In the marine environment, the bioavailability of phosphorus (P) can impact ocean fertility as well as the structure and distribution of phytoplankton communities. Particulate P is of special interest because it is the major mechanism for the transport of P to deep waters and is a source of P to nutrient-depleted surface waters through upwelling and mixing. Changes in nutrient conditions (magnitude and nutrient ratios) can increase the potential for eutrophication and in some cases, facilitate the growth of harmful algal blooms (HABs). One such HAB is the diatom genus Pseudo-nitzschia, which can produce the neurotoxin domoic acid (DA) and has been responsible for numerous marine mammal and bird mortalities on the West Coast of the U.S. The goal of this dissertation is to examine particulate P cycling, Pseudo-nitzschia blooms, and DA export using a 13-year record (August 1993 - August 2006) of sinking particles continuously collected at ~500 m depth by a sediment trap deployed in Santa Barbara Basin (SBB) off the coast of southern California. Sampling included analysis of overlying solutions within sediment trap cups and therefore a detailed examination of possible diagenetic artifacts associated with prolonged sediment trap collection periods.

Total particulate P (TPP), particulate inorganic P (PIP), and particulate organic P (POP) in sinking particles were quantified utilizing a 5-step sequential extraction method. PIP was further divided into four different fractions - loosely-bound, iron-bound, authigenic, and detrital P. Seasonal trends revealed that the relative percentage of POP concentrations increased during upwelling, likely due to increased biological production. However the transport of POP to depth was linked to the availability of both marine (opal) and terrestrially (carbonate) derived mineral fluxes. High particulate organic carbon (POC) to POP ratios suggested rapid and efficient remineralization of POP relative to POC as particles sank through the water column, indicating that this phase is the most likely to contribute to marine production in surface waters via coastal upwelling. As a whole, particulate P fluxes in SBB were dominated by PIP phases and are substantially higher than those measured in sediment traps deployed in other coastal settings. Correlations between the various components of the PIP pool and lithogenic fluxes suggested that a major proportion of the PIP measured in the sediment traps was derived from terrigenous material, likely entering into the basin via river discharge and shelf disturbances.

Measurement of domoic acid concentrations (by liquid chromatography - mass spectroscopy) and fluxes within the SBB sediment trap found concentrations in excess of five times the United States federal shellfish closure limit. Until this research, studies of DA have largely focused on the transfer of toxin in the upper water column food webs, as previous laboratory studies suggested that DA transport to depth was negligible due to photodegradation of the toxin in surface waters. However, sediment trap measurements from this study showed that DA-enriched sinking particles, comprised of phytoplankton aggregate and fecal pellets, rapidly sink (within 3-5 days) to ~ 500 m following toxic Pseudo-nitzschia surface blooms. The time-series SBB sediment trap samples further revealed annual toxin events occurring in the Santa Barbara Basin as early as 1994 (8 years before coastal monitoring in the Basin began), with five high flux events from 1997 to 2007. While in some instances, sediment trap toxin events from the center of SBB coincided with nearshore domoic acid events identified by shellfish monitoring, several events were associated with physical features within central SBB associated with mesoscale eddies. Thus, coastal monitoring may not be sufficient for understanding DA and Pseudo-nitzschia bloom dynamics offshore and their potential effect on benthic biota harvested for human consumption.