Aneesh A. Lotliker

Variability of remote sensing reflectance and implications for optical remote sensing—A study along the eastern and northeastern waters of Arabian Sea

[1] Situated in the eastern and northeastern regions and subjected to similar oceanic processes, the inshore waters north of 20°N and east of 65°E are more optically complex than the offshore waters and waters south of 20°N and east of 65°E. This has been observed through the analysis of variations of the remote sensing reflectance (R rs (λ)) in the optical spectrum of Electromagnetic radiation. The optical complexity has further been studied through the examination of optically active substances (OAS) such as chlorophyll_a, suspended sediment and coloured dissolved organic matter (CDOM). It is found that CDOM is the significant component in making the area optically nonlinear. For the first time multi-spatial/temporal band-ratio algorithms are developed to map OAS from these waters through Ocean Colour Monitor flown on IRS - P4 satellite.

Evaluation of high resolution MODIS-Aqua data for oil spill monitoring

The MODIS - Aqua high-resolution imagery were exploited to detect and monitor oil spills. An evaluation criterion has been established to study its potential. The study focused on two oil spill events: Lake Maracaibo, Venezuela (January 18-20, 2003) and Jiyeh power station oil spill, Lebanon (July 15-31, 2006). The images were examined at level-1B (only geometrically corrected) and level-2 (geometrically and atmospherically corrected) data processing levels. The level-2 data lacked the sufficient contrast range, because of the rigorous atmospheric correction, while the level-1B data were found to be suitable. The 250-m data at 645 and 859 nm and 500-m, interpolated to 250-m, at 469, 555, 1240, and 2130 nm were analyzed. The methodology included examination of individual bands and evaluation of 30 band ratioing combinations to improve the contrast of oil spills in the images. The evaluation criteria were based on both visual and parametric. The metrics involved are: mean contrast function and feature matching. In addition, bi-directional reflectance distribution function (BRDF) at 469, 555, and 645 nm wavelengths, were also evaluated using the same criteria. The study found that at appropriate view-angle, MODIS-Aqua high-resolution is suitable for oil spill detection at 250-m band. When the view-angle is not appropriate, the combination of mid-IR bands with shorter wavelengths improved the feature matching.

Effect of optically active substances and atmospheric correction schemes on remote-sensing reflectance at a coastal site off Kochi

The present study focused on understanding the variability of optically active substances (OASs) and their effect on spectral remote-sensing reflectance (Rrs). Furthermore, the effect of atmospheric correction schemes on the retrieval of chlorophyll-a (chl-a) from satellite data was also analysed. The OASs considered here are chl-a, coloured dissolved organic matter (CDOM), and total suspended matter (TSM). Satellite data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite was used for this study. The two atmospheric correction schemes considered were: multi-scattering with two-band model selection NIR correction (hereon referred as ‘A1’) and Management Unit of the North Sea Mathematical Models (MUMM) correction and MUMM NIR calculation (hereafter referred as ‘A2’). The default MODIS bio-optical algorithm (OC3M) was used for the retrieval of chl-a. Analysis of OASs showed that chl-a was the major light-absorbing component, with highly variable distribution (0.006–25.85 mg m–3). Absorption due to CDOM at 440 nm (aCDOM440) varied from 0.002 to 0.31 m–1 whereas TSM varied from 0.005 to 33.44 mg l–1. The highest concentration of chl-a was observed from August to November (i.e. end of the southwest monsoon and beginning of the northeast monsoon), which was attributed to coastal upwelling. The average value of aCDOM440 was found to be lower than the global mean. A significant negative relationship between aCDOM440 and salinity during the southwest monsoon indicated that much of the CDOM during this season was derived from river discharge. Spectral Rrs was found to be strongly linked to the variability in chl-a concentration, indicating that chl-a was the major light-absorbing component. Satellite-derived spectral Rrs was in good agreement with that in situ when chl-a concentration was lower than 5 mg m–3. The validation of chl-a, derived from in situ Rrs, showed moderate performance (correlation coefficient, R2 = 0.64; log10(RMSE) = 0.434; absolute percentage difference (APD) = 43.6% and relative percentage difference (RPD) = 42.33%). However the accuracy of the algorithm was still within acceptable limits. The statistical analysis for atmospheric correction schemes showed improved mean ratio of measured to estimated chl-a (‘r’ = 1.6), log10(RMSE) (0.49), APD (25.46%), and RPD (17.57%) in the case of A1 as compared with A2, whereas in the case of A2, R2 (0.56), slope (0.26), and intercept (0.27) were better as compared with A1. The two atmospheric correction schemes did not show any significant statistical difference. However the default atmospheric correction scheme (A1) was found to be performing comparatively better probably due to the fact that the concentration of TSM and CDOM was much lower to overcome the impact of chl-a.

