International Journal of Marine Science2016, Vol.6, No.50, 1-9
2
into organic matter. As a constituent of phytoplankton tissue, the fixed carbon may sink, in part, out of the surface
mixed layer; may be transferred in part to higher levels; may be recycled back to carbon dioxide through
respiration. The amount of organic biomass produced per unit area per unit time is called the primary production.
The rate of primary production is one of the major factors controlling the rate at which carbon dioxide will be
driven from the atmosphere into the ocean. The current estimates of primary production by phytoplankton in the
global ocean is between 30-50 gigatonnes of carbon annually which is about 40% of the total global carbon
fixation (Falkowski, 1994; Sakshaug, et al., 1997).
2. Significance of Ocean Biological Deserts
The ability of oceans to sequester large amounts of atmospheric CO
2
through the biological pump has generated
considerable interest in formulating mitigation strategies for reducing the rising concentrations of atmospheric
CO
2
. One such mitigation strategy that has spawned several investigations in recent years is the artificial
fertilisation of oceanic waters characterised by very low phytoplankton biomass. Regions of ocean that contain
low phytoplankton biomass indexed as chlorophyll-concentrations are called “ocean biological deserts”.
The concept of ocean fertilisation is based on the effectiveness of the biological pump to sequester large amount
of atmospheric carbon into deep layers of the oceans. It is widely accepted that the glacial-interglacial cyclic
changes in the CO
2
content were caused by changes in the efficiency of the biological pump (Martin, 1990). As
stated earlier, large phytoplankton populations are supported in sunlit zones enriched by nutrient availability
(Dugdale, 1976). This enrichment or fertilisation occurs naturally when upwelling or vertical mixing entrains
nutrient-rich water to the surface. Fertilization also occurs when weather carries wind-blown dust containing small
amount of iron long distances over the ocean, or iron-rich minerals are carried into the ocean by glaciers, rivers
and icebergs. Due to low solubility under oxidising conditions and not readily bio-available, iron limitations in
certain areas of oceans were known to limit photosynthesis and growth of phytoplankton biomass (Martin et al.,
1991). Further studies indicated that scarcity of iron as a micro- nutrient indeed limited the phytoplankton growth
and overall productivity in these desolate zones, later termed as HNLC (High Nutrient Low Chlorophyll) regions.
About 20% of world’s surface ocean belongs to HNLC regions where macronutrients remain largely unutilised
and low biomass and chlorophyll concentrations prevail. These regions comprise large part of Southern Ocean,
Equatorial Pacific and part of North Pacific.
Ocean fertilisation experiments in macronutrient rich but biologically impoverished (HNLC) regions for
sequestration of carbon dioxide were started in early nineties (Martin et al.,1991). The first “proof of concept”
trial experiment on ocean fertilisation Iron EX1, was carried out near the Galapagos Islands in 1993
demonstrating the generation of phytoplankton bloom by iron addition. A patch of ocean (64 km
2
) near the
Galapagos Islands fertilized with iron sulphate into the surface ocean waters for 18 days produced an intense
bloom causing a large drawdown of carbon dioxide. Since then, several international research teams have
completed eleven ocean trials confirming the iron fertilisation effects (Jin et al., 2008). The most recent open
ocean trial of ocean iron fertilization, dubbed LOHAFEX, was conducted from January to March 2009 in the
South Atlantic.
The surface waters of sub-tropical and tropical oceans have very low concentrations of macro nutrients resulting
in low phytoplankton biomass and are characterised by low rates of primary production and export of particulate
organic carbon (POC) to the deep ocean. These regions are termed as low nutrients low chlorophyll (LNLC) areas
because the magnitude of fluxes in the carbon cycle of these habitats is determined by the supply of inorganic
macronutrients. In the carbon sequestration potential, LNLC areas correspond to the global ocean minima.
However, LNLC regions are important in the global marine carbon export budget since they occupy
approximately 50% of the ocean (Cullen and Boyd, 2008). These oceanic deserts are deficient in one or more
micro or macro nutrients and are classified as HNLC (High nutrient low chlorophyll) and LNLC (low nutrient low
chlorophyll) region.
