Characteristics of particulate organic matter within the photic water column: A case study across the fronts in the indian sector of the Southern Ocean

Abstract During Austral summer 2012, seawater samples were collected from the upper 120 m of the water column, across the frontal zones in the Indian Ocean sector of the Southern Ocean. Suspended particulate organic matter from seawater was characterised, from its organic carbon and nitrogen content, and δ13C(POM). There was a marked difference in δ13C(POM) across the three frontal zones. It was depleted from the Subtropical frontal zone (-24.48 ± 1.94‰) to the Subantarctic frontal zone (-25.53 ± 0.62‰) and the Polar frontal zone (-26.69 ± 1.22‰). Within the water column, Particulate Organic Carbon (POC) and Particulate Nitrogen (PN) decreased with depth, especially below the deep chlorophyll maxima. Principal Component Analysis of δ13C(POM) and the environmental variables showed two clusters, one cluster, within the Subtropical and Subantarctic frontal zones, whereas the other cluster was in the Polar frontal zone. Statistical analysis of δ13C with particulate organic carbon, nitrogen, apparent oxygen utilization, chlorophyll a and phytoplankton community, indicated that biological community structure strongly influenced the particulate organic matter characteristics in the upper water column. The study also revealed that both autotrophic and heterotrophic processes play a significant role in the waters of the Subtropical and Subantarctic frontal zones. Furthermore, physical factors like eddies and advection influence the community structure and food web dynamics of the region. However, in the Polar frontal zone, autotrophic processes were dominant, supported by the abundance of nutrients and prevalence of larger phytoplankton community (diatoms).