N. Anilkumar

Variability of fronts, fresh water input and chlorophyll in the Indian Ocean sector of the Southern Ocean

The aim of this study was to understand the variability in the fronts and water masses, and the effect of melt water on the concentration of chlorophyll (Chl a) in the Indian Ocean sector of the Southern Ocean using hydrographic data collected during the austral summer (February 2010 and 2011). The Southern Subtropical Front (SSTF) and Northern Sub Antarctic Front (SAF1) were found to be further south at 57°30′E than at 47–48°E. This southward shift of the fronts was consistent with the southward meandering (c. 2°) of the Antarctic Circumpolar Current (ACC) core from the western section to the eastern section, which could have been caused by the bottom topography. The intrusion of water masses also differed between the western and eastern transects of the study region as a result of the meandering of the ACC core. Fresh water layer thickness relative to the winter water in 2011 was more compared to that during 2010. This could have been due to the larger amount of sea ice that was present in the winter of 2010, which subsequently melted, resulting in the advection of melt water from the south and west of the study region. In situ observations and satellite data detected a high Chl a concentration (c. 0.38 mg m−3) south of the Northern Polar Front (PF1) in 2011, which was caused by this melt water.

Relative Microalgal Concentration in Prydz Bay, East Antarctica during Late Austral Summer, 2006

Microalgae using a submersible fluorescence probe in water column (up to 100 m) were measured during the austral summer of 2006 (February) in Prydz Bay, East Antarctica (triangular-shaped embayment in the Indian sector of Southern Ocean). Concurrently, environmental parameters such as temperature, salinity and nitrogen (nitrate, ammonium, urea) uptake rates were measured. The concentration of phytoplankton is relatively high due to availability of high nutrients and low sea surface temperature. Phytoplankton community is dominated by diatoms whereas cryptophytes are in low concentration. The maximum concentration of total chlorophyll is 14.87 μg L -1 and is attributed to upwelled subsurface winter water due to local wind forcing, availability of micro-nutrients and increased attenuation of photosynthetically available radiation (PAR). Concentration of blue-green algae is low compared to that of green algae because of low temperature. Comparatively high concentration of yellow substances is due to the influence of Antarctic melt-water whereas cryptophytes are low due to high salinity and mixed water column. Varied concentrations of phytoplankton at different times of Fluoroprobe measurements suggest that the coastal waters of Prydz Bay are influenced by changing sub-surface water temperature and salinity due to subsurface upwelling induced by local winds as also melting/freezing processes in late summer. The productivity is high in coastal water due to the input of macro as well as micro-nutrients.

Primary and new production in the thermocline ridge region of the southern Indian Ocean during the summer monsoon

New and primary production, measured using 15N and 13C tracers, integrated over the euphotic zone (mostly upper 80-100 m) varied from 7.5 to 14.1 mmol N m-2d-1(average 10.8 mmol N m-2d-1) and 2.7 to 17.7 mmol C m-2d-1 (average 11.3 mmol C m-2d-1), respectively, in the tropical India Ocean (TIO). Shoaling of thermocline in the thermocline ridge located in the tropical Indian Ocean (TR) influences new and total productivity by supplying nutrients to the upper layers. At the same time, N-limited conditions prevail to the north of the TR region reducing biological productivity. While higher surface light intensity reduces the surface productivity, it enhances the overall depth-integrated productivity. Productivity at different locations within the region is primarily controlled by the supply of N-nutrients from below. In contrast, at a given location with the available nutrient pool, productivity at different depths is determined by light availability. The region has potential for moderate export production, as do the other parts of the Indian Ocean; the maximum possible f-ratio is found to be 0.56, suggesting that it is capable of exporting a significant part of the total production to the deep, under favorable conditions (i.e., availability of N-nutrients). A comparison with earlier data suggests no significant temporal trend in the productivity here over the past three decades.

Seasonal Surface Chlorophyll a Variability in the Seychelles–Chagos Thermocline Ridge

Seychelles–Chagos Thermocline Ridge (SCTR, 5– 10S, 50–75E) in the southwestern tropical Indian Ocean is a unique area that experiences year-round upwelling. This is a response to the upward Ekman pumping prevalent in the region. Satellite data, model data and objectively analysed Argo temperature/ salinity data have been used to study the seasonal surface chlorophyll a (chl a) variability in SCTR. Variability of surface chl a concentration in SCTR showed a weak semiannual signature. The western part of SCTR (WSCTR, 50–62E) is characterized by higher chl a concentration than the eastern part (ESCTR, 63–75E). Average chl a concentration in WSCTR/ESCTR showed a primary peak in July– August (~0.26/~0.16 mg/m) and a secondary peak in January (~0.14/~0.12 mg/m). Minimum chl a concentration (~0.12/~0.1 mg/m) was observed during March– April and December–January. The high amplitude of chl a variability observed during July–August is associated with weak stratification and deep mixed layer depth (MLD). Deep MLD reaching to nutrient-rich thermocline entrains nutrients to the surface and thereby increases the surface chl a concentration. However, the low surface chl a concentration is a result of shallow MLD in the region. The deep MLD (30–40 m) observed during June–October is dominated by wind mixing and supported by buoyancy mixing. Shallow MLD (<30 m) observed during rest of the year is due to weak wind mixing and high surface buoyancy. The high surface buoyancy is a manifestation of ocean surface warming and presence of low saline surface waters in the SCTR region.