Season :    2012

Voyage :     6

Cruise Name :     06th National Expedition to Southern Ocean

Voyage Classification :     Ocean Science, Southern Ocean Studies

Ship Name :     ORV Sagar Nidhi

Name of Chief Scientist / Leader :     Dr. C.T. Achuthankutty, NCPOR, Goa.

Name of Deputy Chief Scientist / Leader :     Dr. N. Ramaiah, NIO, Goa.

The Southern Ocean (SO) is a source and sink for several intermediate and deep water masses of the world oceans. Many aspects of its circulation, water masses and response to climate change remain unknown primarily due to the lack of high resolution sea truth observations. Since studies in the Indian sector of the Southern Ocean are sparse, it is imperative that more systematic scientific investigations need to be carried out for a comprehensive understanding on the physical and biogeochemical processes vis-à-vis the food web dynamics, for a realistic estimation of biological productivity and carbon cycling. The National Centre for Antarctic and Ocean Research (NCAOR, Goa), as the nodal agency for planning and coordinating the scientific expeditions to the Indian sector of the Southern Ocean, has so far launched five expeditions (January-March 2004 [Pilot Expedition], January-April 2006, January-April 2009, January-March 2010 and January- March 2011), involving about a dozen leading institutions in the country, to understand the complexities of this dynamic ocean. The results obtained from theses expeditions are quite encouraging as they provided sufficient baseline data/information on various physical, biological and biogeochemical processes that are responsible in modulating the global warming and climate variability. Frontal waters are highly complex systems formed at the confluence of distinctly different water masses, resulting in the redistribution of physico-chemical constituents and formation of sharp gradient features. In the Southern Ocean, among the various frontal waters identified, the Subtropical Front (STF) waters show maximum gradients in temperature, salinity and nutrients over a very small area (<200 nautical miles wide). Conversely, the Polar Front (PF) waters receive highly productive Antarctic coastal waters from the south and high nutrient-low chlorophyll (HNLC) waters from the north, resulting in sporadic blooms and high biological biomass. Therefore, during the 5 the expedition (SOE-2011), time-series observations were also carried out at STF and PF, besides the regular sampling at the meridional stations. The time-series and the meridional studies showed distinct spatial distribution of phytoplankton biomass and diel variations, closely linked to nutrient variations. Both phytoplankton biomass (chl a) and mesozooplankton biomass showed a meridional increase with maximum biomass in thePolar Front. Although satellite images showed productive waters in STF, the productivity signatures could not be captured in the in situ observations. This may be due to discrete and few sampling locations within the STF. It appears that such discrete sampling points within the fronts therefore are not enough to explain the prevailing food-web dynamics and biogeochemistry in relation to the oceanographic conditions. Hence, it was proposed to make high resolution sampling in the Subtropical Front during the 6 th Indian Scientific Expedition to the Indian Ocean sector of the Southern Ocean (SOE-2011-12) with the following focal scientific themes: 1. Water column dynamics 2. Biogeochemistry 3. Food web dynamics 4. Atmospheric sciences

The Expedition was very successful and all the planned research activities were accomplished. For more details please refer Cruise report.

From	Depart	To	Arrive
Marmugao, Goa	12/23/2011	Portluis, Mauritius	02/06/2012

Multidisciplinary Parameters: At an interval of 2 degrees (both latitude and longitude), water samples were collected in 24 Niskin bottles of 5 L capacity mounted on a rosette sampler along with CTD, at prefixed depth intervals (surface, 10, 30, 50, 75, 100, 120, 200, 500 & 1000 m and DCM depth as per the fluorescence profile), for measuring all physical (salinity, isotope studies), chemical (DO, DIC, TOC, nutrients, POC, DCHO, PCHO, TEP, SPM) and biological parameters (bacteriological studies, primary production, pigment characterization, phytoplankton diversity, heterotrophic nanoflagellate grazing, photosynthetic irradiance, micro-zooplankton) from 9 stations. Multiple Plankton Net (Hydro-Bios, Germany) was operated up to a depth of 1000 m for collecting meso- zooplankton (>200 μm in size) from 4 depth strata (1000- 500 m, 500 m-bottom of thermocline, thermocline (TC) and mixed layer depth (MLD) from 9 stations (Fig. 2). A WP equivalent net was hauled at the surface layer for collecting the surface meso- zooplankton from all multidisciplinary stations and also from Stn. 7. 2.4 Time Series Sampling: At Stn. 1 (40oS, 58o30’E), a time-series observation was carried out for 2 days, starting at 00.30 hr on 13 th January 2012 and ending at 04.35 hr on 15 th January 2012. Multidisciplinary sampling (Table 1) was carried out at 6 hourly interval and CTD and Microprofiler (MP) observations were carried out at 3 hourly interval. As can be seen from Figure 3, the northern transect of the rescheduled cruise track was located in chlorophyll distributed area in the STF and the meridional transect was cutting through the chlorophyll rich zone and extending to the chlorophyll depleted southern transect. It is hoped that the high resolution sampling strategy adopted for this expedition, both within chlorophyll present and depleted locations in the STF would provide insights in to the hydrodynamic features that are primarily responsible for regulating the biogeochemical cycles and biological productivity processes in this front. 2.5 Atmospheric studies: Aerosol o ptical depth was measured using Microtops Sunphotometer throughout the cruise track at every 30 minute interval, starting from Goa to 53oS and on retun up to 23oS30’ along 57o30’E. Aethalometer was used to measure the ambient black carbon mass concentration starting from Goa to 53oS and on return up to 23oS30’ along 57o30’E. The aerosol differential mass concentration was measured using Quartz Crystal Microbalance. The weather parameters such as temperature, pressure, wind speed and wind direction were determined using Kestrel 400 Pocket Weather Monitor. Garmin GPS 12 was used to geo-tag the data.

The 6 th expedition to the Indian Sector of the Southern Ocean was the 3 rd successive expedition of ORV Sagar Nidhi to this pristine region which was incidentally the 61 st cruise of ORV Sagar Nidhi. The expedition was originally proposed to start on 28 th December 2011 from Port Louis (Mauritius) with the ETA at the 1 st station on 3 rd January 2012 and reach Port Louis on 2 nd February 2012, after completing all the observations and sampling. Accordingly, the ship was to sail from Goa on 15 th December 2011, after loading all the scientific equipments and materials required for the expedition. A total of 40 stations were originally proposed to be sampled, 34 stations with in STF (40oS - 45 oS Lat; 53o30’E - 58o30’E Long) and 6 stations between 50oS - 57oS Lat, along 57o30’E Long (Fig. 1). However, this could not be accomplished due to some unavoidable last minute changes made in the ship’s schedule. The ship was made available for the expedition on 23 rd December 2011 at Goa. The ship sailed on 25 th December 2011 via Galle (Sri Lanka) to the sampling location and was to call on Port Louis on 2 nd February 2012. Due to the rescheduling of the expedition, the ETA at the 1 st station was delayed by almost 10 days and reached on 12 th January 2012 (late night). Also, due to a snag developed to one of the thrusters, the speed of the ship was 2 considerably reduced and the ship reached Port Louis only on 6 th February 2012. As a result, the entire expedition schedule had to be revised and only 21 stations could be sampled due to paucity of time

Please refer Voyage Report

Please refer Voyage Report