Ecosystem state change in the Arabian Sea fuelled by the recent loss of snow over the Himalayan-Tibetan Plateau region

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. The Arabian Sea (AS) is a unique, low-latitude oceanic ecosystem because it is influenced by monsoonal winds that reverse their direction seasonally 1. These reversing winds cause dynamic shifts in surface currents and alterations in the pycnocline, which help fertilize its normally nutrient-depleted surface layers. However, the mechanisms that drive water-column mixing, the upward transport of nutrients and the consequent upsurge of phytoplankton biomass during the summer (Jun.-Sep.) and the winter (Nov.-Feb.) monsoons are different 2-6. During the summer monsoon, nutrient enrichment is via coastal upwelling, which begins when the adjacent land mass becomes warmer relative to the AS, and low pressure develops over the Arabian Peninsula 3,7,8. During this time of the year, strong topographically-steered southwesterly winds blowing over the AS form a low-level atmospheric jet called the Findlater Jet 9. This jet induces a northeastwardly flow of the surface currents, leading to strong upwelling of deep, nutrient-rich waters, causing large phytoplankton blooms along the coasts of Somalia, Yemen, and Oman 3,5,6,10,11. In contrast, during winter, when the Eurasian continent cools, nutrient enrichment is via deep penetrative convective mixing as a result of excessive heat loss from the AS surface layer caused by cold and dry northeasterly winds blowing from the snow-covered Himalayan-Tibetan Plateau (HTP) 4,5. The erosion of the thermocline, and entrainment of deep (100-150 m) nutrient-rich waters into the surface layer of the AS, begins as early as Dec., and has a major influence on winter phytoplankton blooms observable as far south as 14°N 4,11,12 .

Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

“…Follow-up studies also reported decreasing oxygen concentrations in the western and northern Arabian Sea (Piontkovski and Al-Oufi, 2015;Queste et al, 2018). In the northern Arabian Sea, dissolved oxygen concentrations in the surface mixed layer largely reflect the trend seen in the OMZ, as indicated by a compilation of dissolved oxygen data covering the period from the 1960s to 2010 (Gomes et al, 2014).…”

“…We are still uncertain if the recent emergence and persistence of Noctiluca blooms will further intensify the Arabian Sea's OMZ. Satellite-derived Chlorophyll a trends reveal an almost 3-fold increase in phytoplankton biomass, with increases particularly in the northwestern and central Arabian Sea (Goes et al, 2020). With respect to repercussions for the food chain, we are aware that Noctiluca is not a preferred food for most zooplankton but is voraciously grazed upon by gelatinous animals such as salps.…”

“…Nonetheless, we are aware that Noctiluca is not a preferred food for most zooplankton but is a cnidarian such as a jellyfish voraciously grazed upon by gelatinous tunicates such as salps. During our field campaigns, we have seen large swarms of salps, known to be efficient filter feeders, devouring Noctiluca (Gomes et al, 2014) and depositing large pellets. Salp pellets are known to be fast sinking (up to 2700 m d −1 ) and carbon-rich (up to 37 % dry weight), contributing disproportionately to carbon flux compared with other zooplankton (Henschke et al, 2016;Martin et al, 2017).…”

“…In the Arabian Sea, the increasing expansion of hypoxia discussed in Sect. 2.4 both on the shelf and offshore is the predominant driver for a shift in the base of the pelagic ecosystem from mainly autotrophy before 2000 (Garrison et al, 1998(Garrison et al, , 2000Smith et al, 1998a) to greater dependence on mixotrophy more recently (Gomes et al, 2014). A largescale, ongoing study conducted by the National Institute of Oceanography, India, from 2003 onward, in support of India's ocean color program, first documented the appearance of extensive blooms of the green mixotrophic dinoflagellate, Noctiluca scintillans (Noctiluca).…”

“…Noctiluca is a large (∼ 1 mm in diameter) dinoflagellate, capable of sustaining itself via photosynthesis from its green free-swimming endosymbionts, Pedinomonas noctilucae (Wang et al, 2016), and/or by ingestion of exogenous prey (Goes and Gomes, 2016;Gomes et al, 2009Gomes et al, , 2014Prakash et al, 2008Prakash et al, , 2017. Within a decade and a half, Noctiluca, has taken over the once diatom-dominated food chain of the Arabian Sea and the Sea of Oman, forming large green mats that can be observed from space with regular predictability ( Fig.…”