26 Jan 2021

26 Jan 2021

Review status: this preprint is currently under review for the journal BG.

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

Carolin R. Löscher Carolin R. Löscher
  • Nordcee, DIAS, Department of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense M - DK

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 of 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 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, visually difficult to detect cyanobacterial primary producers. 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 two 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 of eukaryotic nitrate-dependent primary producers and the increase of 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 of 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, an effect which would be enhanced by stronger stratification as a result of future warming. Therefore, given a decrease in primary production, the BoB may not be at a tipping point for becoming anoxic, unless external nutrient inputs increase.

Carolin R. Löscher

Status: open (until 23 Mar 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Carolin R. Löscher

Carolin R. Löscher


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Short summary
The Bay of Bengal (BoB) is classically seen as an ocean region with low primary production, which has been predicted to even decrease further. Here, the importance of such a trend is used to explore what could happen to the BoB's low oxygen core waters if primary production decreases. Lower biological production leads to less oxygen loss in deeper waters by respiration, thus it could be that oxygen will not further decrease and the BoB will not become anoxic, different to other low oxygen areas.