Elgar Desa

A Small Autonomous Surface Vehicle for Ocean Color Remote Sensing

This paper provides a study on the development and the use of a small autonomous surface vehicle (ASV) that automatically follows programmed mission transects, while measuring sensor outputs along the tracks. It discusses the mechanical construction of the ASV, the distributed architecture of controller area network (CAN)-based nodes for science and vehicle payloads, high-speed radio-frequency (RF) communications, the performance of the heading autopilot, global positioning system (GPS)-based guidance algorithm, and the mission programming technique. The field trials of the ASV, performed off the coast of Goa, India, are focused on retrieving the 2-D spatial distribution of surface chlorophyll, which is one of the useful parameters in characterizing the nature of calibration-validation (CALVAL) sites for ocean remote sensing needs. A further benefit of ASVs is that they can be built at a low cost and used in monitoring applications of diverse coastal ecosystems.

Satellite and ship studies of phytoplankton in the northeastern Arabian during 2000-2006 period

Sequence of the images from IRS P4 / OCM satellite and extensive shipboard sampling programme are used to understand the seasonal variation of phytoplankton abundance and types in the Northeastern (NE) Arabian Sea and Lakshadweep Sea. An appreciable degree of spatial and temporal variability is observed in chlorophyll a distribution from November to April months, as well as coastal and offshore stations, indicating marked seasonality in phytoplankton distribution in NE Arabian Sea. During November month (fall intermonsoon) average chlorophyll a (Chl a) by fluorometer was (0.799 mgm^-3) and by OCM it was 0.584 mgm^-3. The higher chlorophyll a observed was due to Trichodesmium (cyanobacteria) blooms. During December the average chlorophyll a was 0.34 mgm^-3 also due to Trichodesmium filaments in water column. During January onwards winter cooling led to increase in nutrients which enhanced chlorophyll a value to 0.64 mgm^-3 due to growth of flagellates (as seen by high chlorophyll b besides chlorophyll a) in water column. February, March and April supported moderately high chlorophyll value (0. 3 to 0.5 mgm^-3) due to growth of prasinophytes (as seen by pigment prasinoxanthin) and blooms of the Noctiluca miliaris. Time series monitoring of Noctiluca bloom was also conducted using OCM based chlorophyll images in NE Arabian Sea. During February chlorophyll a retrieved by OCM was 0.3 to 0.9 mgm^-3. Pigment analysis of water samples indicated the equal important of accessory pigment like zeaxanthin, prasinoxanthin, beta-carotene. The relevance of these pigments estimated by HPLC like zeaxanthin (cyanobacteria), fucoxanthin (diatoms), peridinin (dinoflagellates) is presented and discussed. Similarly, exercise is conducted in Lakshadweep waters where Trichodesmium related peak in chlorophyll a was observed during March onwards in OCM data. The average chlorophyll a in NE Arabian Sea at surface during November was (0.726 mgm^-3), December (0.34 mgm^-3), January (0.723 mgm^-3), February (0.344 mgm^-3), March (0.963 mgm^-3) and April 0.665 mgm^-3. Similar trend was observed in primary productivity estimates. The attempt is made to work out seasonality in the productivity of the Arabian Sea using OCM derived chlorophyll and relation of enhancement in productivity due to development of winter blooms in the Arabian Sea. The environmental conditions (temperature, wind, nutrients and mixed layer depth) affecting these blooms responsible for year to year variation in bloom biomass and productivity is also presented in detail.

In Situ Profiling of Eastern Arabian Sea Coastal Waters Using a New Autonomous Vertical Profiler

The autonomous vertical profiler (AVP) presented here offers a fast, cost-effective, optimized approach to profiling in coastal waters. It consists of a hands-free, slightly buoyant, motor-driven in situ robot profiler that requires no operator skill or deployment gear, but fulfills the requirements of repetitive profiling of the water column. It uses standard oceanographic sensors to measure vertical structure at high resolution ( ~ 10 cm) in waters as shallow as 5 m and as deep as 200 m. The detailed engineering design, hydrodynamics, safety systems, endurance, and performance of the AVP relative to other profilers are covered in this paper. Proof-of-concept experiments in coastal waters present a clear picture, for the first time, of stable interactions between constant density contours and chlorophyll maxima in the waters of the Coral Island of Kavaratti (in the Lakshadweep Archipelago). The AVP has also been used in deep dives to 200 m in shelf regions of the Arabian Sea to record the presence of anoxic layers and phytoplankton existing in low-oxygen conditions.

