Abstract
The Western Antarctic Peninsula (WAP) is a rapidly changing region with receding sea-ice cover. This region generally has increased phytoplankton productivity on the continental slope with a decrease observed in off-shelf waters located further northward. This study aims to improve the understanding of the distribution of iron (Fe) in this climatically important oceanic region during early sea-ice retreat, as well as the impact of receding ice cover on Fe concentrations and the importance of Fe and its sources at the beginning of the phytoplankton growth season. Five ocean transects were sampled along the Palmer Long Term Ecological Research (Pal-LTER) grid with an additional oceanward extension to access off-shelf waters during the austral spring (October and November) of 2014. High Fe inputs into the entire water column from ice melt as well as from sediments increased dissolved Fe (DFe) and total dissolvable Fe (TDFe) concentrations across the shelf (off-shelf DFe: 0.38 ± 0.30 nmol/L and TDFe: 2.23 ± 2.95 nmol/L versus shelf DFe: 1.54 ± 1.38 nmol/L and TDFe: 19.47 ± 23.82 nmol/L). The combination of meteoric meltwater and shallow sedimentary sources is strongest over the shelf and increased landward towards the WAP. Additionally, a winter sea-ice formation signature was detected in inner shelf waters that appeared to contribute to DFe concentrations. Relatively warm Circumpolar Deep Water (CDW) that flows onto the shelf through troughs is likely modified by non-reductive sedimentary input of Fe into the water column. The increase in Fe concentrations in WAP waters in early spring could trigger enhanced phytoplankton productivity across the shelf, although the highest productivity levels were observed in off-shelf waters, likely related to improved light conditions following receding sea-ice cover. The relatively high productivity levels in WAP off-shelf waters are presumably caused by two factors: (1) supply from below, and (2) the transport of Fe from the shelf to the off-shelf region. However, 80–90% of Fe introduced into shelf waters was removed from the upper water column prior to reaching off-shelf waters, reducing the influence of the coastal Fe source. Low concentrations of DFe (0.24 ± 0.26 nmol/L) in off-shelf waters (upper 100 m) of the WAP coincide with relatively elevated chlorophyll a concentrations (0.66 ± 0.56 μg/L), implying that uptake of available DFe had already occurred prior to sampling. Our results imply that the horizontal supply of DFe from the shelf as well as total DFe supply into the surface mixed layer might not be sufficiently high to support productivity in the off-shelf waters of the WAP, underlining the development of an Fe deficit early in the growth season.
•Major sources of Fe in spring to WAP waters are meteoric meltwater, sea-ice formation and non-reductive sedimentary input.•The majority of Fe input is constrained to the shelf region.•Estimated Fe supply to the open ocean is barely sufficient to support observed NPP.•A deficit of Fe develops early in the growth season.