A multi-sensor autonomous platform for fast ocean observation studies

Variations in dissolved oxygen (DO), chlorophyll-a, temperature and salinity play an important role in the outcome of Primary Productivity [PP] levels in the oceans. The vertical structure and spatial distribution of these variables in time can help in providing insights on the role of PP as an indicator of ecosystem health. The DO values in Arabian sea where the studies have been undertaken, generally vary from 220 μM/L at surface to near zero as depth exceeds 150 M. Oceanic waters that have less that 0.2 μmole/L DO are known as Oxygen Minimum Zones (OMZ) or dead zones. The spatial and seasonal variations of the OMZ need to be tracked for better understanding of the cause of this phenomenon. It is well known that conventional technique of lowering instrumented packages and water sampling devices from a research vessel is a slow process resulting under sampling of spatial and temporal resolution of processes, and therefore limiting the scope of sampling oxygen deficient waters. A new platform Autonomous Vertical Profiler (AVP) has been developed at CSIR National Institute of Oceanography in Goa, India that can rapidly and autonomously collect repetitive data to 200m water depth. It is equipped with sensors to measure conductivitytemperature-depth (CTD), dissolved oxygen (DO), PAR, chlorophyll-a, turbidity and backscatter. A typical profile is obtained within ten minutes. The ability to collect profiles and transmit data to remote places over a radio or satellite link makes the AVP an appropriate tool for ocean observations related to climate, water quality studies etc. The AVP can be deployed for 12 days with four profiles per day to a depth of 100m and it is being enhanced to 30 days endurance. This enables several dives to be executed in a short duration to determine the movement of OMZ with respect to depth. Compared to the AVP, ARGO floats are designed for deep water operations with cm/sec descent rates and limited sensor payloads, although this situation is now improving with the advent BIO ARGO floats equipped with sensors for DO, and chlorophyll-a. The AVP’s initial tests were carried out in fresh water bodies at Amthane Dam, Goa and at Tillari dam, Maharashtra. AVP’s subsequent deployments were at locations off Goa, off Ratnagiri, in Maharashtra and in Coral Islands of Lakshadweep Archipelago from depths of 30 m to 200 meters. It was generally observed that DO values declined rapidly beyond 30 meters to 100 meters depth and by 150m were practically zero indicating anoxic waters. During a cruise onboard RV Sindhu Sankalp the AVP was deployed off Kavaratti Island, Lakshadweep, India to a depth of 200 m. The data indicates presence of very low DO values, possibly the presence of an OMZ signature. Similarly, during a mid December 2010 cruise on RV Sindhu Sankalp, an OMZ was detected at 200m close to shelf waters off Goa (West cost of India). This again points to the possible intrusion of the OMZ waters from the Central Arabian Sea to coastal waters of the West coast of India. *Corresponding author, CSIR National Institute of Oceanography, Dona Paula, Goa 403004. 1 sanjeev@nio.org The detection of anoxic waters found by many AVP profiles proves conclusively its use in the detection of low oxygen waters in rapid ocean observation studies. Currently, the AVP is also being used in satellite sensor validation experiments for the OceanSat 2 satellite launched by India, and in other wide ranging observations of anoxia in freshwater dams and in coastal waters off Goa, Vizag, and Kochi.