Colimitation by light, nitrate, and iron in the Beaufort Sea in late summer

As part of the GEOTRACES International Polar Year program, in September 2009 we performed a grow‐out experiment to investigate the potential for nitrate, light, and iron (Fe) limitation of Beaufort Sea phytoplankton. Nutrients, phytoplankton biomass/composition, and Fe and C uptake rates were measured regularly. Nitrate additions enhanced phytoplankton growth, demonstrating that the late summer phytoplankton community was N limited. In the treatments with additions of nitrate but not Fe, total chlorophyll a doubled with each increase in light level, indicating the community was also light limited. In nitrate enriched treatments, colimitation of primary production by Fe and light was observed at light levels ≤10% surface irradiance (Io), which corresponded to depths ≥33 m. The higher Fe demand associated with low irradiance was supported by 10‐ to 100‐fold higher Fe:C assimilation ratios. Furthermore, the Fe:C assimilation ratios of phytoplankton with an Fe addition (1 nmol L−1) were ∼2‐fold higher than those with no Fe added at 1%Io, indicating severe Fe deficiency at in situ Fe (0.154 nmol L−1) and low light. The average light intensity experienced by phytoplankton in the 1%Io treatments corresponded to the light intensity at ∼73 m, which is well below the 44 m euphotic depth at this station in late summer, but coincides with the depth where the subsurface chlorophyll maximum is often found in this region (∼70 m). These results suggest that in addition to nitrate and light, Fe availability may control primary productivity in the Beaufort Sea, and seasonal changes in light, nitrate, and Fe availability may differentially control Arctic phytoplankton growth. Key Points NO3‐, light and Fe co‐limitation was found in the Beaufort Sea in Aug/Sept 2009 Seasonality of NO3‐, light, and Fe may control Beaufort Sea primary productivity Global warming may lead to increased Fe and NO3‐ limitation in the Beaufort Sea