Nutrient dynamics at the juncture of Agulhas Return Front and subtropical front in the Indian sector of the Southern Ocean during the austral summer

Abstract

The northern boundary of the Indian sector of Southern Ocean is marked by the juncture of the Agulhas Return Front and the southern Subtropical Front and is a hydrographically dynamic region. During austral summer, a time series study (13th to 15th January 2012) was conducted in this region (40°S, 58°30′E) to discuss the episodic biochemical changes with respect to nutrient dynamics within the upper (120 m) water column. The observed nitrate was depleted in the upper 50 m. However, there was an influx of nitrate at 6 and 18 h of Day-1. Even though the nitrate was exceptionally low, the system was moderately productive, suggesting the rapid utilization of nitrate at this location. Also, the Degree of Nutrient Consumption, was high at 50 m (~0.4), compared to the rest of the water column indicating the presence of aged/old upwelled waters at 50 m. The observed influx of nutrients could be attributed to the wind induced mixing as well as the wind stress curl driven pumping at the periphery of the anticyclonic eddy at the location. Although the influx of nutrients was low, there was the rapid utilization of nutrients, which triggered a switch in the phytoplankton community structure and subsequently influencing the biogeochemical cycle.