ecosystem health.;In Chapter 3, I investigated the influence of an offshore bank, Cordell Bank (CB), and surrounding hydrographic measurements within this persistent coastal upwelling region on the abundance of two seabird taxa: Cassin's auklet, Ptychorhamphus aleuticus, and dark shearwater (sooty, Puffinus griseus and pink-footed, Puffinus creatopus). I analyzed spatial patterns of the abundance for these two taxa using negative binomial regression models. While both taxa were associated with CB, auklets and shearwaters were found to forage within different sub-regions over and surrounding CB. Abundance of Cassin's auklet was significantly higher in waters offshore and over CB, corresponding with a maximum abundance in surface salinities around 33.30. In contrast, dark shearwater abundance was significantly greater in both lower (33.5) salinity waters. Higher shearwater abundance was also found in regions with elevated surface temperature gradients, possibly denoting fronts or transition zones between water masses. Understanding seabird dynamics near CB yields insight to the influence banks may have on top predator foraging in strong, persistent upwelling regions. (Abstract shortened by UMI.).