Short-term Biogeochemical Influence of a Diatom Bloom on the Nutrient and Trace Metal Concentration in South San Francisco Bay Microcosm Experiments.

Two laboratory microcosm experiments were conducted to mimic an annual spring diatom bloom in South Sao Francisco Bay by isolating the phytoplankton community from the benthic grazing pressure to induce a phytoplankton bloom. The purpose of these experiments was to isolate the impact of a spring diatom bloom on the nutrient and trace metal geochemical cycling. Microcosms were created in 2.5 L incubation bottles and subjected to one of 4 treatments (control, copper [Cu] addition, manganese [Mn] additions, and both Cu and Mn addition) to investigate the toxicity of Cu on the resident plankton and the potential antagonistic effects of Mn on reducing Cu toxicity. Dissolved macronutrient (nitrate + nitrite, phosphate. and silicate), and dissolved and particulate trace metal (Cu, Ni, Mn) concentrations were monitored it: the grow-out incubations on a daily basis. Chlorophyll a concentrations were also monitored over the course of the experiment and used to calculate diatom-specific growth rates. In the experiments containing ambient South San Francisco Bay surface waters, average specific growth rates were nit the order of 1.1 d^-1. The induced diatom blooms resulted to significant removal of macronutrients from the microcosms over the course of the experiments. Our research supports previous suggestions that dissolved Ni and Cu concentrations in South San Francisco Bas have a very low biological availability as a result of organic chelation. Ni(EDTA)^2- has been to be the dominant dissolved Ni species by other researchers and Cu speciation analyses from this study and others indicate that >99% of the dissolved Cu in South San Francisco Bay is strongly chelated as CuL₁. The free cupric ion concentration was on the order of 10^-12 M. Marked removal of dissolved Mn was observed in the control treatments, well exceeding expected dissolved Nm removal by diatom uptake. Additions of 375 nM Cu resulted in the complete titration of the chelating ligand (L₁) concentrations. The elevated [Cu²⁺] (≈ 10^-8 M) appeared to have a toxic effect on the diatom community observed in the significant decreases in their specific growth rates (μ = 0.4 d^-1). The suppression of dissolved Nm removal front solution was also observed in treatments spiked with high levels of dissolved Cu, providing support that Mn precipitation was due to biologically mediated oxidation not phytoplankton assimilation. The observed geochemical behavior in the concurrent Cu and Mn addition treatments provide evidence in support of Mn alleviation of Cu toxicity. The biological role in the ambient short-term biogeochemical cycling of Cu and Ni in Smith San Francisco Ray appears to be minimal due to the inert character of the organic ligand-metal complexes. A significant portion of the annual macronutrient and Mn cycling occurs as a result of spring diatom blooms in South San Francisco Bay. [ABSTRACT FROM AUTHOR]