Reviews and syntheses: Trends in primary production in the Bay of Bengal – is it at a tipping point?

Abstract

Abstract. Ocean primary production is the basis of the marine food web, sustaining life in the ocean via photosynthesis, and removing carbon dioxide from the atmosphere. Recently, a small but significant decrease in global marine primary production has been reported based on ocean color data, which was mostly ascribed to decreases in primary production in the northern Indian Ocean, particularly in the Bay of Bengal. Available reports on primary production from the Bay of Bengal (BoB) are limited, and due to their spatial and temporal variability difficult to interpret. Primary production in the BoB has historically been described to be driven by diatom and chlorophyte clades, while only more recent datasets also show an abundance of smaller cyanobacterial primary producers visually difficult to detect. The different character of the available datasets, i.e., direct counts, metagenomic and biogeochemical data, and satellite-based ocean color observations, make it difficult to derive a consistent pattern. However, making use of the most highly resolved dataset based on satellite imaging, a shift in community composition of primary producers is visible in the BoB over the last 2 decades. This shift is driven by a decrease in chlorophyte abundance and a coinciding increase in cyanobacterial abundance, despite stable concentrations of total chlorophyll. A similar but somewhat weaker trend is visible in the Arabian Sea, where satellite imaging points towards decreasing abundances of chlorophytes in the north and increasing abundances of cyanobacteria in the eastern parts. Statistical analysis indicated a correlation of this community change in the BoB to decreasing nitrate concentrations, which may provide an explanation for both the decrease in eukaryotic nitrate-dependent primary producers and the increase in small unicellular cyanobacteria related to Prochlorococcus, which have a comparably higher affinity to nitrate. Changes in community composition of primary producers and an overall decrease in system productivity would strongly impact oxygen concentrations of the BoB's low-oxygen intermediate waters. Assuming decreasing nitrate concentrations and concurrent decreasing biomass production, export, and respiration, oxygen concentrations within the oxygen minimum zone would not be expected to further decrease. This effect could be enhanced by stronger stratification as a result of future warming and thus possibly counteract oxygen decrease as a direct effect of stratification. Therefore, given a decrease in primary production, the BoB may not be at a tipping point for becoming anoxic, unless external nutrient inputs increase.

The Bay of Bengal exposes abundant photosynthetic picoplankton and newfound diversity along salinity‐driven gradients

“…Importantly, our results support a proposal suggesting that picophytoplankton dominate in the Bay of Bengal relative to micro‐ and nanoplankton size fractions, based on pigment analyses (Sarma et al, 2016, 2020). Moreover, our study supports the projection that picophytoplankton will continue to increase concurrent with decreases in larger eukaryotic phytoplankton (Löscher, 2021). Our insights provide a springboard for more targeted studies and experimentation to understand picophytoplankton niche partitioning in the Bay of Bengal, monsoonal influences, and community transitions.…”

“…Recent publications have pointed to the potential importance of Prochlorococcus in the Bay of Bengal, including possible amplification due to climate change (Gregg & Rousseaux, 2019; Löscher, 2021). Here, we examined the 13 Prochlorococcus lineages previously resolved through phylogenetic analysis of the ITS, and termed HLI to VI and LLI to VII (Biller et al, 2015; Johnson et al, 2006; Rocap et al, 2003).…”

“…Additionally, a satellite imaging‐based study concluded there has been a shift in the primary producer community from larger eukaryotic phytoplankton to picocyanobacteria, specifically Prochlorococcus , in response to decreasing nitrate concentrations (Gregg & Rousseaux, 2019). This community shift has further been projected to critically impact bulk primary production, picoeukaryotic taxa (Chen et al, 2022), export production, and the expansion and intensity of the oxygen minimum zone in the Bay of Bengal (Löscher, 2021). While these studies shed light on possible scenarios and, in some cases, particular phytoplankton community members, they have generally not been performed in a depth‐resolved integrated manner nor over the latitudinal expanse of the bay, which has regional variations that are especially sensitive to changes in the global water cycle.…”

Insights into Prokaryotic Community and Its Potential Functions in Nitrogen Metabolism in the Bay of Bengal, a Pronounced Oxygen Minimum Zone

“…Heavy rainfall and the associated river discharge in summer (June to September) cause a considerable drop in surface water salinity ( 18 ). This, together with increased surface water temperature, induces strong stratification of the water column, limiting nutrient flux from subsurface water to the ocean surface, therefore making the BoB an area of relatively low primary production compared to the Arabian Sea ( 19 ). The BoB does not span a broad region with absolute anoxia as the other three OMZs do, and no indication of nitrogen loss has been observed although oxygen levels are below 1 to 2 μM ( 20 ).…”

Atmospheric gas-phase composition over the Indian Ocean

“…At the same time, the near-surface area will experience increased ventilation due to the predicted stronger winds. On the contrary, a review in this issue summarizes evidence indicating a significant, but small, reduction in primary production in the northern Indian Ocean (Löscher, 2021). An alarming decrease of up to 20 % in marine phytoplankton during the past 6 decades has been identified in the western Indian Ocean (Roxy et al, 2016) driven by enhanced ocean stratification and suppressed nutrient mixing from subsurface layers.…”