Phytoplankton (chl-a) Biomass Seasonal Variability in the Gulf of Mannar, South India: A Remote Sensing Perspective

Abstract

The phytoplankton is being a primary producer that lies at the base of food web for aquatic flora and fauna of marine and coastal ecosystems. The study describes remote sensing applications for assessment of phytoplankton (chl-a) biomass variability and its major influencing factors (sea surface temperature, salinity, waves and currents) in the Gulf of Mannar (GoM), southeast coast of India. Multi-temporal Landsat ETM+ images acquired on 2016 and 2017 are used for mapping assemblage of phytoplankton (chl-a) and its concentration at site-specific level. Band combination analysis along with mathematically derived coefficient of determination values has been used to extract phytoplankton (chl-a) concentration using spectral reflectance properties (0.4–10 μm). Multispectral images acquired on different times are used for the mapping of phytoplankton concentration with its seasonal variability. Highest assemblage of phytoplankton (chl-a) is measured at concentration of 48.8 μg/L, and it reflects 0.25% at wavelength of 0.55 μm (green) and 1.0 μm (SWIR), respectively. Coastal water comprises higher chl-a concentration which can observe majority wavelength in blue (0.45 μm) and red (0.65 μm) that distinguish phytoplankton from coastal water. The result reveals that chl-a concentration has significantly decreased to 36.20 μg/L, and this reflects 0.16% of wavelength at 0.55 μm (green). The chl-a concentration is further decreased to 33.33 μg/L, and it reflects 0.17% at the wavelength of 0.55 μm. Seasonal assessment shows higher chl-a concentration during pre-monsoon. Coastal water in shallow depth (<5.0 m) area has estimated higher chl-a concentration within a distance of 1.0 km from the shore during post-monsoon. However, the chl-a concentration has significantly decreased in monsoon due to highly fluctuating hydrodynamic conditions that reduce availability of nutrients. Spatial variability of chl-a assemblage is mainly regulated by changing salinity and sea surface temperature. The coastal waters with a salinity level of 33.67 psu (practical salinity unit) at the temperature of 26.44 °C is found favour higher phytoplankton concentration (48.88 mg/m3) in the post-monsoon, whereas considerable reduction of primary production to 17.10–33.33 mg/m3 at the salinity level of 33.0–34.5 psu and this has been observed during monsoon. The phytoplankton concentration increases to 26.85–36.20 mg/m3 at the salinity of 34.0–35.5 psu with the optimum temperature range of 28.0–30.5 °C during pre-monsoon. Sea surface temperature (SST) involves growth and productivity of phytoplankton (chl-a) in coastal waters. The phytoplankton productivity increases up to 33.33–48.48 mg/m3 in the coastal water, and SST ranges from 26.0 to 28.0 °C; however, it is decreased to 22.5–24.5 mg/m3 at the SST level of 28.5–30.0 °C. Growth and productivity of phytoplankton have increased in various parts during post-monsoon than monsoon and pre-monsoon because of the occurrence of optimum SST and salinity in coastal water and prevailing favourable hydrodynamic forces and climatic conditions. It is observed that phytoplankton (chl-a) concentration is gradually decreased with an increase of depth and distance in the Gulf of Mannar region.