Did dilution limit the phytoplankton response to iron addition in HNLCLSi sub-Antarctic waters during the SAGE experiment?

An in situ iron addition experiment (SAGE) was carried out in high-nitrate low-chlorophyll low-silicic acid (HNLCLSi) sub-Antarctic surface waters south-east of New Zealand. In contrast to other iron addition experiments, the phytoplankton response was minor, with a doubling of biomass relative to surrounding waters, with the temporal trends in dissolved iron and macronutrients instead dominated by physical factors such as mixing and dilution. The initial increase in patch surface area indicated a lateral dilution rate of 0.125 d−1, with a second estimate from a model of the decline in peak SF6 concentration yielding a higher lateral dilution rate of 0.16–0.25 d−1. The model was tested on the SOIREE SF6 dataset and provided a lateral dilution of 0.07 d−1, consistent with previous published estimates. MODIS ocean colour images showed elevated chlorophyll coincident with the SF6 patch on day 10 and 12, and an elevated chlorophyll filament at the SAGE experiment location 3–4 days after ship departure, which provided additional lateral dilution estimates of 0.19 and 0.128 d−1. Dissolved iron at the patch centre declined by 85% within two days of the initial infusion, of which dilution accounted for 50–65%; it also decreased rapidly after the 2nd and 3rd infusions but remained elevated after the fourth infusion. Despite decreases in nitrate and silicic acid from day 7 and 10, respectively, the final nutrient concentrations in the patch exceeded the initial concentrations due to supply from lateral intrusion and mixed-layer deepening. The low Si:N loss ratio suggested that the observed limited response to iron was primarily by non-siliceous phytoplankton. Algal growth rate exceeded the minimum dilution rate during two periods (days 3–6 and 10–14), and coincided with net chlorophyll accumulation. However, as the ratio of algal growth to dilution was the lowest reported for an iron addition experiment, dilution was clearly a significant factor in the SAGE experiment recording the lowest phytoplankton response to mesoscale iron addition.