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Abstract: The metabolic activity and growth of phytoplankton taxa drives their ecological function and contribution to biogeochemical processes. We present the first quantitative, taxon‐resolved silica production rates, growth rates, and silica content estimates for co‐occurring diatoms along two cross‐shelf transects off the California coast using the fluorescent tracer 2‐(4‐pyridyl)‐5‐((4‐(2‐dimethylaminoethylaminocarbamoyl)methoxy)phenyl)oxazole, and confocal microscopy. Taxon contribution to total diatom community silica production was predominantly a function of the surface area (SA) of new frustule that each taxon created as opposed to cell abundance or frustule thickness. The influential role of SA made large diatoms disproportionately important to community silica production over short time scales (< 1 d). In some cases, large taxa that comprised only ∼ 15% of numerical cell abundance accounted for over 50% of total community silica production. Over longer time scales relevant to bloom dynamics, the importance of SA declines and growth rate becomes the dominant influence on contribution to production. The relative importance of SA and growth rate in relation to silica production was modeled as the time needed for a smaller, faster‐growing taxon to create more SA than a larger, slower‐growing taxon. Differences in growth rate between the taxa effected the model outcome more than differences in SA. Shifts in relative silica production among taxa are time restricted by finite resources that limit the duration of a bloom. These patterns offer clues as to how taxa respond to their environment and the consequences for both species succession and the potential diatom contribution to elemental cycling. [ABSTRACT FROM AUTHOR]

Abstract: A 3‐day shipboard incubation experiment was conducted in the California Current System in July 2014 to investigate the cycling of iron (Fe), copper (Cu), nickel (Ni), cadmium (Cd), manganese (Mn), cobalt (Co), lead (Pb), and scandium (Sc) under a range of light and particle conditions. Filtered (< 0.2 μm) and unfiltered treatments were incubated under the following light conditions: Dark, light (“UV”), and light without the ultraviolet (UV) wavelengths (“noUV”). The experiment was sampled for carbon and Fe uptake rates, dissolved trace metal concentrations (Fe, Cu, Ni, Cd, Mn, Co, Pb, Sc), Fe and Cu speciation, size‐fractionated concentrations of Cd and Fe, and diatom community composition from DNA sequencing. Exposure to UV light increased phytoplankton Fe uptake in the first 24 h of the incubation relative to the noUV treatment, suggesting that a fraction of the ambient ligand‐bound Fe was photoreactive. Fe‐binding organic ligand production was observed in the unfiltered light treatments in association with increasing chlorophyll a, and evidence for Cu‐binding ligand production in these treatments was also observed. Biological uptake of Cd and Co was observed along with scavenging of dissolved Pb. Manganese appeared to be rapidly oxidized by Mn‐oxidizing bacteria with concomitant drawdown of dissolved Ni. Scandium displayed similar trends to Fe, reinforcing the limited observations of the physicochemical similarities between these two elements in seawater. Overall, this study highlights distinct impacts of photochemical processes, scavenging, and biological effects on marine trace metal cycling in an environment characterized by seasonal upwelling. [ABSTRACT FROM AUTHOR]