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The Bay of Bengal is traditionally considered to be a less productive basin compared to the Arabian Sea. We explore the reasons for this in the central Bay during summer. Copious rainfall and river water freshen the upper layers of the Bay by 3–7 psu during summer, and SST was warmer by 1.5–2°C than in the central Arabian Sea. This leads to a strongly stratified surface layer. The weaker winds over the Bay are unable to erode the strongly stratified surface layer, thereby restricting the turbulent wind‐driven vertical mixing to a shallow depth of <20 m. This inhibits introduction of nutrients from below, situated close to the mixed layer bottom, into the upper layers. While advection of nutrients rich water into the euphotic zone makes the Arabian Sea highly productive, this process is unlikely in the Bay of Bengal.

Reliable data on biological characteristics from the Bay of Bengal are elusive. In this paper, we present results on physics, chemistry and biology simultaneously measured

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.

We initiated and mapped a diatom bloom in the northeast subarctic Pacific by concurrently adding dissolved iron and the tracer sulfur hexafluoride to a mesoscale patch of high-nitrate, low-chlorophyll waters. The bloom was dominated by pennate diatoms and was monitored for 25 d, which was sufficiently long to observe the evolution and termination of the bloom and most of the decline phase. Fast repetition-rate fluorometry indicated that the diatoms were iron-replete until day 12, followed by a 4-5-d transition to iron limitation. This transition period was characterized by relatively high rates of algal growth and nutrient uptake, which pointed to diatoms using intracellularly stored iron. By days 16-17, the bloom was probably limited simultaneously by both iron and silicic acid AcknowledgementsWe thank the captains, crews, and participating scientists onboard the vessels John P Tully, El Puma, and Kaiyo Maru during this study. We also thank Bill Crawford, Sheila Tows, and Frank Whitney (Institute of Ocean Sciences, Sidney, Canada) for shore-side logistical support. We acknowledge the support of Maurice Levasseur (University of Laval, Quebec, Canada) in providing unpublished data for this manuscript. We thank NASA and Orbimage for the provision of SeaWiFS satellite images presented in Figs.