Coupling between upper ocean layer variability and size-fractionated phytoplankton in a non-nutrient-limited environment

Abstract

We describe the coupling between upper ocean layer variability and size-fractionated phytoplankton distribution in the non-nutrient-limited Bransfield Strait region (BS) of Antarctica. For this purpose we use hydrographic and size-fractionated chlorophyll a data from a transect that crossed 2 fronts and an eddy, together with data from 3 stations located in a deeply mixed region, the Antarctic Sound (AS). In the BS transect, small phytoplankton (< 20 µm equivalent spherical diameter [ESD]) accounted for 80% of total chl a and their distribution appeared to be linked to cross-frontal variability. On the deepening upper mixed layer (UML) sides of both fronts we observed a deep subducting column-like structure of small phytoplankton biomass. On the shoaling UML sides of both fronts, where there were signs of restratification, we observed a local shallow maximum of small phytoplankton biomass. We propose that this observed phytoplankton distribution may be a response to the development of frontal vertical circulation cells. In the deep, turbulent environment of the AS, larger phytoplankton (> 20 µm ESD) accounted for 80% of total chl a. The proportion of large phytoplankton increases as the depth of the upper mixed layer (Z UML ), and the corresponding rate of vertical mixing, increases. We hypothesize that this change in phytoplankton composition with varying Z UML is related to the competition for light, and results from modification of the light regime caused by vertical mixing.

Influence of oceanographic features on spatial and interannual variability of phytoplankton in the Bransfield Strait, Antarctica

“…Sangrà et al, 2011Sangrà et al, , 2014. Although the presence of such oceanographic features has been reported within the BS, particularly with an emphasis on their influence on phytoplankton community structure variability (Sangrà et al, 2014), only the Peninsula Front was identified in the present work. The inability to observe both the Bransfield Front and mesoscale anticyclonic eddies in this work could be as a result of the low spatial resolution coverage that exceeded the mean local first baroclinic Rossby radius Table 3 Abundance ranges (minimum and maximum, cells L −1 ) of the main phytoplankton taxa on four cruises (2003, 2004, 2008 and 2009) of deformation (Rd) (~10 km; Chelton et al 1998).…”

“…These contrasting, transitional waters were previously shown to generate a noticeable, physical gradient across the BS, composed by the Peninsula Front (close to the AP), the Bransfield Front (close to the SSI) and an inter-frontal anticyclonic eddy field between those fronts (e.g. Sangrà et al, 2011Sangrà et al, , 2014. Although the presence of such oceanographic features has been reported within the BS, particularly with an emphasis on their influence on phytoplankton community structure variability (Sangrà et al, 2014), only the Peninsula Front was identified in the present work.…”

“…As a result, the subsequent transitional waters can be detected within the surface layer: transitional water with Weddell influence (TWW) and transitional water with Bellingshausen influence (TBW), with each showing particular thermohaline structures ( Fig. 1) (Amos, 2001;López et al, 1999;Sangrà et al, 2011Sangrà et al, , 2014Zhou et al, 2006). The dynamic interaction between SBSC and GSC produces mesoscale processes, such as anticyclonic eddies, which become separated from those currents by the establishment of two oceanographic fronts: the Bransfield Front (BF) and the Peninsula Front (PF) (Sangrà et al, 2011(Sangrà et al, , 2014.…”

“…1) (Amos, 2001;López et al, 1999;Sangrà et al, 2011Sangrà et al, , 2014Zhou et al, 2006). The dynamic interaction between SBSC and GSC produces mesoscale processes, such as anticyclonic eddies, which become separated from those currents by the establishment of two oceanographic fronts: the Bransfield Front (BF) and the Peninsula Front (PF) (Sangrà et al, 2011(Sangrà et al, , 2014. The distribution of those waters, fronts, anticyclonic eddies and their respective thermohaline structures cause variations in the standing stock and composition of the phytoplankton community across the BS (García-Muñoz et al, 2013;Mura et al, 1995;Sangrà et al, 2014).…”

The influence of the geomorphological and sedimentological settings on the distribution of epibenthic assemblages on a flat topped hill on the over-deepened shelf of the western Weddell Sea (Southern Ocean)

“…This hypothesis remains speculative as the dispersal route would be opposite to the generally westward flowing currents in the Antarctic Sound. It can, however, not be ruled out due to the turbulent hydrographic conditions in the Antarctic Sound (Sangrà et al, 2014). Colonisation by larvae from the southeast and southern Weddell Sea via the Weddell Gyre is also possible but less likely because of the much longer distance between the source region and the Nachtigaller Hill.…”