A 'shallow bathtub ring' of local sedimentary iron input maintains the Palmer Deep biological hotspot on the West Antarctic Peninsula shelf

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'.

Keywords: Antarctic; biogeochemistry; iron; manganese; phytoplankton; sediment.

Figure 1.

Map of PD study region off the Antarctic Peninsula. Stations designated YY-# where YY is cruise year and # is the event number were sampled from surface to bottom in 2015 (closed squares and blue triangle) and 2016 (closed circles). The Palmer LTER grid station 600.040 in the deepest part of the canyon was occupied in each year (red circle). Station E (blue triangle) was sampled over a time-series in January–March 2015, in surface waters only.

Figure 2.

Hydrographic data from the eight stations, with shallower stations in (a–d) and deeper canyon stations in (e–h). Plotted left to right are (a,e) potential temperature, (b,f) salinity, (c,g) % meteoric water contribution and (d,h) % sea ice melt. A T–S plot for these stations is shown as inset on lower salinity graph. Station colour designations are consistent with figure 3.

Figure 3.

Vertical distribution of (a,e) dissolved Fe (dFe), (b,f) total particulate Fe (TpFe), (c,g) dissolved Mn (dMn) and (d,h) total particulate Mn (TpMn). Bottom sample collected at approximately 10 m off bottom in each profile. Station 16-394 (solid black circles) runs off scale, but is shown in full detail in figure 4. Open circles indicate canyon head stations sampled on the north and south flanks late in the research cruise (7–8 February), 27 days after the nearby stations (figure 1) indicated by filled circles of the same colour.

Figure 4.

Vertical profiles of dissolved (open symbols) and particulate (closed symbols) Fe (squares) and Mn (circles) at southern coastal station 16-394. Bottom is at approximately 300 m. (Online version in colour.)

Figure 5.

Comparison of the vertical profiles of dissolved Fe (dFe), labile particulate Fe (LpFe) and total particulate Fe (TpFe) at stations 15-24, 16-414 and 16-445 (a–c). Note that particulate Fe concentrations are much larger than dissolved and are shown on bottom x-axis. (d–f) the same variables for Mn at the same stations, and all values on the top x-axis. (Online version in colour.)

Figure 6.

Time-series of dissolved Fe and Mn, meteoric water contribution (left y-axis) and mixed layer depth (right y-axis) at Station E in early 2015. Note that mixed layer depth is plotted increasing down to be more visually intuitive. (Online version in colour.)