Environmental dynamics of red Noctiluca scintillans bloom in tropical coastal waters

An intense bloom of red Noctiluca scintillans (NS) occurred off the Rushikulya estuarine region along the east coast of India, an important site for mass nesting events of the vulnerable Olive Ridley sea turtle. At its peak, densities of NS were 3.3×10(5) cells-l(-1), with low relative abundance of other phytoplankton. The peak bloom coincided with high abundance of gelatinous planktivores which may have facilitated bloom development by their grazing on other zooplankton, particularly copepods. Ammonium concentrations increased by approximately 4-fold in the later stages of bloom, coincident with stable NS abundance and chlorophyll concentrations in the nano- and microplankton. This increase likely was attributable to release of intracellular ammonium accumulated through NS grazing. Dissolved oxygen concentrations decreased in sub-surface waters to near hypoxia. Micro-phytoplankton increasingly dominated chlorophyll-a biomass as the bloom declined, with diminishing picoplankton abundance likely the result of high predation by the ciliate Mesodinium rubrum. Together, these data illustrate factors that can disrupt ecosystem balance in this critically important Indian coastal region.

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.

BoBBLE (Bay of Bengal Boundary Layer Experiment): Ocean-atmosphere interaction and its impact on the South Asian monsoon

AbstractThe Bay of Bengal (BoB) plays a fundamental role in controlling the weather systems that make up the South Asian summer monsoon system. In particular, the southern BoB has cooler sea surface temperatures (SST) that influence ocean–atmosphere interaction and impact the monsoon. Compared to the southeastern BoB, the southwestern BoB is cooler, more saline, receives much less rain, and is influenced by the summer monsoon current (SMC). To examine the impact of these features on the monsoon, the BoB Boundary Layer Experiment (BoBBLE) was jointly undertaken by India and the United Kingdom during June–July 2016. Physical and biogeochemical observations were made using a conductivity–temperature–depth (CTD) profiler, five ocean gliders, an Oceanscience Underway CTD (uCTD), a vertical microstructure profiler (VMP), two acoustic Doppler current profilers (ADCPs), Argo floats, drifting buoys, meteorological sensors, and upper-air radiosonde balloons. The observations were made along a zonal section at 8°N b...AbstractThe Bay of Bengal (BoB) plays a fundamental role in controlling the weather systems that make up the South Asian summer monsoon system. In particular, the southern BoB has cooler sea surfac...

Performance of Semianalytical Algorithm and Associated Inherent Optical Properties in Coastal Waters of North Western Bay of Bengal

