R. K. Sarangi

Cover: Remote sensing of Trichodesmium blooms in the coastal waters off Gujarat, India using IRS‐P4 OCM

Abundant phytoplankton blooms can usually be recognized from far away due to discoloration of the sea surface. Occasionally the algae grows very fast or ‘blooms’ and accumulates into dense, visible...

Remote Sensing Observations on Impact of Phailin Cyclone on Phytoplankton Distribution in Northern Bay of Bengal

Remote sensing-based analysis has been carried out to study the impact of “Phailin” cyclone on ocean phytoplankton distribution off Odisha coast and on the northern Bay of Bengal water. Oceansat-2 Ocean Colour Monitor (OCM) and MODISTerra sensors-derived chlorophyll images have been generated over the study area during October 2013. There has been observation of drastic change in the chlorophyll concentration of the north-western Bay of Bengal water with effect of the cyclone “Phailin,” which hit Gopalpur, Odisha coast on 12 October night at around 21:00 h IST. The postcyclone images of Oceansat-2 OCM data and retrieved chlorophyll concentration were observed to be very high, 3.0-4.0 mg/m3 in coastal water, which was around 0.5-1.0 mg/m3 during precyclone. Similarly, the postcyclone chlorophyll was around 0.80-1.50 mg/m3 in the offshore water, unusually high compared to precyclone concentration (<;0.60 mg/m3). The cyclone track of India Meteorological Department (IMD) has been superimposed on chlorophyll images and observed as proxy along the chlorophyll front with algal bloom features. There has been observation of cooling in the northern Bay of Bengal and off Odisha coast with impact of the induced upwelling, entrainment and mixing of the water column, as evident from the sea surface temperature (SST) data analysis. Interesting features such as cold core eddies and fronts have been visualized. This study is important to assess the impact of Phailin cyclone on ocean productivity on the local and basin scale and its deviation from seasonal trend as well.

Vertical diffuse attenuation coefficient (Kd) based optical classification of IRS-P3 MOS-B satellite ocean colour data

The optical classification of the different water types provides vital input for studies related to primary productivity, water clarity and determination of euphotic depth. Image data of the IRSP3 MOS-B, for Path 90 of 27th February, 1998 was used for deriving vertical diffuse attenuation coefficient (Kd) and an optical classification based onKd values was performed. An atmospheric correction scheme was used for retrieving water leaving radiances in blue and green channels of 412, 443, 490 and 550 nm. The upwelling radiances from 443 nm and 550 nm spectral channels were used for computation of vertical diffuse attenuation coefficientKd at 490 nm. The waters off the Gujarat coast were classified into different water types based on Jerlov classification scheme. The oceanic water type IA (Kd range 0.035-0.040m-1), type IB (0.042-0.065 m-1), type II (0.07-0.1m-1) and type III (0.115-0.14m-1) were identified. For the coastal waters along Gujarat coast and Gulf of Kachchh, Kd(490) values ranged between 0.15 m-1 and 0.35 m-1. The depth of 1% of surface light for water type IA, IB, II and III corresponds to 88, 68, 58 and 34 meters respectively. Classification of oceanic and coastal waters based onKd is useful in understanding the light transmission characteristics for sub-marine navigation and under-water imaging.

Inter-annual and seasonal variations in hydrological parameters and its implications on chlorophyll a distribution along the southwest coast of Bay of Bengal

