Nitrogen and the North East Arabian Sea

By: R Ramesh
Nitrogen an essential macronutrient of the oceanic ecosystem, can limit the biological production if not available in a suitable form or concentration. This article looks at the role of nitrogen in the waters of the Arabian Sea.
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Most plants and bacteria produce energy that the rest of the ecosystem can use, and are the primary producers; and, the rate at which they produce the energy is labelled productivity which plays a crucial role in the global carbon cycle. Roughly half of this productivity occurs in the oceans, by microscopic plants called plankton. Thus, ocean primary productivity (OPP) is defined as the carbon per unit time and unit area set by the ocean phytoplankton’s photosynthesis. New productivity is part of the oceanic primary productivity supported by nitrates, brought into the euphotic (sunlit) zone from deeper waters through a physical processes; while regenerated productivity is productivity supported by ammonium and urea, derived from biological processes occurring within the euphotic zone.

Regenerated production is fuelled by nitrogen recycled biologically in the euphotic zone, ammonium (NH) being the principal form. New production, on the other hand, is fuelled by external sources, primarily in the form of nitrate (NO3) supplied principally by vertical mixing from below the euphotic zone (R C Dugdale and J J Goering (1967); Uptake of new and regenerated forms of nitrogen in primary productivity; Limnol. Oceanogr.).

The availability of the nutrients is influenced by biological and physical processes like upwelling, mixing, advection and diffusion. In the sunlit zone, under a steady state – i.e. the absence of significant lateral transport, the loss of nitrogen through sinking, mixing or predation is balanced by the gain of ‘new’ nitrogen. In addition to the nitrate flux to the euphotic zone, nitrogen fixation can also be a substantial factor in the new production. N2 fixation is particularly important in the tropical and sub tropical oceans.

The relevance of productivity

A study was conducted in the north eastern (NE) Arabian Sea to measure the ‘new’ and ‘regenerated’ productivity in the waters using the 15N tracer technique. The importance of differentiating between the two types of productivity lies in the fact that only new productivity is relevant to the sequestration of carbon from the atmosphere to the deep ocean.

The Study

In the study, 15N based new and regenerated productivity in the north eastern (NE) Arabian Sea has been monitored from 2003 to 2007 using the 15N tracer technique. A variety of geochemical and biological processes are active in this small basin; e.g., high rates of primary productivity are sustained (a) during the winter monsoon due to the entrainment of nutrients to the mixed layer by convective overturning and (b) during the summer monsoon, due to coastal upwelling. Oxygen deficiency in middle layers of the water column is a result of such episodic events of high productivity. Loss of oxidised form of nitrogen to the atmosphere by intense denitrification and anaerobic ammonium oxidation is also observed. Trichodesmium blooms are frequently seen in the area and nitrogen fixation within such blooms can be substantial. In addition to the Trichodesmium blooms, presence of diazotrophic -proteobacteria in the Arabian Sea has also been reported, which are thought to release fixed nitrogen to the ocean water, which then becomes available to the other non-diazotrophic species.

During winter (November–February), dry and cool northeasterly trade winds intensify evaporation and cool the sea surface, triggering sinking and convective mixing of surface water. This mixing leads to the deepening of the mixed layer and an upward transport of nutrients into the surface waters, leading to winter bloom. Satellite based studies reveal that the winter bloom in the region generally appears by the end of February and disappears by the end of March every year.

Fig 1. Sampling locations at which new productivity measurements were carried out during the period 2003 to 2007 in the NE Arabian Sea. The horizontal dashed line separates sampling locations occupied from regions 20-25oN (Region 1) and 10-20oN (Region 2). Sampling periods are shown in the legend.
Fig 1. Sampling locations at which new productivity measurements were carried out during the period 2003 to 2007 in the NE Arabian Sea.
The horizontal dashed line separates sampling locations occupied from regions 20-25oN (Region 1) and 10-20oN (Region 2). Sampling periods are shown in the legend.