Comparison of measured and satellite-derived spectral diffuse attenuation coefficients for the Arabian Sea

We present here the results of our study comparing the spectral diffuse attenuation coefficients K d(λ) measured in the Arabian Sea with those derived from the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) using three algorithms, of which two are empirical-data-driven and one is semi-analytical. The measurements were carried out in all water types and the mean values of the measured spectral K d(λ) are 0.105, 0.092, 0.077, 0.082, 0.110 and 0.490 m−1 for wavelength λ at 412, 443, 490, 510, 555 and 670 nm, respectively. This profile corresponds to a chlorophyll value of about 1 mg m−3. The maximum values of the measured K d correspond to waters with chlorophyll of about 8 mg m−3. Though the satellite-derived K d(λ) are found to be overestimated in all bands, we have observed good correlations between the measured and satellite-derived values in all bands, and excluding the band at 670 nm, the mean absolute percent deviations are observed to be less than 50% in all bands. The performance of the data-driven empirical methods was found to be consistent in all the bands, except at the red band of 670 nm, which is uncorrelated with the measured values and has large errors. The performances of the empirical methods depend on the accuracy of the band ratios of the retrieved remote sensing reflectance. Though the performance of the semi-analytical algorithm is found to be spectrally varying, with large positive bias observed in the blue regions, this algorithm is recommended for hyperspectral applications. The performance of the semi-analytical algorithm could be improved by having a robust algorithm to accurately derive spectral inherent optical properties of absorption and backscattering coefficients from the satellite data.

The detection of annual hypoxia in a low latitude freshwater reservoir in Kerala, India, using the small AUV Maya

The Idukki Reservoir at an altitude of 748m covering an area of 53 km 2 is surrounded by tropical forests in the Western Ghats in the southwestern Indian state of Kerala.. We used the small AUV Maya with onboard sensors of dissolved oxygen, chlorophyll, turbidity, temperature and depth to monitor the water quality environment of Idukki Reservoir in May 2006. The use of AUVs in confined spaces like small lakes and reservoirs is new and uncommon requiring extra safety to be implemented. As this is a prototype AUV, we shall describe in brief key aspects of the vehicle attributes namely; its novel mechanical design, the autopilots which control the heading and cruising depth, safety and endurance of the platform. The data acquired by Maya revealed an acute oxygen deficiency at 21m, a mid-water low turbidity layer between 10 to 15m, and a prominent chlorophyll maximum in the thermocline region of Idukki waters at 6m. These experiments were repeated a year later in May 2007 are in agreement with the 2006 findings of hypoxia unambiguously. These are the first observations of hypoxic processes using a small AUV in any Indian lake. The relevance of these results in freshwater systems show similarities to hypoxia in saline coastal waters of the west coast of India and are discussed briefly.

Basin scale distribution of Trichodesmium spp. in the Arabian Sea using Oceansat I/OCM

