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. The Sub-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. Despite 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.