Fe and Zn effects on the Si cycle and diatom community structure in two contrasting high and low-silicate HNLC areas

We compared the importance of Fe, Zn and Si availability for diatom growth and silicification through microcosm enrichment experiments in two contrasting HNLC systems of the Sub-Arctic and Sub-Antarctic Pacific. The Bering Sea was characterized by low Fe and Zn concentrations (<0.2 nM) but relatively high silicic acid (5.6–15.9 μM). The addition of 0.25 nM Fe induced a 2–7-fold increase in diatom cell abundance (Pseudo-nitzschia sp. and Cylindrotheca closterium), an increase in Chl a, biogenic silica, and particulate organic carbon and nitrogen, and a 2–3-fold decrease in the average cellular Si content. Zn had no impact on biomass parameters or diatom community structure in this region. b-Antarctic Zone (SAZ) was a low Si-HNLC system, with initial silicic acid levels of 0.45 μM and Fe and Zn concentrations <0.03 nM. Si was the proximate limiting factor controlling diatom growth, followed by a secondary role for Fe on non-siliceous phytoplankton. In this region, we also found evidence for Zn-mediated changes in diatom community structure. The presence of Zn (+1 nM) shifted the community away from a large colonial pennate (Pseudo-nitzschia sp.) towards a smaller and less silicified solitary pennate (Cylindrotheca closterium), potentially prone to more rapid silica dissolution in the surface layer. e the dominance by the same two diatom genera, these two high-latitude regimes exhibited different nutrient limitation scenarios. Diatom growth in the Bering Sea was strongly Fe-limited, while the SAZ was mainly limited by Si and only secondarily by Fe.