Abstract
The recent trend of global warming has exerted a disproportionately strong influence on the Eurasian land surface, causing a steady decline in snow cover extent over the Himalayan-Tibetan Plateau region. Here we show that this loss of snow is undermining winter convective mixing and causing stratification of the upper layer of the Arabian Sea at a much faster rate than predicted by global climate models. Over the past four decades, the Arabian Sea has also experienced a profound loss of inorganic nitrate. In all probability, this is due to increased denitrification caused by the expansion of the permanent oxygen minimum zone and consequent changes in nutrient stoichiometries. These exceptional changes appear to be creating a niche particularly favorable to the mixotroph, Noctiluca scintillans which has recently replaced diatoms as the dominant winter, bloom forming organism. Although Noctiluca blooms are non-toxic, they can cause fish mortality by exacerbating oxygen deficiency and ammonification of seawater. As a consequence, their continued range expansion represents a significant and growing threat for regional fisheries and the welfare of coastal populations dependent on the Arabian Sea for sustenance.
Highlights
The Arabian Sea (AS) is a unique, low-latitude oceanic ecosystem because it is influenced by monsoonal winds that reverse their direction seasonally[1]
Using contemporary Chlorophyll a (Chl a, a proxy for phytoplankton biomass) data obtained from satellite and field studies, we attempt to explain why the response of the AS ecosystem under a scenario of weaker winter convective mixing and enhanced stratification, differs substantially from what has been projected in previous studies[20,22]
Consistent with Coupled Model Intercomparison Project Phase 5 (CMIP5) projections shown earlier[20], mixed layer depth (MLD) trends from Global Ocean Data Assimilation System (GODAS) show a weakening of winter convective mixing (Fig. 1a), but notably at a rate (0.28 m yr−1) much faster than the 0.15 m yr−1 rate predicted in the CMIP5-based study[20]
Summary
The Arabian Sea (AS) is a unique, low-latitude oceanic ecosystem because it is influenced by monsoonal winds that reverse their direction seasonally[1]. Analysis of nitrate concentration data, starting with the earliest measurements from the IIOE cruises of the 1960s up to more recent data collected during our Noctiluca bloom study cruises, show that the AS is experiencing a significant loss of nitrate inventories (Fig. 3a) within the upper euphotic column; a trend that in all likelihood, is being fostered by enhanced water column denitrification and ammonification on account of the AS’s expanding permanent OMZ45.
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