Role of mesoscale eddies on the variability of biogenic flux in the northern and central Bay of Bengal

Abstract

Mesoscale cyclonic eddies (cold core) are though known to enhance the phytoplankton biomass in the Bay of Bengal, their contribution to the carbon export to the deep ocean are yet to be quantified. Using biogenic flux data obtained from the sediment traps, located at 17°27′N, 89°13′E (northern Bay of Bengal trap: NBBT) and 13°07′N, 84°24′E (central Bay of Bengal trap: CBBT), we explored the variability of biogenic flux in response to cyclonic eddies. Temporal variation of the biogenic flux at NBBT (1994–1998) and CBBT (1993–1996) showed four distinct peaks with no well‐defined seasonal pattern. The inverse relationship between the high flux events and sea surface height anomaly (SSHA) along with the dominant periodicity of 10–15 weeks from the Chi‐square analysis of SSHA confirmed the role of eddies in the observed higher biogenic fluxes. During the eddy events, enhanced opal flux suggested eddy‐driven new production, while lower carbonate to opal ratio along with high organic carbon indicated large export via diatoms. Finally, we show that 42% of the total organic carbon collected at mid‐depth (∼1000 m) in the Bay of Bengal is contributed by eddy‐driven processes via diatom bloom making it a regional sink for CO2.

Effects of Freshwater Stratification on Nutrients, Dissolved Oxygen, and Phytoplankton in the Bay of Bengal

“…Because strong stratification inhibits exchange between surface waters and those at depth, intense forcing, such as that during cyclones, may be required to erode the stratification and inject nutrients into the euphotic zone to enhance primary production (Madhu et al, 2002;Maneesha et al, 2011;Latha et al, 2015). Studies indicate that cold-core eddies play an important role in uplifting nutrients to enhance primary production (Prasanna Kumar et al, 2004;Vidya and Prasanna Kumar, 2013). Recent work suggests that frontal zones in the Arabian Sea harbor high phytoplankton biomass due to increased nutrient input through vertical mixing and advection (Roy et al, 2015;Vipin et al, 2015).…”

Dissolved silicon and its isotopes in the water column of the Bay of Bengal: Internal cycling versus lateral transport

“…This result is suggestive of continuous steady supply of dissolved Si (and other nutrients) from subsurface waters during the period of this study. Some of the recent studies have underscored the importance of eddy pumping as a source of nutrients to surface waters of the BoB (Prasanna Kumar et al, 2004Kumar et al, , 2007Nuncio and Prasanna Kumar, 2012;Vidya and Prasanna Kumar, 2013). The satellite altimetry data from AVISO (www.aviso.oceanobs.com) during November 2008, the period of our sampling, have identified two cyclonic (upwelling) meso-scale eddies.…”

“…The sources that contribute dissolved Si to this layer include (i) supply from the G-B river system and SGD, (ii) in situ dissolution of dust/river borne particles, (iii) vertical advection/diffusion, and (iv) lateral advection. In addition, some of the earlier studies have highlighted the important role of eddy pumping to support new production in the BoB (Gomes et al, 2000;Prasanna Kumar et al, 2004Nuncio and Prasanna Kumar, 2012;Vidya and Prasanna Kumar, 2013). Vidya and Prasanna Kumar (2013) estimate that 42% of organic carbon flux at depth $1000 m of the BoB is due to diatom bloom caused by eddy driven processes.…”

“…Earlier studies have suggested that much of the nutrients including Si supplied by various rivers to the BoB are consumed in estuaries and therefore do not reach the open bay (Qasim, 1977;Prasanna Kumar et al, 2002). Another important source of Si and nutrients to surface waters of the BoB is meso-scale eddies Prasanna Kumar et al, 2004Nuncio and Prasanna Kumar, 2012;Chen et al, 2013;Vidya and Prasanna Kumar, 2013), the relative significance of these two modes of nutrient supply to support the biological production in the open BoB however, is a topic of debate.…”

Evidence for enhanced chlorophyll-a levels in the Bay of Bengal during early north-east monsoon

“…In contrast, during North-East monsoon season, the Bay is typically characterized by high productivity compared to that in summer owing to thin freshwater lens at the surface, which can be churned by the relatively weaker monsoonal winds (~ 5 m/s). The role of Eddies in enhancing the productivity over the Bay has been highlighted in several studies (Narvekar and Prasanna Kumar, 2006;Kumar et al, 2007;Vidya and Kumar, 2013). Albeit, there exist a difference in terms of the abundance of chlorophyll-a and fixation of carbon during primary production.…”