The oceanic phytoplankton play an important role in linking the Southern Ocean pelagic ecosystem to the global biogeochemical cycle and modulating climate change. Phytoplankton is the base of the oceanic food web, supplying organic matter for all other organisms in the marine environment. They are responsible for contributing more than 50 per cent of the oxygen in the world’s atmosphere that we utilise for breathing. Even though the oceanic phytoplankton account for less than 1 per cent of the photosynthetic biomass on the earth, they contribute roughly 50 per cent of the world’s total primary production (combining ocean and land) by transforming nearly 45-50 billion tonnes of inorganic carbon into their cells (Field et al. 1998), which upon death and decay, sink down to be deposited as seafloor sediments. The process is known as the ‘biological pump’. The phytoplankton and other organisms in the sunlit zone pump about 15 per cent of the organic material synthesised each year to the deep ocean. Once it reaches the deep ocean, ~0.1 per cent of it gets trapped in sediment becoming a source of fossil fuel in the timescale of several million years (Laws et al. 2001). If the upper ocean biological pump stops pumping carbon down to the ocean interior, atmospheric levels of CO2 would in time rise by another 200 ppm thereby accelerating global warming further (Falkowski, 2012). Each oceanic province of the world ocean is unique in influencing the global climate with their implication for drawing down the atmospheric CO2.
The Southern Ocean is known as the largest high-nutrient low-chlorophyll (HNLC) region of the world’s oceans, playing a significant role as a sink for atmospheric CO2. Mesoscale iron fertilisation investigations revealed strong influence of micronutrient iron on phytoplankton biomass and community composition in the Southern Ocean (Coale et al. 2004). Direct deposition of micronutrient iron from mineral dust is low into the Southern Ocean. There are locations in the vicinity of islands where iron from islands and surrounding shallow plateau enhances phytoplankton biomass (Venables et al. 2010) owing to Island Mass Effect (IME). This can contribute to the productivity and fishery resources in and around islands and also for global CO2 budget. Satellite remote sensing offers a great scope to explain the phytoplankton blooms of the Southern Ocean on a regional scale.
Field observations are sparse around the Southern Ocean because of adverse weather condition, navigational hazards, remote location, and inhospitable environment with high sea states because of the strong winds. Satellite remote sensing of the Southern Ocean colour provides synoptic and time series coverage of near surface chlorophylla-a concentration dynamics affected by seasonality. The synoptic coverage of chlorophyll-a concentration is widely used to investigate the dynamics of regional oceanographic features such as fronts, eddies, gyres, upwelling zones, plumes and surface current patterns. These oceanographic features are useful to study the Southern Ocean ecosystem which supports large assemblages of phytoplankton, zooplankton, seabirds, seals, and whales. This signifies the need for the retrieval of chlorophyll-a from ocean colour sensors with greater accuracy over the Southern Ocean. However the retrieval is complicated at regional and local scales as the spectral inherent optical properties (IOPs) of the ocean influencing the ocean colour are complex.
Phytoplankton bloom in the Southern Ocean was first reported by Hart (1942) and subsequently was monitored effectively using the satellite remote sensing images because of its synoptic, receptivity, and multispectral characteristics (Jena, 2016). Analysis of Aqua-Moderate Resolution Imaging Spectro-looradiometer (MODIS) data over the Southern Ocean shows that the monthly evolution of phytoplankton bloom areal extent varies from 1.1 to 18.1 million sq km. In general, the bloom extent was minimal during the austral or southern winter (1.1 to 1.4 million sq km) and was ascribed to light limitation possibly caused by lower solar elevation angle, cloudiness, and intensive sea ice coverage. The extent moderately increased during austral spring (2.7 to 8.8 million sq km), reaching at its peak during austral summer (13.8 to 18.1 million sq km), which could be attributed to the conducive environment with the availability of optimum light condition (Venables and Moore, 2010), sedimentary source of iron from shallow bathymetric region (<1 km) that fertilises the sunlit zone enhanced continental dust advection and supply of iron from the marginal ice zone through sea ice melting during the austral summer. Subsequently, the extent declined in austral autumn (2.7 to 7.3 million sq km), possibly due to decrease of light availability for phytoplankton growth. The Southern Ocean contributes nearly 60 per cent of global ocean phytoplankton blooms during December and January (austral summer). The dominant region of bloom occurrence is located in the Atlantic sector of the Southern Ocean, followed by the Pacific and Indian sector (Fig. 1). The satellite observations provide evidence of phytoplankton blooms resulting from natural sources (mainly in the coastal/shelf waters, downwind of Patagonia and Australia/New Zealand).
In addition, the wind controls the phytoplankton blooms in the Southern Ocean by mixing the water column, induces upwelling, and modifies the light availability. The variability of Chlorophyll-a concentration is known to peak at wind speeds of 5 m per second and generally declines as wind speed increases (Fitch and Moore, 2007). The increase in wind speed leads to deepening of mixed layer, and thereby a decrease in mean light level available for phytoplankton photosynthesis. The mechanism explains the strong inverse relationship between wind speed and bloom occurrence in the Southern Ocean, because the light-limited phytoplankton in a deeper mixed layer would be less likely to actually bloom, achieving high Chlorophyll-a concentrations.
The role of dust also has a major contribution for the phytoplankton bloom formation in the Southern Ocean. For example, the widespread observed bloom in the Atlantic sector (Fig. 1) could be ascribed to availability of iron-rich dust input from Patagonia desert (Gasso and Stein, 2007), in addition to the sedimentary source of iron from the islands and surrounding plateau (Venables and Moore, 2010). The very location of Patagonia desert at the southern end of South America is influenced by the prevailing strong westerly winds, due to the presence of the Andes mountain range. Low pressure events move northward into the land mass frequently during spring and summer, generating storms with wind speeds exceeding 70 km per hour (Gasso and Stein, 2007). Generally, the dust advection from Patagonia is more frequent during austral summer that leads to the observed blooms condition in the Atlantic sector (Fig. 1).
However, in absence of any major source of dust input, the sedimentary source of iron from shallow topography or from the Islands plays a major role in explaining the observed blooms in the vicinity of the Crozet and Kerguelen Islands and its surrounding plateau (Fig. 2). These Islands are situated in the Polar Frontal Zone of the Southern Ocean and are characterised by occurrence of distinct annual phytoplankton bloom in the waters surrounding it.
Phytoplankton – Endnote
The satellite derived chlorophyll-a concentration in the Southern Ocean has some systematic uncertainties owing to its different bio-optical characteristics, in addition to sparse match-up observations (in situ and satellite) being used for algorithm development and validation. An optimised bio-optical algorithm is required on the basis of absorption coefficient to account for the differences in bio-optical characteristics for different groups of phytoplankton. The algorithm is essential mainly for the monitoring of diatom dominated phytoplankton blooms and for precise estimation of the oceanic role in the biogeochemical cycle of carbon, specifically the importance of the Southern Ocean as a sink for atmospheric CO2. Continuous long-term ocean colour observations are needed for regular monitoring of Antarctic marine ecosystem.
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