A Spatio-Temporal Context for the Phytoplankton Community Patterns of the Galapagos Archipelago and the Northwest Florida Shel

Phytoplankton play a pivotal role in the global carbon cycle and structuring of marine ecosystems. With the shifts in taxonomic composition susceptible to climatic variations and the increasing detrimental effects of harmful algal blooms (HABs), the identification of phytoplankton community distributions is important. This dissertation addressed the physical and chemical forcing of phytoplankton dynamics in the eastern equatorial Pacific and characterized an offshore, near-bottom dinoflagellate niche in the Northwest Gulf of Mexico.;In Chapters 2 and 3, seasonal and ENSO related interannual variability of chlorophyll a concentrations and phytoplankton community distributions were examined in the Galapagos Archipelago using HPLC, CHEMTAX, and FlowCAM analyses. Spatiotemporal patterns in abundance and community distribution varied depending on the seasonal and ENSO cycle. Maximum chlorophyll a concentrations occurred in the equatorial upwelling zone west of Isabela during normal conditions and decreased ten-fold during the 2006/07 El Nino. Diatoms and haptophytes contributed the most to chlorophyll a during normal ENSO conditions. The most significant seasonal variability was the 10-fold increase in diatoms, specifically Corethron criophilum, at the southern end of the Canal de Bolivar. Cold, nutrient-rich, upwelled waters to the west of Isabella were composed mostly of Corethron criophilum during all surveys. However, abundance and composition varied within the Canal de Bolivar. The northern region consisted of a diverse group of mostly dinoflagellates and some diatoms, while the southern region was mainly comprised of diatoms. Typical for El Nino conditions, chlorophyll a was at a minimum throughout the Archipelago beginning in July 2006 and continued until the beginning of 2007. During the 2006/07 El Nino, phytoplankton community distributions were similar throughout the Archipelago and haptophytes and cyanobacteria were the dominant groups relative to chlorophyll a. Chlorophyll a concentration and phytoplankton abundance in the upwelling region decreased dramatically during the Nino-Nov06 survey and Pseudonitzchia sp. was the dominant species north and west of Isabella. In early 2007, a chlorophyll a plume occurred and extended beyond 95°W. In May 2007, the Nina-May07 survey captured a decrease in phytoplankton biomass prior to the 2007/2008 La Nina when distributions varied spatially, and diatoms and pelagophytes were the dominant groups relative to chlorophyll a found in the upwelling zone.;In Chapter 4, we characterized the hydrography of the Northwest Florida shelf, the concentration and composition of phytoplankton communities across the shelf, and near-bottom chlorophyll maximum areas possibly related to HAB initiation. Summer light and nutrient conditions along the Northwest Florida shelf provide a "Benthic Dinoflagellate Migration" niche for algal populations, such as Karenia brevis, that may serve as seed populations for surface blooms under upwelling-favorable conditions. Identifying the phytoplankton community in the context of cross-shelf hydrography is critical to understanding the competitive interactions and onshore transport. A pycnocline existed between 10--20 m depth, the 1% light level penetrated to ∼45 m depth, and nitrate-nitrite concentrations started increasing about 10 m above the sediment interface out to 50-m depth and then increased below 40-m depth to the 65 m isobath. A chlorophyll a maximum occurred near-bottom between 20--30 m. Gyroxanthin-containing dinoflagellates that include Karenia brevis occurred across the shelf in near-surface and near-bottom waters. Nearsurface cells were found in low concentrations and near-bottom dinoflagellates were most abundant between the 20--30 m isobath where the nitrate-nitrite concentration was 1--4 muM. Below the euphotic zone, cells were present near-bottom to the 60 m isobath where light was limited (<1% PAR) but nitrate-nitrite was highest (up to 6 muM). A pattern consistent with diel vertical migration of the dinoflagellates between the sediment at night and the lower euphotic zone during the day was observed at the 50-m isobath time-series station.