Meteoric water promotes phytoplankton carbon fixation and iron uptake off the eastern tip of the Antarctic Peninsula (eAP)

Abstract Both bioavailable iron (Fe) and water stability have been proposed to regulate primary production in large parts of the Southern Ocean. Freshwater is not only an important factor regulating water stability, but is also an important source of dissolved iron (DFe) for phytoplankton growth. In this study, stable oxygen isotopic composition (δ18O) and salinity were used to distinguish meteoric water (glacial discharge and precipitation) from sea ice meltwater in the northernmost part of the eastern Antarctic Peninsula (eAP). The carbon fixation rate (CFR) and Fe uptake rate (FeUR) were measured via 14C and 55Fe tracer assay, respectively. We observed positive responses of FeURs to the fraction of meteoric water and sea ice meltwater, and a stronger correlation between FeUR and meteoric water, indicating that freshwater input could relieve the Fe limitation in this area, and meteoric water was likely to be the main source of DFe. In addition, we also observed that both meteoric water and sea ice meltwater increased CFR, but CFR only showed slightly positive response to sea ice meltwater. This suggested that the primary effect of increased sea ice meltwater was to enhance the stability of water column, providing a favorable condition for phytoplankton growth. Since the potential meteoric water inventory is much greater than sea ice meltwater and meteoric water input is expected to rise further as climate warming continues as mentioned in Meredith et al’s studies (2008, 2010), the meteoric water is likely increasingly important in Antarctic coastal zones. Smaller phytoplankton responded more obviously to the freshwater input, and we speculate that this would influence on the carbon export in the ambient and nearby oceanic areas. The low Fe demand (Fe:C ratio = 8.04 μmol mol−1) suggested that phytoplankton in the eAP were adapted to a low Fe environment. In addition, primary production estimated from high Fe:C ratios would underestimate the real situation in the southern ocean. These findings help to understand how meteoric water and sea ice meltwater could affect the Antarctic ecosystem in response to climate change.