Seasonal and inter-annual variability of hydrological parameters and its impact on chlorophyll distribution was studied from January 2009 to December 2011 at four coastal stations along the southwest Bay of Bengal. Statistical analysis (principal component analysis (PCA), two-way analysis of variance (ANOVA) and correlation analysis) showed the significant impact of hydrological parameters on chlorophyll distribution in the study area. The ranges of different parameters recorded were 23.8–33.8°C (SST), 4.00–36.00 (salinity), 7.0–9.2 (pH), 4.41–8.32 mg/L (dissolved oxygen), 0.04–2.45 μmol/L (nitrite), 0.33–16.10 μmol/L (nitrate), 0.02–2.51 μmol/L (ammonia), 0.04–3.32 μmol/L (inorganic phosphate), 10.09–85.28 μmol/L (reactive silicate) and 0.04–13.8 μg/L (chlorophyll). PCA analysis carried out for different seasons found variations in the relationship between physico-chemical parameters and chlorophyll in which nitrate and chlorophyll were positively loaded at PC1 (principal component 1) during spring inter-monsoon and at PC2 (principal component 2) during other seasons. Likewise correlation analysis also showed significant positive relationship between chlorophyll and nutrients especially with nitrate (r=0.734). Distribution of hydrobiological parameters between stations and distances was significantly varying as evidenced from the ANOVA results. The study found that the spatial and temporal distribution of chlorophyll was highly dependent on the availability of nutrients especially, nitrate in the southwest Bay of Bengal coastal waters.

Chlorophyll — A Concentration Along West Coast of India Using IRS-P3 MOS-B Data

Remote sensing of ocean colour yields information on the constituents of sea water, such as the concentration of phytoplankton pigments, suspended sediments and yellow substances. It is well understood that the study of ocean colour is significantly related with the primary production and zonation of potential fishing sites in coastal and oceanic waters. The major pigment constituent is predominated by chlorophyll-a (ocean colour pigment of phytoplankton). The chlorophyll mapping on regular basis plays a major role in assessing water quality and classifying different water types. IRS P-3 MOS-B satellite data for three consecutive passes of path 94, during the period of January-February 1997 have been used to derive chlorophyll-a concentration. The present study emphasizes on the chlorophyll mapping using IRS-P3 MOS-B data for the coastal and offshore water of Maharashtra coast, India.

Spatiotemporal variability of MODIS-Aqua-derived aerosol and its impact on surface chlorophyll-a in the Indian coastal and offshore waters

Abstract Observation was made to understand the spectral behavior of aerosol from the NASA moderate resolution imaging spectroradiometer (MODIS-Aqua) sensor data. The 869 nm central wavelength band MODIS-Aqua-derived images were retrieved and interpreted. The aerosol concentration was found to be higher during December (Northeast monsoon) over the Bay of Bengal (BoB). Its characterized distinct plume with higher aerosol optical depth (AOD) ranged between 0.05 and 0.25. The coastal aerosol concentration was higher than the offshore region, with decreasing trend ( AOD ∼ 0.05 ). To understand aerosol’s link with ocean biological productivity in terms of phytoplankton pigment, the ocean surface chlorophyll was retrieved from 8-years monthly datasets (2002 to 2010). MODIS-Aqua-derived monthly scale aerosol and chlorophyll data were analyzed in the northern Indian Ocean (NIO) region during July 2002 to June 2010. Their interrelationship was found to be significant ( r 2 = 0.663 and 0.584) over pockets in the BoB and Arabian Sea. Land to seaward movement of wind and aerosol plays a significant role to increasing ocean productivity. This study is very important and interesting to understand the precise composition of aerosol dust like iron (Fe) and increase in sulfate ions, nitrates, etc., and their link to phytoplankton biomass and the related productivity in the Indian water and attempted for the first time in the NIO water.

Impact assessment of the Japanese tsunami on ocean-surface chlorophyll concentration using MODIS-Aqua data

