Trace metals in the central California Current upwelling system
The objective of this dissertation was to develop and utilize a multi-element analysis method to determine sources and distributions for a suite of trace metals in the central California Current upwelling regime (cCCS). Chapter 1 involved the development of this multi-element method and its use in the U.S. GEOTRACES inter-calibration efforts. Chapters 2 and 3 investigated sources of Fe and its importance as a bottom up control on phytoplankton growth of the cCCS. Coastal surface Fe concentrations were related to continental shelf width and upwelling strength, and benthic boundary layer Fe concentrations were high in regions with a wide shelf and/or very low oxygen concentrations. Several regions with narrow continental shelves (Big Sur Coast and Pt. Arena to Cape Mendocino) demonstrated evidence for Fe limitation of diatom blooms (Chapter 2). The dominant Fe source to the offshore transition zone (TZ) is from the transport of coastally upwelled waters offshore via filaments, though Fe is rapidly drawn down as these waters move offshore. The TZ thus exhibits residual NO3- concentrations (5-15 μg L-1), very low Fe (<0.2 nmol kg-1), and relatively low and constant chlorophyll concentrations (1-2 μg L-1). Additional Fe delivery via offshore wind curl induced upwelling and/or vertical mixing is not sufficient to accompany NO3- delivered to the surface. Thus, the TZ is a broad region of the cCCS exhibiting evidence for Fe limiting conditions (Chapter 3). Chapter 4 presented seasonal sources and distributions for a suite of trace metals (Mn, Fe, Co, Ni, Cu, Zn, and Cd) relative to macronutrients in the cCCS. Upwelling sources of Ni, Zn, and Cd were from the ocean interior and internal biogeochemical cycling. Conversely, Mn, Fe, Co, and Cu had an external source to upwelling waters in the continental shelf sediments. There is an increased upwelling source of Co and Mn later in the summer as shelf sediments become highly reducing. Surface Fe, Zn, Cd, and Co showed evidence for preferential drawdown relative to NO3- indicating a changing metal to carbon assimilation ratio in the environment.