 

Some results of the study

New productivity during winter: The possible sources of nitrate during winter are the deepening of the mixed layer, upwelling, and atmospheric deposition. It has been estimated that the contribution of atmospheric deposition to new production could be at best 2.5 per cent of the total. It has been shown that the deepening of mixed layer (not upwelling) is the main cause of the observed entrainment of nitrate and the resultant higher productivity during the season. The mid layer depth (MLD) increases to 100-120 m during winter from its inter-monsoon value of 30-40 m. The calculated excess nitrate input and the observed new productivity values reveal that the former is able to sustain the winter bloom for more than a month. In the later part of winter, the mixed layer starts shoaling, which dwindles the supply of nutrients to the surface. This turns the system back again towards oligotrophic conditions, reflecting inter-monsoon, lower nitrate concentrations and nitrate uptake rates in the mixed layers.

Spatial variation in new productivity during winter: Vertical profiles of temperature and a decreasing trend from south to north in sea surface temperature (SST) during winter suggest that winter mixing is more prominent in the northern Arabian Sea (region between 20 to 25°N hereafter referred to as Region 1) than in the region between 10 to 20°N (hereafter referred to as Region 2) (Fig. 1). Almost 0.5°C per degree latitude decrease from south to north has been reported during this season. The observed deeper mixed layer and higher nitrate in Region 1 relative to those in Region 2 also supports this inference. However, new productivity values in both the regions are comparable. Rixen et. al., in their work titled ‘Deep ocean fluxes and their link to surface ocean processes and the biological pump’ suggested in 2005, that the southern boundary of the monsoon-driven high flux region is situated somewhere between 10°N and 15°N in the open Arabian Sea, which is further south than the boundary demarcated by the current study.

It is difficult to pinpoint the exact reason for the comparable new productivity in both the regions. For new productivity, other than physical processes, regeneration of nitrogen occurring in situ can also be a significant source of nutrients for the surface layer of the ocean. A. Yool et. al. (2007) in the paper, ‘The significance of nitrification for oceanic new production’ published in Nature, suggested that for much of the world oceans a substantial fraction of the nitrate taken up is generated through recent nitrification (the oxidation of ammonium to nitrate mediated by nitrifying bacteria) near the surface. Thus, a significant part of the available nitrate could account for regenerated rather than new production. This can have a significant impact in the calculation of the f-ratio, which is the fraction of the total primary production fuelled by nitrates as against that fuelled by other nitrogen compounds. Ammonium regeneration is another source of regenerated nitrogen to the euphotic zone.

Winds, SST, MLD and mixed layer nitrate show similar variations between these regions. This difference shows that the hydrodynamic and meteorological parameters control new productivity and f-ratios in these regions. These results indicate a significant spatial variability in the upward nitrate supply and in its subsequent uptake during winter in the Arabian Sea.

N-uptake characteristics during spring inter-monsoon: Region 2 was relatively more productive than Region 1. New productivity values observed here are comparable to the values from north west (NW) Arabian Sea and lower than that from other parts of the Arabian Sea. Low wind strength, shallow MLD and low mixed layer nitrate during spring resulted in low productivity in the NE Arabian Sea. The results suggest that a large spatial variation exists in the uptake of different nitrogenous nutrients between the NW, central and NE Arabian Sea.

Endnote

The NE Arabian Sea has a significant potential for higher new productivity during blooms. New productivity is mainly controlled by hydrodynamic and meteorological parameters in the region. Higher f-ratios during blooms point to the strong coupling between surface and sub-surface layers. The input of nitrate varies during the different months of winter. In general, its peak values are centred in December–January. Later, it decreases as the mixed layer starts shoaling. It is found that the entrained nitrate into the surface layers could sustain the observed new production and winter bloom for a period lasting more than a month. Diurnal cycling of the mixed layer could be the reason of the underutilisation of the entrained nitrate during the early winter. Overall, new productivity and f-ratio along with wind speed, SST, nitrate, residence time of nitrate, and chlorophyll a vary intra-seasonally, spatially and from year to year. These variations are mainly limited to the northern part of Arabian Sea. It was observed that the amplitude of sub-seasonal variation is higher than that of the inter-annual variation. During spring, the region returns to its oligotrophic character and exhibits lower nitrate values with highly stratified surface waters.

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