The Bay of Bengal is traditionally considered to be a less productive basin compared to the Arabian Sea. Despite the contrasting chlorophyll and primary productivity pattern, sediment trap data shows that annual fluxes of organic carbon reach comparable rates in both the basins. The traditional mechanisms of nutrient supply to the upper ocean waters cannot account for this. We propose eddy pumping as a possible mechanism of vertical transfer of nutrients across the halocline to the oligotrophic euphotic zone during summer monsoon when upper ocean is highly stratified. This would induce rapid biological uptake and in turn significantly increase biological production. In the northern Bay, riverine input acts as an additional source of nutrients and augments the subsurface nutrient injection to the euphotic zone by eddy pumping. Notwithstanding this, the lower than expected primary production in the north suggests the possible role of riverine sediment in limiting the sunlight for photosynthesis.
[1] Lake Superior has exhibited a continuous, centurylong increase in nitrate whereas phosphate remains at very low levels. Increasing nitrate and low phosphate has led to a present-day severe stoichiometric imbalance; Lake Superior's deepwater NO 3 À :PO 4 3À molar ratio is 10,000, more than 600 times the mean requirement ratio for primary producers. We examine the rate of [NO 3 À ] increase relative to budgets for NO 3 À and fixed N. Nitrate in Lake Superior has continued to rise since 1980, though possibly at a reduced rate. We constructed whole-lake NO 3 À and N budgets and found that NO 3 À must be generated in the lake at significant rates. Stable O isotope results indicate that most NO 3 À in the lake originated by in-lake oxidation. Nitrate in the lake is responding not just to NO 3 À loading but also to oxidation of reduced forms of nitrogen delivered to the lake. The increasing [NO 3 À ]:[PO 4 3À ] stoichiometric imbalance in this large lake is largely determined by these in-situ processes.
Using in situ data collected during 1992-1997, under the Indian programme of Joint Global Ocean Flux Study (JGOFS), we show that the biological productivity of the Arabian Sea is tightly coupled to the physical forcing mediated through nutrient availability. The Arabian Sea becomes productive in summer not only along the coastal regions of Somalia, Arabia and southern parts of the west coast of India due to coastal upwelling but also in the open waters of the central region. The open waters in the north are fertilized by a combination of divergence driven by cyclonic wind stress curl to the north of the Findlater Jet and lateral advection of nutrient-rich upwelled waters from Arabia. Productivity in the southern part of the central Arabian Sea, on the other hand, is driven by advection from the Somalia upwelling. Surface cooling and convection resulting from reduced solar radiation and increased evaporation make the northern region productive in winter. During both spring and fall inter-monsoons, this sea remains warm and stratified with low production as surface waters are oligotrophic. Inter-annual variability in physical forcing during winter resulted in one-and-a-half times higher production in 1997 than in 1995.
Situated in similar latitudes and subjected to similar atmospheric forcing, the tropical basins of the Arabian Sea looses fresh water due to excess evaporation over precipitation while Bay of Bengal receives freshwater via excess rain and river run off. The hydrological imbalance thus created on an annual scale will have to be balanced by the inter‐basin exchange. In winter this happens through the intrusion of Bay of Bengal waters into the Arabian Sea, when the southward flowing East India Coastal Current carrying low salinity waters from the northern Bay feeds into the West India Coastal Current flowing north along the shelf in the Arabian Sea. Advection of nutrients by this intrusion triggers enhanced levels of chlorophyll near the southern part of the western shelf of India and may play a role in altering the biogeochemistry of this intense hypoxic region.
Phytoplankton blooms mediated by the oceanic supply of nutrients is a well-understood phenomenon in the Arabian Sea (AS), while the role of dust deposition in enhancing phytoplankton is less explored. In this paper, we show that during winter monsoon the central Arabian Sea (CAS), away from the realm of active winter convection, supports episodic phytoplankton blooms. These blooms cannot be fully explained by the oceanic input of nutrients through processes such as advection and mixing in the upper ocean. Using satellite images, we tracked about 45 dust storms over the AS during the winter monsoons of 2002-2003 to 2010-2011 of which only eight were followed by chlorophyll enhancements. We used a regional climate model to get possible fluxes of dust and the amount of nutrients (nitrate, phosphate, and iron) that can be derived from the dust depositions. Additionally, we used published in situ nutrients data in conjunction with carbon: nitrogen: phosphorus and iron: carbon molar ratios to compute the potential requirements of different nutrients for the eight cases of chlorophyll enhancements. It is likely that the deepening of the mixed layer can incorporate nitrate and phosphate, but not enough iron from the subsurface waters leading to potential iron limitation. Although, all the phytoplankton blooms within CAS were observed following episodic dust events, only four blooms can be attributed to dust depositions. Our work shows that phytoplankton blooms fueled by episodic dust storms are important in driving the interannual variability in chlorophyll in a region away from active winter convection.
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