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We investigated the effects of copper (Cu) and iron (Fe) availability on the growth rates, cellular Cu content, and steady-state Cu uptake rates of eight species of centric diatoms (coastal and oceanic strains). Whereas Fe and Cu availability had a significant effect on the growth rates of both costal and oceanic diatoms, an interaction between Fe and Cu availability and growth rates was only observed for the oceanic diatoms. Determination of cellular Cu : carbon (C) quotas using the radiotracers 67 Cu and 14 C revealed that under Cu-sufficient conditions oceanic diatoms had elevated Cu : C ratios relative to coastal strains, regardless of Fe availability. Two species (one oceanic and one coastal) significantly increased their Cu demands in response to Fe limitation, indicating upregulation of the Cu-dependent high-affinity Fe uptake system in these organisms. The changes in cellular Cu : C ratios were accompanied by variations in steady-state Cu uptake rates. Thus, in some cases Cu uptake rates appear to be regulated by the cell in response to Fe availability. Rates of Cu acquisition also responded significantly to Cu variability. The variation in Cu uptake was more closely correlated with changes in total Cu concentration in the medium than in inorganic, free Cu concentrations, implying that organic Cu complexes may be bioavailable to diatoms. These findings indicate a greater biological role for Cu than was previously thought in open ocean regions.

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.

No abstract

Palmer Deep (PD) is one of several regional hotspots of biological productivity along the inner shelf of the West Antarctic Peninsula. The proximity of hotspots to shelf-crossing deep troughs has led to the 'canyon hypothesis', which proposes that circumpolar deep water flowing shoreward along the canyons is upwelled on the inner shelf, carrying nutrients including iron (Fe) to surface waters, maintaining phytoplankton blooms. We present here full-depth profiles of dissolved and particulate Fe and manganese (Mn) from eight stations around PD, sampled in January and early February of 2015 and 2016, allowing the first detailed evaluation of Fe sources to the area's euphotic zone. We show that upwelling of deep water does not control Fe flux to the surface; instead, shallow sediment-sourced Fe inputs are transported horizontally from surrounding coastlines, creating strong vertical gradients of dissolved Fe within the upper 100 m that supply this limiting nutrient to the local ecosystem. The supply of bioavailable Fe is, therefore, not significantly related to the canyon transport of deep water. Near shore time-series samples reveal that local glacial meltwater appears to be an important Mn source but, surprisingly, is not a large direct Fe input to this biological hotspot.This article is part of the theme issue 'The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change'.

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Abstract.A high resolution time-series analysis of stable carbon isotopic signatures in particulate organic carbon (δ 13 C POC ) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting δ 13 C POC in the coastal western Antarctic Peninsula sea ice environment. The study covers two austral summer seasons in Ryder Bay, northern Marguerite Bay between 2004 and 2006. A shift in diatom species composition during the 2005/06 summer bloom to near-complete biomass dominance of Proboscia inermis is strongly correlated with a large ∼10 ‰ negative isotopic shift in δ 13 C POC that cannot be explained by a concurrent change in concentration or isotopic signature of CO 2 . We hypothesise that the δ 13 C POC shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms (CCMs) in different diatom species. Specifically, very low δ 13 C POC in P. inermis may be caused by the lack of a CCM, whilst some diatom species abundant at times of higher δ 13 C POC may employ CCMs. These short-lived yet pronounced negative δ 13 C POC excursions drive a 4 ‰ decrease in the seasonal average δ 13 C POC signal, which is transferred to sediment traps and core-top sediments and consequently has the potential for preservation in the sedimentary record. This 4 ‰ difference between seasons of contrasting sea ice conditions and upper water column stratification matches the full amplitude of glacial-interglacial Southern Ocean δ 13 C POC variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary δ 13 C POC . We also find significantly higher δ 13 C POC in sea ice than surface waters, consistent with autotrophic carbon fixation in a semi-closed environment and possible contributions from post-production degradation, biological utilisation of HCO − 3 and production of exopolymeric substances. This study demonstrates the importance of surface water diatom speciation effects and isotopically heavy sea ice-derived material for δ 13 C POC in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based δ 13 C POC in the sedimentary record.

Biogeochemical cycling in high-latitude regions has a disproportionate impact on global nutrient budgets. Here, we introduce a holistic, multidisciplinary framework for elucidating the influence of glacial meltwaters, shelf currents, and biological production on biogeochemical cycling in high-latitude continental margins, with a focus on the silica cycle. Our findings highlight the impact of significant glacial discharge on nutrient supply to shelf and slope waters, as well as surface and benthic production in these regions, over a range of timescales from days to thousands of years. Whilst biological uptake in fjords and strong diatom activity in coastal waters maintains low dissolved silicon concentrations in surface waters, we find important but spatially heterogeneous additions of particulates into the system, which are transported rapidly away from the shore. We expect the glacially-derived particles-together with biogenic silica tests-to be cycled rapidly through shallow sediments, resulting in a strong benthic flux of dissolved silicon. Entrainment of this benthic silicon into boundary currents may supply an important source of this key nutrient into the Labrador Sea, and is also likely to recirculate back into the deep fjords inshore. This study illustrates how geochemical and oceanographic analyses can be used together to probe further into modern nutrient cycling in this region, as well as the palaeoclimatological 3 approaches to investigating changes in glacial meltwater discharge through time, especially during periods of rapid climatic change in the Late Quaternary.