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Southern Ocean

The Southern ocean is the largest high nutrient low chlorophyll (HNLC) region in the global ocean. The surface waters of the Southern Ocean have been widely identified as being replete in macronutrients (nitrogen and phosphorous) that phytoplankton are unable to utilize fully. []. The high macronutrient concentration in this ocean is attributed to large upwellings of nutrient rich deep water [] primarily due to ekman transport [] as a part of the global circulation Global ocean circulation. But phytoplankton do not utilize the macro nutrients completely as their respiration is limited by micro nutrients like iron [; ] that are available only in low concentrations due to low dust inputs and low solubility of iron which drops the iron:nitrate ratio in the upwelled water below the threshold value required by the phytoplankton [; ], thus restraining their growth, as made manifest by generally low concentrations of chlorophyll a (chl a) This leads to a high nutrient low chlorophyll situation in these regions.

But some parts of the Southern Ocean appear to have adequate available iron concentrations to support strong biological blooms. Major locations in the Southern Ocean exhibiting where this occurs are Crozet, Kerguelen islands and downstream of South Georgia []. There have been numerous studies to understand the underlying processes behind this []like CROZEX project ( CROZet natural iron bloom and Export) [] at the Crozet Islands, Project KEOPS 1 in the wake of the Kerguelen plateau [] and explorations in the Southern Drake Passage region [].The common factor among these three areas is that they are located in the vicinity of the shelf regions of antarctic continent and islands on the Southern Ocean. The micronutrients required for the algal blooms is believed to be acquired from the shelves themselves.[].

Temperature in the Southern Ocean increases outwards from the Antarctic continent where the surface waters are as cold as -2°C towards the outer boundaries of the ocean where the temperature is of the order of 10°C and more. In contrast to this, variation in salinity across the surface waters is less but the minor differences in temperature and salinity set up gradients that drive the Southern Ocean currents along with effects wind. Antarctic Circumpolar Current flowing east, which is the world's largest current is the dominant current in the Southern Ocean. There are also smaller, westward flowing currents along the continental shelf. Due to the low surface temperatures at the Southern Ocean, salinity gradient is the major driver of vertical circulation at these latitudes. These dense waters slowly flow north towards the Pacific as a part of the global ocean circulation that transports heat and dissolved compounds such as carbon dioxide and oxygen.[]

There are 4 different water masses in the southern ocean, namely: Antarctic Surface Water which is found at the surface between the Antarctic continent and the Polar Front; Antarctic Bottom Water which is formed around Antarctica and is the densest water is found at the bottom of the ocean; North Atlantic Deep Water which is a water mass that forms in the North Atlantic Ocean and flows into the Southern Ocean along the global ocean conveyor belt, where it mixes to become part of Circumpolar Deep Water; Antarctic Intermediate Water, with relatively low salinity and found mostly at intermediate depths (around 1000 m), Circumpolar Deep Water which is relatively warm and saline, penetrates the continental shelves and occurs all around Antarctica. It is often separated into two, Upper and Lower Circumpolar Deep Water. It forms most of the water circulating in the Antarctic Circumpolar Current. []

The role of light penetration as a cause of HNLC regions in the Southern Ocean has also been analysed by studying the effects of mixed-layer averaged light availability on phytoplankton concentrations in High Nutrient High Chlorophyll areas and transferring these results to study the potential impacts of light limitation in HNLC areas. It was proven that light limitation was not a significant constraint on the phytoplankton concentration in these regions. []

The majority of the macro nutrients present in Southern ocean is brought up by the upwelling deep water. Besides Iron, there are other micronutrients that possibly affect the growth of phytoplankton in the southern ocean. The most important of them are Zinc and Cobalt. But Iron still remains the pivotal micronutrient for phytoplankton growth and community structure in the region. []

The primary source of Iron in the surface waters of the Southern Ocean is the upwelling deep water. Thus it has been hypothesised that the iron inputs, and in turn, the primary production is sensitive to changes in iron inputs to the North Atlantic. Studies even suggest that a major portion of the iron in the Southern Ocean could be derived from Saharan dust deposited in the Atlantic Ocean. []

Micronutrient deficiency severely limits productivity in the Southern Ocean, except in areas close to the Antarctic shelf where micronutrients are believed to be obtained from the shelf itself. There are a number of fish species in these waters, ranging from krill to multiple whale species. [] For phytoplankton, availability of iron not only regulates productivity, but also their community structure[].The low temperatures prevailing at the Southern Ocean is also believed to have a negative impact on the growth rate of Algae. Grazing by Herbivores such as Krill, Copepods and Salps are also believed to contain the algae population in these regions. Primary production is very intense and short lived in polynyas and permanent sea-ice zones. During this period, the zooplankton biomass is very low; therefore, most phytoplankton and other species of algae are not consumed and sink to the bottom, sustaining the benthic population which in turn become food sources for Penguins and Weddell seals[].