National Aeronautics and Space Administration (NASA) moderate resolution imaging spectroradiometer (MODIS) Aqua-retrieved chlorophyll concentration data sets have been analyzed and interpreted, as have changes in chlorophyll concentrations associated with the tsunami that occurred on March 11, 2011, in Sendai, Tohoku (latitude 38.3°N, longitude 142.4°E). The eight-day and three-day composite data sets were archived from the NASA MODIS website to generate chlorophyll images during March 2011, before, during, and after the tsunami. The chlorophyll concentration variability was studied with reference to satellite images, a transect line, eight points, and mean plots over two subsets in the coastal and offshore water. An increase in chlorophyll concentration of up to 5.0     mg / m 3 along the transect line and in the range of 1.0 – 4.3   mg / m 3 in the coastal water points and subset ( 1 0 × 1 0 ) was observed. The chlorophyll mean ranged from 0.289 to 1.237     mg / m 3 and 0.123 to 0.271     mg / m 3 for subsets 1 and 2, respectively. The offshore water subset was about 300 km from the tsunami site, and the chlorophyll range has not shown more gradients in post-tsunami images, as was evident from subset-2 mean data. Data from March 2010 and 2009 was processed for the same locations to comply with any seasonal trend. But the recent 2011 tsunami had a sudden episodic impact on the coastal water of Japan, enhancing chlorophyll concentration without being tallied with seasonal trends. So, the episodic effect seemed to be significant instead of being a seasonal trend. As observed from field photographs and previous tsunami events, water receded from the land, along with a tsunami return wave that probably carried nutrients and minerals, and even upwelling would have enriched the phytoplankton growth and sudden chlorophyll concentration increase in the post-tsunami period found convincing in satellite images. The anomaly of chlorophyll observed was significant, with p < 0.02 during 2011. The interannual study does not support the drastic change than the non-tsunami years and significance is not seen with p -values observed from t -testing. So the catastrophic and episodic event has had an impact on the ecosystem of Japanese water, enhancing the ocean’s chlorophyll biomass, and hence, productivity. This is the first attempt to study the chlorophyll variability of the effects of a tsunami on the Japanese coast.

Time-series analysis of chlorophyll-a, sea surface temperature, and sea surface height anomalies during 2003–2014 with special reference to El Niño, La Niña, and Indian Ocean Dipole (IOD) Years

ABSTRACT The present study was carried out to find the variability of chlorophyll-a (chl-a) concentration, sea surface temperature (SST), and sea surface height anomalies (SSHa) during 2003–2014, covering the Bay of Bengal (BoB) and Arabian Sea (AS) waters. These parameters were linked with El Niño, La Niña, and Indian Ocean Dipole (IOD) years. The observed results during 2003–2014 were evaluated and it was found that the monthly mean value for 12-year data ranged as follows: chl-a (0.11–0.46 mg m−3), SST (27–31 °C), and SSHa (−0.2 to 20 cm). The annual mean range of chl-a for 12-year data was 0.1–0.23 mg m−3, the SST range was 27–28 °C, and the SSHa range was 2.14–13.91 cm. It has been observed that with the SST range of 27–28 °C and the SSHa range of 7–9 cm, the chl-a concentration enhanced to 0.20–0.23 mg m−3. With a higher SST range of 28–29 °C and with a positive SSHa range of 11–14 cm, the chl-a concentration appeared to be low (0.17–0.18 mgm−3). During normal years, SSHa was positive with the >5 to <10 cm range during the months of April–June, which coincided with an increase in SST, >2 to <4 °C. During the normal years, SSHa (>−0.2 to <−10 cm) was observed to be negative during October–December, with a decrease in SST (<3 °C) observed. The high monthly mean chl-a concentration (>0.3 to <0.5 mg m−3) was noticed during December–February in the BoB and AS. Compared to the BoB chl-a range (<0.4 mg m−3), a high chl-a concentration was observed in AS (>0.4 mg m−3). However, during the phenomenon years, the study area had experienced low chl-a (<0.2 mg m−3), high SST (>5 °C), and more positive SSHa (>10 to <20 cm) during January–March and October–December in AS and BoB. The present study infers that a positive IOD leads to low chl-a concentration (<2 mg m−3) and low primary productivity in AS. El Niño caused the down-welling process, it results in a low chl-a concentration (<1 mg m−3) in BoB and AS. La Niña caused the upwelling process, and it results in a high chl-a concentration (>2.0 mg m−3) in BoB and AS. In the recent past years (2003–2014), the intensity and frequency of El Niño, La Niña, and IOD have been increasing, evidenced with few studies, and have impacts on the Indian Ocean climate. Therefore, the influences of the relative changes of these phenomena on the BoB and AS need to be understood for productivity assessment and ocean state monitoring.