Increase in the atmospheric CO2 due to fossil fuel usage, change in agricultural practices and deforestation is balanced by draw down by the land and ocean biota. About 104.9 Pg (1 Petagram = 1 Gigaton= 1015 grams) of carbon is fixed per year by the global biota, out of which about 46.2 per cent (48.5 Pg) is taken up by the oceans alone. Here we show how the Nitrogen-15 (15N) tracer technique can be used to quantify this export flux of carbon into the deep sea.
The Indian Ocean has been identified as a net sink for atmospheric CO2. It takes up nearly 330-430 Tg (tetragram) C (carbon)/year by solubility, accounting for nearly 20 per cent of the global oceanic uptake of CO2. Most of this air-to-sea CO2 flux is temperature driven and occurs south of 20ºS, after this latitude, the sea surface temperature decreases drastically. In contrast, the northern Indian Ocean (north of 35ºS) has been recognised as a net source of CO2 to the atmosphere; with an estimated outward flux of nearly 240 Tg C/year. Biological uptake of CO2 by the Indian Ocean has been estimated to be ~750-1320 Tg C /year. Proposed estimates of CO2 loss from the warmer northern Indian Ocean range from 150 to 500 Tg C /year. The central and eastern Arabian Sea alone contribute an average of ~45 Tg C yr/year to the atmosphere.
Total and new production
Ocean biota mainly consists of single-celled micro-organisms called phytoplankton, present in the upper, sunlit layer of the ocean called the euphotic zone. In the presence of sunlight phytoplankton converts inorganic CO2 into organic carbon through photosynthesis (Fig.1).
The amount of carbon thus fixed by phytoplankton through the synthesis of organic carbon, measured as carbon per unit volume of water per unit time, is termed primary production. Once formed, this organic matter faces the immediate possibility of decomposition back to CO2, phosphate, ammonia and other nutrients through consumption by herbivorous zooplankton and degradation by bacteria. Some portion of the primary production is, however, exported to deeper waters through higher trophic levels. A small part (~1 per cent) might also end up in the sediments. This process by which sinking of organic matter effectively removes CO2 from atmosphere to the deeper ocean is known as the ‘biological pump’.
Availability of sunlight is one of the major limiting factors of primary productivity, as light intensity decreases exponentially with depth. The general limit of light penetration, even in open ocean waters, is approximately 100-150 m. Apart from sunlight, CO2 and H2O, minute quantities of elements such as N, P, Fe, Si etc., are also essential for phytoplankton growth, the absence of which limits photosynthesis and primary production.
Supply of nitrogenous nutrients is considered to be the major limiting factor of oceanic primary production in many regions. Primary production is further partitioned on the basis of the nitrogen source: ‘new’ production supported by nitrate brought into the euphotic zone from the deep, riverine and atmospheric inputs; and, ‘regenerated’ production supported by ammonium and urea, derived from biological processes occurring within the euphotic zone. Ammonium and urea can circulate indefinitely under a quasi-steady state or form an ideal closed system if there is no loss from the phytoplankton population. But there are losses through the sinking of particulate matter, mixing and by predation by zooplankton in the real ocean and therefore other sources of nitrogen are needed. The sum of the losses, is balanced by nitrogen fixation or by any other possible sources of non-regenerated nitrogen.
The ratio of new to total production is called the f ratio (varies from 0 to 1), which defines the strength of the biological pump. It represents the probability that a nitrogen atom is assimilated by phytoplankton due to new production. New and regenerated productivity may be directly measured using the 15N tracer technique. Isotopic ratio of even low concentrations of nitrogen can now be measured with sufficiently high precision using an isotope ratio mass spectrometer with improved electronics, vacuum system and ion optics.
15N studies in northern Indian Ocean
New productivity measurements were carried out in the western and central Arabian Sea under U S Joint Global Ocean Flux Study (US JGOFS) programme. Our group too has been carrying out such measurements in the eastern part of the Arabian Sea independently from 2003 onwards. In addition, we have also completed new production measurements in the Bay of Bengal (Table 1). A significant seasonal and geographical variation in the new production and f-ratios are observed in the Arabian Sea. A large variation in the N-uptake rate, ranging from 0.1 to 13 mmol N m-2/day has also been reported during the spring inter-monsoon and the summer monsoon for the northern Arabian Sea.
Very high new production has been found in the north-eastern Arabian Sea during winter with a lower f ratio, averaging around 0.19. A general trend of spatial increase in the new production from south to north has been observed. In addition, we have found the presence of two different biogeochemical provinces in eastern Arabian Sea during the late winter monsoon: less productive southern and more productive northern regions. This may be the effect of more intense winter cooling towards the north. The southern sector is characterised by low column N-uptake and very low f-ratio.
New production during the pre-monsoon in the Bay of Bengal was higher than that in the post-monsoon (Table 1). Overall, new production for the region during the pre-monsoon was almost twice the average value observed during the post-monsoon period. However, the average f ratio estimated for the entire region increased to 0.70 (±0.1) during pre-monsoon as compared to 0.5 during the post-monsoon.
In the end all the measurements in this and other seas cumulatively will tell us how the export of carbon from the atmosphere is changing. The calculation of a precise rate of global warming will ensure remedial actions accordingly.