Planktonic food web structure at SSTF and PF in the Indian sector of the Southern Ocean during austral summer 2011

Abstract

This study aims to describe the planktonic food web structure with respect to phytoplankton biomass (chlorophyll a) and prevailing environmental conditions at the South Subtropical Front (SSTF) and the Polar Front (PF) in the Indian sector of the Southern Ocean. Sampling was carried out at each front for 72 hrs, at 6-hr intervals, during the austral summer 2011. Considerable variations were observed in the hydrography between these two fronts. A strong temperature minimum layer was observed at the PF. Although the surface primary production and chlorophyll a values showed similar trends at both the fronts, the water column values of these parameters showed major disparities. The phytoplankton composition also revealed marked difference between the fronts. A deep chlorophyll maximum concordant with the upper limit of the temperature minimum layer was prominent at the PF. The microzooplankton abundance at the SSTF was twice as high as at the PF. The mesozooplankton biovolume and population density also showed considerable variations between these fronts. Noticeable diel variations were observed in the surface mesozooplankton biovolumes at both the fronts and the copepod Pleuromamma gracilis showed active diel vertical migration at SSTF. Both the grazing and senescence indices showed significant variations between these fronts, suggesting a disparity in the ecological efficiency of the two regions. The variability observed in the plankton community structure with respect to the hydrography and the biological components measured suggests that a multivorous food web at the SSTF and a conventional food web at the PF prevailed during the period of study.

Dynamics of an intense diatom bloom in the Northern Antarctic Peninsula, February 2016

“…The HPLC analysis allowed separation, identification, and quantification of three types of Chl a degradation products: chlorophyllide a (Chlide a), pheophytin a (Phytin a), and pheophorbide a (Pheide a). The relative content of those degradation products can be used as a proxy for grazing pressure and for senescence of phytoplankton cells (e.g., Jeffrey 1974;Mendes et al 2012;Pillai et al 2018). Thus, the sum of the Phytin a and Phide a was used as a proxy of zooplankton assemblage grazing, while Chlide a was used as senescence index of the phytoplankton community.…”

“…4-6). In the Southern Ocean, this grazing index has been found to be greater than 10% when associated with diatom blooms (e.g., Mendes et al 2012;Pillai et al 2018), indicating the prevalence of active grazing pressure during these high biomass events. However, mean Chl a in this study was approximately 10-fold higher in Gerlache than in the other regions, which would indicate a high phytoplankton growth rate and, therefore, a higher grazing rate would be expected in the area (e.g., Gutiérrez-Rodríguez et al 2009).…”

Changes in Phytoplankton Communities Along the Northern Antarctic Peninsula: Causes, Impacts and Research Priorities

“…Phytoplankton communities are expected to change across the Southern Ocean (Deppeler and Davidson, 2017) and monitoring their response is critical to predict the cascading effects of climate change within the NAP ecosystem. Despite top-down control having an important role in regulating the phytoplankton community composition and biomass during the austral summers (e.g., Mendes et al, 2012;Pillai et al, 2018), the bottom-up control effects have been more pronounced in both WAP (Venables et al, 2013;Saba et al, 2014) and NAP (Costa et al, 2020). This bottom-up control has been associated with sea ice cover season, which prevents wind mixing during winter and, along with glacial melting, provides meltwater to stabilize the upper ocean during summer, supporting phytoplankton growth (Venables et al, 2013).…”

Biological responses to change in Antarctic sea ice habitats

“…Observations of the faecal pellets of salps suggest that some of these larger particles are passed through the digestive tract relatively intact and are therefore lost to epipelagic food webs as the heavy pellets sink rapidly out of the euphotic zone (Cabanes et al, 2017;Iversen et al, 2017;Pauli et al, 2021a). They appear to flourish in warmer waters in the Southern Ocean and in regions of low to moderate chlorophyll a concentration (Loeb et al, 1997;Pakhomov et al, 2002;Pillai et al, 2018). In the sea ice zones of the Southern Ocean, information about salps is limited but it is noted that in the Antarctic Peninsula region, where temperatures are warming much faster than the global average, they replace krill on a regular basis and this appears to be contingent on sea ice conditions (Pakhomov et al, 2002;Atkinson et al, 2004).…”