Abstract
The Ross Sea supports massive phytoplankton blooms and exhibits the highest marine primary productivity rates due to seasonal relief of iron and light limitations. Although Earth System Models project regional increases in temperature and iron supply, there is uncertainty regarding the response of different phytoplankton communities. To assess this, minicosm experiments of 7–8 days duration were conducted in the Ross Sea during austral summer 2021 on natural phytoplankton communities at three locations across a gradient of trophic conditions from off‐shelf low‐productivity waters to bloom conditions on the shelf. Four experimental treatments simulated different scenarios based on model projections: (a) control/in situ conditions, (b) elevated temperature (+2°C), (c) +2°C and low‐Fe addition (0.25 nmol L−1), and (d) +2°C and high‐Fe addition (0.5 nmol L−1), with the response of phytoplankton biomass (Chlorophyll‐a, particulate organic carbon, particulate organic nitrogen, and bSi), elemental stoichiometry (POC:PON, POC:Chla, and POC:bSi), and 18S rRNA gene‐based taxonomic composition determined. +2°C produced no significant response in any parameter at any location; however, iron + temperature increased Chlorophyll‐a three‐times more strongly on the continental shelf than off‐shelf, driven by increases in nano‐ and microphytoplankton. Changes in taxonomic community composition were observed in specific phytoplankton strains in iron + temperature treatments, with the diatoms Fragilariopsis sp. and Pseudo‐nitzschia sp. the most responsive taxa on the continental shelf, and low‐abundance diatoms responding in the off‐shelf. Species‐level taxonomic analysis revealed regional variability in phytoplankton community response to projected increases in iron and temperature which, together with variable biomass increases, has implications for the future trophic state and biogeochemical cycling in the Ross Sea region.