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.

KEYWORDS:

• Mesozooplankton
• phytoplankton
• microzooplankton
• diel vertical migration
• South Subtropical Front
• Polar Front

ABBREVIATIONS:

• ANOVA: analysis of variance
• chl a: chlorophyll a
• chlide a: chlorophyllide a
• CTD: conductivity–temperature–depth recorder
• DCM: deep chlorophyll maximum
• DO: dissolved oxygen
• HPLC: high-performance liquid chromatography
• PC: principal component
• PCA: principal component analysis
• PF: Polar Front
• phytin a: pheophytin a
• phide a: pheophorbide a
• SO: Southern Ocean
• SSS: sea surface salinity
• SST: sea surface temperature
• SSTF: South Subtropical Front

Acknowledgements

We are thankful to the Director of the National Centre for Antarctic and Ocean Research for support and encouragement. The help rendered in the implementation and completion of this study by the cruise participants and scientists at the National Centre for Antarctic and Ocean Research is acknowledged. This is National Centre for Antarctic and Ocean Research contribution number 28/2017.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Ministry of Earth Sciences, Government of India.