Mixed layer variability and chlorophyll <i>a</i> biomass in the Bay of Bengal

Abstract

Abstract. The mixed layer is the most variable and dynamically active part of the marine environment that couples the underlying ocean to the atmosphere and plays an important role in determining the oceanic primary productivity. We examined the basin-scale processes controlling the seasonal variability of mixed layer depth in the Bay of Bengal and its association with chlorophyll using a suite of in situ as well as remote sensing data. A coupling between mixed layer depth and chlorophyll was seen during spring intermonsoon and summer monsoon, but for different reasons. In spring intermonsoon the temperature-dominated stratification and associated shallow mixed layer makes the upper waters of the Bay of Bengal nutrient depleted and oligotrophic. In summer, although the salinity-dominated stratification in the northern Bay of Bengal shallows the mixed layer, the nutrient input from adjoining rivers enhance the surface chlorophyll. This enhancement is confined only to the surface layer and with increase in depth, the chlorophyll biomass decreases rapidly due to reduction in sunlight by suspended sediment. In the south, advection of high salinity waters from the Arabian Sea and westward propagating Rossby waves from the eastern Bay of Bengal led to the formation of deep mixed layer. In contrast, in the Indo-Sri Lanka region, the shallow mixed layer and nutrient enrichment driven by upwelling and Ekman pumping resulted in chlorophyll enhancement. The mismatch between the nitrate and chlorophyll indicated the inadequacy of present data to fully unravel its coupling to mixed layer processes.

Examining features of enhanced phytoplankton biomass in the Bay of Bengal using a coupled physical‐biological model

“…A large area of high Chl a (1-2 mg m 23 ) at the head of the strait (88N-48N, 958E-1008E) was seen in both model (Figures 4a-4c) and ocean color (Figures 4d-4f) distributions. This is corroborated in the surface Chl a distribution of Narvekar and Prasanna Kumar [2014] for in situ data for March-May. These high Chl a concentrations are remnants of a strong bloom that arises during the northeast winter monsoon [Tan et al, 2006] in the Malacca Strait.…”

“…It has generally been accepted that north of 158N coastal phytoplankton biomass is generally low (0.2 mg m 23 ) [Narvekar and Prasanna Kumar, 2006] especially in summer because the enormous pulsed freshwater and sediment discharge from the Ganges-Brahmaputra Rivers dampen upwelling [Shetye et al, 1991] Our model shows very little enhancement of phytoplankton at the region closely influenced by the Ganga-Brahmaputra river complex because phytoplankton were severely light-limited due to cloud cover and the immense load of suspended sediments [Milliman and Meade, 1983]. Close to the mouth of the large rivers, in spite of high nitrate concentrations, up to 3 lM from river runoff [Narvekar and Prasanna Kumar, 2014], the maximum Chl a measured only up to 0.5 mg m 23 .…”

“…The two different processes that lead to the area of high phytoplankton biomass are delineated in the results of Narvekar and Prasanna Kumar [2014] Figures 9a-9c) show that the development of this Journal of Geophysical Research: Oceans 10.1002/2015JC011508 cyclonic gyre, referred to as the Bay of Bengal Dome by Vinayachandran and Yamagata [1998], starts in November (Figure 9a), attains maximum strength in December (Figure 9b), and begins to dissipate in January ( Figure 9c). The western rim of this gyre is the equatorward EICC (Figures 3b and 3d) which strengthens by November.…”

Marine Dynamics and Productivity in the Bay of Bengal

“…14.2). Observational evidence on chlorophyll and nutrient levels is limited (Narvekar and Kumar 2014;Kumar et al 2010), but satellite data shows evidence of two chlorophyll peaks, in July-August and in DecemberFebruary, with the second being higher (Martin and Shaji 2015). The seasonal cycle of phytoplankton growth can be explained by a combination of nutrient and light limitation.…”

“…Summer brings stronger stratification but also a supply of riverborne nutrients that trigger a bloom, which is then suppressed as sediment in the water limits the supply of light. The waters clear during the autumn and the north-east winds of winter lead to mixing of the water column and a new supply of nutrients, leading to the second and stronger bloom (Narvekar and Kumar 2014;Kumar et al 2010). The winter bloom starts later and persists longer in the north-east than the north-west; this has been attributed to a combination of the eastward advection of riversourced nutrients and an upwelling of nutrients due to subsurface currents in the east in winter (Martin and Shaji 2015).…”