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

Abstract

KEY POINTS: Coupled Bio-Physical Model configured for the Indian Ocean Description of areas of enhanced phytoplankton biomass validated by remotely sensed chlorophyll a Model provides vertical cross sections of phytoplankton not available by remote sensing ABSTRACT A coupled bio-physical ocean model is used to describe areas of enhanced phytoplankton biomass, seen remotely sensed observations, in the otherwise oligotrophic environment of the Bay of Bengal. The model is based on the Naval Coastal Ocean Model (NCOM), which is oneway coupled to the 13-component Carbon, Silicate, and Nitrogen Ecosystem (CoSiNE) model and configured for the Indian Ocean. Model results are compared and evaluated against a set of in situ shipboard observations as well as ocean color data acquired from several remote sensing platforms. The model is shown to successfully simulate the seasonal cycle of phytoplankton, the markedly contrasting scenarios of phytoplankton distribution in the north versus the south Bay of Bengal, and the biological impact from the 1997/1998 Indian Ocean Dipole (IOD) event. The model simulation provides us with vertical cross sections of phytoplankton biomass from summer and winter blooms in the southwest of the bay; information not found in remotely sensed data. It also successfully reproduces the timing of the onset of the blooms and their spatial extent, thereby providing a measure of its potential for augmenting in situ and remotely sensed observations to improve understanding of the dynamics of primary producers and carbon cycling in one of the most poorly sampled regions of the world's oceans.

Quantifying Tropical Cyclone's Effect on the Biogeochemical Processes Using Profiling Float Observations in the Bay of Bengal

“…The main consequence of the presence of salt stratification in the near‐surface layer in the BoB leads to the formation of barrier layer; the layer between base of the mixed layer depth (MLD) and the isothermal layer depth (ILD) (Girishkumar, Ravichandran, & Han, ; Girishkumar et al, ; Lukas & Lindstrom , ; R. R. Rao & Sivakumar, ; Thadathil et al, , ; Thangaprakash et al, ; Vinayachandran et al, ). As reported in earlier studies, both barrier layer and near‐surface salt stratification can play a significant role in the seasonal evolution of phytoplankton variability in the BoB (Gomes et al, , ; Kumar et al, ; Lévy et al, ; Narvekar & Kumar, , ). The strong haline stratification inhibits upward flux of nutrient‐rich subsurface water into the euphotic zone, which leads to significantly low biological productivity in the BoB, as compared with the Arabian Sea (Gomes et al, ; Kumar et al, ; Narvekar & Kumar, ).…”

“…The strong haline stratification inhibits upward flux of nutrient‐rich subsurface water into the euphotic zone, which leads to significantly low biological productivity in the BoB, as compared with the Arabian Sea (Gomes et al, ; Kumar et al, ; Narvekar & Kumar, ). Another important factor affecting the biological productivity during the summer monsoon (June–September) in the BoB is the presence of monsoonal‐cloud cover, which hinders the incoming shortwave radiation (Gomes et al, , ; Madhu et al, ; Madhupratap et al, ). Subsequently, large quantities of suspended sediments transported by the rivers draining into the BoB may significantly reduce downward penetration of solar radiation in the water column (Gomes et al, , ; Madhupratap et al, ; Madhu et al, ).…”

“…Another important factor affecting the biological productivity during the summer monsoon (June–September) in the BoB is the presence of monsoonal‐cloud cover, which hinders the incoming shortwave radiation (Gomes et al, , ; Madhu et al, ; Madhupratap et al, ). Subsequently, large quantities of suspended sediments transported by the rivers draining into the BoB may significantly reduce downward penetration of solar radiation in the water column (Gomes et al, , ; Madhupratap et al, ; Madhu et al, ). However, recent observational and modeling studies show that during favorable conditions, there is considerable enhancement of chlorophyll in the near‐surface layer at certain regions in the BoB on the seasonal scale (Currie et al, ; Girishkumar et al, ; Gomes et al, ; Kumar et al, ; Lim et al, ; Murtugudde et al, ; Narvekar & Kumar, ; Vinayachandran, ; Vinayachandran et al, , ; Vinayachandran & Mathew, ).…”

“…Subsequently, large quantities of suspended sediments transported by the rivers draining into the BoB may significantly reduce downward penetration of solar radiation in the water column (Gomes et al, , ; Madhupratap et al, ; Madhu et al, ). However, recent observational and modeling studies show that during favorable conditions, there is considerable enhancement of chlorophyll in the near‐surface layer at certain regions in the BoB on the seasonal scale (Currie et al, ; Girishkumar et al, ; Gomes et al, ; Kumar et al, ; Lim et al, ; Murtugudde et al, ; Narvekar & Kumar, ; Vinayachandran, ; Vinayachandran et al, , ; Vinayachandran & Mathew, ). Also, there are notable efforts to document the interannual variability of the chlorophyll in the BoB (Girishkumar et al, ; Gomes et al, ; Martin & Shaji, ; Murtugudde et al, ; Vinayachandran & Mathew, , and references therein).…”

No nitrogen fixation in the Bay of Bengal?

“…The high iron (Fe) requirement of N 2 -fixing microbes (60 times higher compared to other marine organisms; Gruber and Galloway, 2008) limits N 2 fixation in large parts of the ocean (Moore et al, 2013). However, eolian Fe fluxes to surface waters of the southern BoB were estimated to be comparable to those detected underneath Saharan dust plumes in the Atlantic (290 ± 70 µmol m −2 yr −1 ; Grand et al, 2015a). Indeed, dissolved Fe (dFe) accumulates in the BoB OMZ, reaching comparably high concentrations of up to 1.5 nM (Grand et al, 2015b;Chinni et al, 2019).…”