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Remote Sensing

Phytoplankton in the Northeastern Arabian Sea exhibit seasonality

Using a variety of instruments on both ship and satellite, phytoplankton off the coast of India can be monitored as they respond to environmental conditions.
22 March 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0616

The reversal of monsoon winds brings about changes in the circulation pattern of the Arabian Sea1–3 that can cause seasonality in the abundance of phytoplankton, micro-organisms that carry out photosynthesis and are an important part of marine food webs.4 Phytoplankton seasonality is also connected to altered coastal processes related to monsoons.5,6 During the southwest (SW) monsoon, the west coast of India experiences an upwelling of nutrient-rich water. Toward the end of the SW monsoon the offshore component of wind stress recedes, so upwelling does as well. During the northeast (NE) monsoon, surface currents reverse direction, moving northward along the coast carrying warm, low saline, low nutrient waters from the equatorial region. These environmental changes cause phytoplankton seasonality in the Arabian Sea along the west coast of India.

Since the early 1940s, microscopy has been the method of choice for studying phytoplankton diversity.7–14 However, pigment distributions can also be used to verify the presence of different algal groups.15–17 In this study we used a varied sampling strategy both offshore and close to the coast to comprehensively investigate the seasonality of phytoplankton during the SW (June–August)5 and NE (November–February)18–21 monsoons as well as during the inter-monsoon period (March–May) when the water column becomes highly stratified and devoid of nutrients.22

In addition to shipboard data, we also collected information using the Indian Remote Sensing Satellite P4 (IRS-P4), which has an onboard ocean color monitor (OCM). Figure 1 presents the station positions during various cruises. A variety of different methods were used to quantify various parameters, including chlorophyll a (Chl a) by fluorometer,23 pigments by high performance liquid chromatography (HPLC),17 phytoplankton taxonomy by microscopy,24 primary productivity by the 14C technique,23 and OCM data with OC-2 algorithms.25

Figure 1. Location of sampling stations in the Arabian Sea
Table 1. Chlorophyll a at the surface and in the water column
Table 2. Primary productivity at the surface and in the water column
Table 3. Dominant phytoplankton species in the Northeastern Arabian Sea
Results and discussion

Both at the surface and in the water column, the number of phytoplankton of different species varied considerably depending on the time of year. In November cyanobacteria were the major phytoplankton population (69.21%). The average surface counts were high, 15.831× 104L-1, as were column counts (180.69×106m-2). High counts of Trichodesmium  were recorded in the column in December, although surface counts were low. Mainly due to a Noctiluca bloom, January and February counts were moderately high in the water column. The high phytoplankton counts during March and April were due to a Trichodesmium bloom. During April as much as 80% of the phytoplankton were Trichodesmium  species, particularly Trichodesmium erythraeum.

Table 1 presents estimates of the Chl a concentrations during various months. Chl a was high during November (due to cyanobacteria), February (due to Noctiluca), and April and May due to Trichodesmiumerythraeum (see Table 3). Similar trends were observed in the column Chl a. As shown in Table 2, the average primary productivity throughout the water column was low during November, high in December and January, and low again in February. The hike in Chl a (surface and column) recorded in March and April increased the primary productivity throughout the water column (see Table 2).

Figure 2.Distribution of pigments in different months. Chl a: chlorophyll a. Chl b: chlorophyll b. Chl c: chlorophyll c. Fuco: fucoxanthin. ß-caro: ß-carotene. Per: peridinin.

Figure 3.Chlorophyll a from OCM in the NE Arabian Sea in different months.

In November and December, the depletion of nutrients upwelled during the SW monsoon coincided with development of high concentrations, or blooms, of T. thiebautii at the surface in November and at the subsurface in December. Measurements of elevated zeaxanthin and ß-carotene confirmed this (see Figure 2). The ocean color images mirrored the low Chl a concentrations and phytoplankton counts in December (see Figure 3). In January, blooms composed of a mixture of several species of dinoflagellates contributed to the high Chl a observed in the ocean color data and to the high concentrations of peridinin measured by HPLC. This mixed dinoflagellate bloom was followed by an intense N. miliaris bloom in February, which caused the elevated levels of Chl b and Chl c. The symbiotic prasinophyte Pedinomonas noctilucae that N. miliaris harbors as an endosymbiont caused high levels of prasinoxanthin. T. erythraeum blooms occurred in association with the N. miliaris bloom as well as afterward, during March and April. The highest primary productivity, observed during April, was due to Trichodesmium bloom. These results confirm that phytoplankton populations in the Arabian Sea exhibit seasonality due to environmental changes related to monsoons.

We appreciate the encouragement and support of National Institute of Oceanography director S. R. Shetye for this work. This study was carried out under the Ocean Color Project with financial assistance from the Space Application Center (SAC) in Ahmedabad. A SAC Fellowship to S. G. Parab and S. Pednekar is gratefully acknowledged.

S. G. Prabhu Matondkar
Biological Oceanography Division,
National Institute of Oceanography
Panjim, India
Rashmin M. Dwivedi
Marine and Water Resources Group,
Space Application Center
Ahmedabad, India
Sushma Parab, Suraksha Pednekar, Antonio Mascarenhas
National Institute of Oceanography
Dona Paula, India
Mini Raman, Sanjay Singh
Space Application Center
Ahmedabad, India