Trichodesmium spp. is widely spread in the Arabian Sea. It form dense patches. During 2000-2005 (5 years period) extensive sampling was done in the Arabian Sea covering large area and different months starting from November to May. Three prominent sites are identified as Trichodesmium bloom sites in the Arabian Sea: 1) Lakshadweep waters 2) Off Goa and 3) Off Gujarat area. Bloom of around 100 km2 area with 4 to 400737 filamentsL-1 concentrations are recorded. Two species of Trichodesmium are encountered based upon seasonality and environmental conditions. OCM derived chlorophyll a during bloom was as high as 0.5 to 1 mgm-3, at time increased upto 5 mgm-3 and depends upon number of filaments in water. The Trichodesmium features were identified at 869, 670, 555nm in OCM data. Trichodesmium was detected as stripes and eddies in OCM images. The bloom patches appear darker which is taken as measurement of spread of the bloom in water. Total 133 stations are covered during 5 years period out of which 63 stations showed presence of Trichodesmium with discolouration of water. In offshore water Trichodesmium was detected as early as November and continued upto April month whereas in the coastal water Trichodesmium prevailed from February to May. The seasonality of these blooms was observed with respect to inshore/offshore and two species of Trichodesmium is discussed with the help of OCM data processed for chlorophyll a during Trichodesmium growth period, at 3 identified sites in the Arabian Sea.

Secchi depth analysis using bio-optical parameters measured in the Arabian Sea

Secchi depth provides the oceanographer with the first hand information about transparency and penetration of light in the water. Here we present results of the Secchi depth and the optical properties measured in the Arabian Sea. Our analyses show spatial and temporal variability of Secchi depth and their dependence on the optical properties beam attenuation and diffuse attenuation the biological parameter of Chlorophyll. The in-situ measured inherent and apparent optical properties have been used to understand the underwater light properties and their relations to the Secchi depth in various water types. The Secchi depth model is validated using the measured optical properties. We also present an empirical method to determine Secchi depth from the satellite ocean color sensor, and the application of the same to the IRS-P4 OCM is found to provide comparable results to the measured values.

Empirical algorithm to estimate the average cosine of underwater light field at 490 nm

The average cosine of the underwater light field μ(λ), where λ is the wavelength, is an apparent optical property (AOP) that describes the angular distribution of radiance at a given point in water. Here, we present a simple algorithm to determine the average cosine at 490 nm, μ(490), which was developed using the measured optical parameters from the eastern Arabian Sea and coastal waters off Goa. The algorithm is validated using measured optical parameters. This algorithm, based on a single optical parameter, performed better compared with other empirical algorithms in determining the average cosine of underwater light field. The absorption coefficient at 490 nm, derived as an application of μ(490), compared well with the synthetic optical data and optical data measured from other regions.

A simple method to minimize orientation effects in a profiling radiometer

Marine optical parameters required for ocean color satellite applications must be measured with high accuracy and errors within the permissible limits. These stringent requirements demand careful measurements of optical parameters. Though the free-fall radiometer is found to be a better option for measuring underwater light parameters as it avoids the effects of ship shadow and is easy to operate, the measurements demand profiling the radiometer vertical in water with minimum tilt. Here we present the results of our observations on the tilts of the radiometer from the measurements in the Arabian Sea. Since there is hardly any study carried-out on the tilt of the profiling radiometer, the result of this study will help in the better design of such marine instruments. The tilt of the radiometer near the surface of the water is attributed to the mode of deployment and environment parameters, while the tilt at depth of the water is influenced by the density variations of the water. Here we also demonstrate a method of deploying the instrument that minimizes the tilt of the instrument at the surface layer of the water.

Cross-calibration of IRS-P4 OCM satellite sensor

We present here the cross calibration of ocean color satellite sensor, IRS-P4 OCM using the radiative transfer code, with SeaWiFS as a reference. Since the bands of IRS-P4 OCM are identical to those of SeaWiFS and SeaWiFS has been continuously and rigorously calibrated, SeaWiFS is used as a reference for the cross calibration of IRS-P4 OCM. Calibrations coefficients for each band of IRS-P4 OCM are derived by comparing the actual radiances detected by the satellites at top of the atmosphere and those obtained from the radiative transfer simulations of IRS-P4 OCM and SeaWiFS. The chlorophyll a values derived using the calibrated IRS-P4 OCM are found to be comparable with those derived from SeaWiFS and in close agreement with the measured values. The relative root mean square error (RRMSE) between measured chlorophyll a and those derived from the satellites are found to be 0.28 and 0.26 for SeaWiFS and IRS-P4 OCM respectively.