Is the Bay of Bengal at a tipping point?

The Bay of Bengal (BoB) has the long-stood enigma of an oxygen minimum zone, which maintains traces of oxygen without becoming fully anoxic. This may, besides biological feedback loos stabilizing the low oxygen concentrations in those waters, have to do with low primary production in the BoB’s surface waters and the lack of subsequent respiration of organic material in intermediate oxygen poor waters. 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 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, however, this is not consistent with newer data, which instead show an abundance of smaller, visually difficult to detect cyanobacterial primary producers. By combining the available metagenomic and biogeochemical datasets with satellite-based ocean color observations, a pattern can be derived showing a shift in community composition of primary producers 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. 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. A potential change in primary producer community composition and especially a decrease in primary production in the BoB may thus have a significant impact on the distribution of low oxygen waters in this basin and may possibly mitigate their further expansion, therefore arguing against the BoB being at a tipping point to develop full anoxia.