Controls on dissolved and particulate iron distributions in surface waters of the Western Antarctic Peninsula shelf

Abstract

The Western Antarctic Peninsula (WAP) displays high but variable productivity and is also undergoing rapid change. Long-term studies of phytoplankton communities and primary production have suggested transient limitation by the micronutrient iron (Fe), but to date no data have been available to test this hypothesis. Here, we present the first spatially extensive, multi-year measurements of dissolved and particulate trace metals in surface waters to investigate the key sources and sinks of Fe in the central WAP shelf. Surface samples of dissolved and particulate metals were collected throughout the 700 x 200 km grid of the Palmer Long-Term Ecological Research program in three consecutive austral summers (2010-2012). Iron concentrations varied widely. Both dissolved and particulate Fe were high in coastal waters (up to 8 nmol kg-1 and 42 nmol kg-1 , respectively). In contrast, very low Fe concentrations (< 0.1 nmol kg-1) were widespread in mid-to outer-shelf surface waters, especially in the northern half of the sampling grid, suggesting possible Fe limitation of primary production on the shelf. Sea ice and dust inputs of Fe were minor, although their relative importance increased with distance from shore due to the larger near-shore sources. Sedimentary inputs were inferred from manganese distributions; these were more significant in the northern portion of the grid, and showed interannual variation in intensity. Overall, the interannual distribution of Fe was most closely correlated to that of meteoric water (glacial melt and precipitation). Although the Fe concentrations and relative contributions of dissolved and particulate Fe attributed to meltwater were variable throughout the sampling region, increasing glacial meltwater flux can be expected to increase the delivery of Fe to surface waters of the coastal WAP in the future.

Light Is the Primary Driver of Early Season Phytoplankton Production Along the Western Antarctic Peninsula

“…Although many stations had dFe concentrations that hovered around the growth‐limiting level of 0.1 nM (Sedwick et al, ), we observed no evidence of dFe stress in phytoplankton (Arrigo et al, ). Early‐season iron sources in the WAP include sedimentary input from both deep wintertime mixing throughout the water column (Annett et al, ) and horizontal transport from coastlines (Sherrell et al, ), upwelling of Circumpolar Deep Water onto the WAP shelf (Dinniman et al, ; Measures et al, ), sea ice melt (Annett et al, ; Wang et al, ), glacial melt (Annett et al, , ), and precipitation (Annett et al, ). As the season progresses, sea ice melt and warming stratify the water column (Smith et al, ), lifting light limitation but making it more difficult to mix dFe into surface waters.…”

“…As the summer approaches and temperatures increase, ice melt, solar heating, and reduced winds shoal the ML and phytoplankton experience high, and sometimes excessive, light (Alderkamp et al, ). Concurrently, coastal meteoric (glacial meltwater and precipitation) and offshore sea ice melt inputs provide a spatially heterogeneous dFe supply that can continue to fuel some phytoplankton growth (Annett et al, , ). Surface ML light is expected to change in the future as upper ocean circulation, stratification, and sea ice cover continue to undergo rapid and dramatic changes (Martinson et al, ; Meredith et al, ; Moffat & Meredith, ; Schofield et al, ; Stammerjohn et al, ).…”

“…In the long‐term future, the combination of these physical changes will likely lead to a deepening of the ML as sea ice is lost and winds increase, resulting in more variable and overall lower light in the ML (Carvalho et al, ; Meredith et al, ; Schofield et al, ; Turner et al, ; Vernet et al, ). Although dFe inputs are likely to increase with warming, a deeper ML would dilute available dFe (Annett et al, ). Overall, the net change in dFe concentration depends on a number of unconstrained factors, including wind mixing, mixed layer depth (MLD), sea ice and glacial melt, sedimentary inputs, and biological demand (Annett et al, ; Sherrell et al, ).…”

Testing the Canyon Hypothesis: Evaluating light and nutrient controls of phytoplankton growth in penguin foraging hotspots along the West Antarctic Peninsula

“…Two different treatments were set: one surface (S) with “surface” water only, and one mix (M) where equal volumes of “surface” and “deep” water were mixed, totaling equivalent 1.0 L volumes in each treatment. Given that no phytoplankton were present in the “deep” water, half of the “surface” water in the S treatments was filtered through an acid‐cleaned, 0.2 μ m filter (Acropak‐200, Pall) inside of a self‐fabricated clean room bubble (Annett et al ) so that all treatments started with the same phytoplankton biomass. For each treatment, three replicates were spiked with 100 μ L of a solution of 17.9 μ mol L −1 Fe 3+ made from an FeCl 3 stock standard diluted into 0.24 mol L −1 ultrapure hydrochloric acid to give a final Fe addition of 1.8 nmol L −1 (“+” treatments, with no significant effects on solution pH), and three bottles were left unaltered as controls.…”

“…Seawater samples for iron analysis were collected using the TMC rosette composed of 12 Teflon‐coated Niskin‐X bottles mounted on a polyurethane‐coated aluminum frame that was free of metal anodes and included epoxy encased lead weights (Annett et al ; Sherrell et al ). The rosette was deployed using a plastic‐coated aramid conducting cable through an anodized aluminum sheave.…”