The present study was carried out with an aim to assess two semianalytical algorithms, Garver–Siegel–Maritorena (GSM) and Generalized IOP (GIOP) and their association with Inherent Optical Properties (IOP) such as phytoplankton absorption coefficient (aph), detritus absorption (adg) and particulate backscattering (bbp). Ten years of Moderate Imaging Spectroradiometer-Aqua (MODISA) data was processed to retrieve Chlorophyll_a (Chl_a) from GSM and GIOP. Subsequently the Rrs from MODISA was inverted to compute aph, adg and bbp. Both the algorithms showed similar trend at an annual scale. However in magnitude, Chl_a from GSM was three times higher than that from GIOP. Further the Chl_a retrieved from GSM was closer to in situ values. The variability of aph and adg was similar at long-term time scale. The peak observed in variability of aph and adg during pre-monsoon was due to the occurrence of bloom whereas the peak during monsoon was due to nutrient discharge from river. The observed peak in bbp during monsoon can be also attributed to the increasing concentration of total particulate matter from river as well as from bottom re-suspension. The poor association of IOP with Chl_a explains the limited accuracy of satellite retrieved Chl_a from these semianalytical algorithms. Hence it is require generating IOP at the regional scale and tuning the semianalytical model for better accuracy.

An Ephemeral Dinoflagellate Bloom during Summer Season in Nearshore Water of Puri, East Coast of India

The present paper reports on the phenomenon of pinkish-red discoloration of the nearshore water of Puri, Odisha on 12th May 2016. Many local newspapers covered this event, as Puri city is a major tourist and pilgrimage place on the east coast of India. Field observations were carried out in order to provide a scientific basis to the event and to elicit possible causes of this discoloration. Taxonomic analysis of the phytoplankton samples revealed the dominance of red colored dinoflagellate species Gonyaulax polygramma, contributing 90% to total phytoplankton population. The localized concentration of G. polygramma was responsible for the pinkish-red discoloration of nearshore water. The exact factor that lay behind the genesis of this bloom could not be delineated due to the short period of its persistence. But two factors - upwelling and anthropogenic nutrient influx - can be viewed as the main cause for this ephemeral bloom. Non-hypoxic conditions in the coastal water following the ephemeral bloom event indicated no significant risk of ecological deterioration to the ambient medium.

Response of phytoplankton community and size classes to green Noctiluca bloom in the northern Arabian Sea

A comprehensive analysis on the phytoplankton ecology with special reference to different phytoplankton size classes was carried out at green Noctiluca scintillans (hereafter Noctiluca) bloom and non-bloom locations in offshore waters of the northern Arabian Sea. At the bloom locations, green Noctiluca represented a dense mono-specific proliferation with average cell density of 10.16 ± 5.806 × 104 cells-L-1 and relative abundance share of 98.63%. Active photosynthesis through prasinophytic endosymbiont was depicted from net community production magnitude reaching 85.26 mgC/m3/Day under low prey abundance. Parallel swarming of Porpita porpita, a voracious copepod feeder signified the competitive advantage of Noctiluca to have the phytoplankton prey. Average concentration of picophytoplankton biomass was eleven times lower in surface waters of non-bloom stations in comparison to bloom. Higher N:P ratio in subsurface waters of non-bloom stations signified non-utilization of nitrogenous nutrients. Green Noctiluca bloom onset subsequent to diatom rich conditions was evident from spatio-temporal ocean colour satellite imageries.

Zooplankton Distribution in Coastal Water off Gopalpur, North-Western Bay of Bengal

An interannual study on zooplankton abundance, biomass, and species composition was carried out during different seasons in two local coastal water types off Gopalpur, north-western Bay of Bengal. Although, Type-1 was observed with higher zooplankton abundance in comparison to Type-2, pattern of variation followed similar seasonal trends in both water types during individual years. Well pronounced seasonality was observed in zooplankton distribution. Zooplankton community was composed of 217 holoplankton and 22 meroplankton. The holoplankton community was predominated by copepod in terms of species diversity and abundance followed by hydrozoa, tintinnida, malacostraca, gastropoda, chaetognatha and chordata. The meroplankton were represented by larval forms viz. bivalve veliger, brachyuran zoea larvae, caridean larvae, copepod nauplii, fish egg and gastropod veliger. Dominance of copepod species viz. Acrocalanus longicornis, Paracalanus aculeatus and Paracalanus parvus were observed frequently in both water types. The species richness was higher in Type-1 in comparison to Type-2 during both the years. Salinity regimes and availability of phytoplankton prey influenced the distribution and species composition of zooplankton assemblage.