Surface chlorophyll concentrations in relation to the Antarctic Polar Front: seasonal and spatial patterns from satellite observations

Abstract

Satellite ocean color data from the Sea Viewing Wide Field of View Sensor (SeaWiFS) are used to investigate phytoplankton bloom dynamics at the Antarctic Polar Front (PF). Satellite sea surface temperature (SST) data are used to map the location of the PF at weekly timescales. Elevated chlorophyll within the PF often appears as a narrow band that occupies only a portion of the SST gradient across the PF. Phytoplankton blooms within the PF occur most frequently during the month of December and are unevenly distributed within the Southern Ocean. Elevated chlorophyll concentrations at the PF are most frequently observed where the current is interacting with large topographic features.Mesoscale physical processes, including meander-induced upwelling and increased eddy mixing, where the PF encounters large topographic features likely leads to increased nutrient flux to surface waters in these regions. The highest mean chlorophyll values associated with the PF occur where the front comes into contact with relatively shallow waters along the North Scotia Ridge and at Kerguelen Plateau. Iron input from sedimentary sources likely plays an important role in these regions.Over seasonal timescales it appears likely that light-limitation prevents phytoplankton blooms at the PF during winter and spring months. PF blooms are observed most commonly during December when surface radiation peaks and mixed layer depths are rapidly shoaling. Even during December, when the light regime would seem to be favorable, PF blooms are largely restricted to regions where enhanced nutrient fluxes to surface waters due to frontal dynamics are likely. During late summer, nutrient limitation due to depletion of iron and possibly silicate largely prevent blooms at the PF. In the fall, deepening mixed layers would provide some relief from nutrient limitation but likely lead again to light-limitation of growth rates and the prevention of blooms. D

Imprint of Southern Ocean eddies on chlorophyll

“…The clearest case for a substantial contribution of changes in biogeochemical rates on δChl was found for the northern ACC The associated negative δChl of anticyclones is consistent with the mechanism of an amplification of large-scale prevailing light limitation in deep mixed layers (Boyd, 2002;Moore and Abbott, 2002;Venables and Meredith, 2009;Fauchereau et al, 2011): Anticyclones tend to deepen isopycnals, causing deeper mixed layers of several tens of meters and weaker mixed layer stratification, especially in winter (Song et al, 2015;Hausmann et al, 2017;Dufois et al, 2016). Hence, phytoplankton within 15 the mixed layer will be exposed to reduced mean radiation in anticyclones as compared to ambient waters, and vice versa for cyclones.…”

You are what you eat: describing the foraging ecology of southern elephant seals (Mirounga leonina) using blubber fatty acids

“…The boundary between the SE and E-APF regions was often unclear, and the circumpolar APF acts as a diffuse boundary between water bodies with different salinities, temperatures and nutrient concentrations (Orsi et al 1995). Although different algal and animal communities populate the geographical zones to the north and south of the APF (Moore & Abbott 2002), this front is characterized by a high degree of short-and long-term latitudinal variation between 40°S and 60°S (Knox 1994). This latitudinal variation may blur the distinction of the assigned areas across seasons (Orsi et al 1995).…”

Dissolved iron in the vicinity of the Crozet Islands, Southern Ocean

“…Several previous studies (Croot et al, 2004a;de Baar et al, 1999;de Jong et al, 1998 ; Löscher, 1999 ; Sarthou et al, 1997) have inferred iron fertilised phytoplankton blooms around island systems in the Southern Ocean such as South Georgia (Korb et al, 2004;Moore and Abbott, 2002), Kerguelen (Blain et al, 2001(Blain et al, , 2002Bucciarelli et al, 2001Moore and Abbott, 2002), Bouvet (Croot et al, 2004a) and the Crozet islands (Metzl et al, 1999; Sedwick et al, 2002).…”