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

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 impacts of anthropogenic climate change on the monsoonal system have elicited a large number of studies in recent years 8,13,14 . Most of these have focused on the summer monsoon, because of its implications for rainfall patterns and water supply in countries downstream of the summer monsoon 13,[15][16][17] ; and especially because of its consequences for coastal upwelling, biological productivity and fisheries in countries bordering the AS 3 . In contrast, the response of the boreal winter component of the monsoon cycle to global warming has received far less attention, despite knowledge that it has a significant impact on annual precipitation patterns over the HTP region and over southeast India and Sri Lanka 18 , and on winter convective mixing, responsible for sustaining the large phytoplankton blooms of winter, which contribute significantly to enhancing the rich fisheries potential of the AS 4,5,12,19 .
A recent study 20 , based on results from Coupled Model Intercomparison Project Phase 5 (CMIP5), predicts that the AS will experience significant weakening of winter monsoon winds by the turn of the 21st century due to enhanced warming of the dry Arabian Peninsula relative to the southern Indian Ocean. Weaker winds would result in a substantial reduction in oceanic heat loss to the atmosphere and robust weakening of convective mixing, which the authors 20 postulate would lead to a reduction in the productivity and size of winter phytoplankton blooms in the AS.
Here we present data on more contemporaneous changes in winter convective mixing using mixed layer depth (MLD) outputs from the Global Ocean Data Assimilation System (GODAS) 21 , a real time analysis and reanalysis system used for retrospective analysis, and for monitoring of present-day oceanic conditions. 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 .

Results and Discussion
Consistent with CMIP5 projections shown earlier 20 , MLD trends from GODAS also 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 . Despite certain limitations 21,23,24 , GODAS provides more realistic and robust outputs of MLDs than CMIP5, because it is a real-time ocean analysis and reanalysis system, in which model outputs are constrained by in-situ observations, unlike the unconstrained ensemble climate projections of CMIP5. Outputs from GODAS show that since 1980, winter-time average AS basin-wide MLDs have decreased by>11 m, a change that has been accompanied by a warming of winter monsoon winds (~0.012 °C yr −1 ), a decline in the strength of these winds (~0.006 m s −1 yr −1 ) and an increase in their relative humidity (0.1% yr −1 ) (Fig. S1a-c). Collectively, these changes have resulted in an increase in net-heat flux from the atmosphere into AS surface waters (Fig. S1d) that indicates an increase in the upper AS ocean heat content (OHC) since 2000 25 . Convective mixing is driven primarily by buoyancy destabilizing forces that include cooling and/or evaporation and that cause surface ocean waters to become colder, saltier and denser than the underlying waters. The process is greatly enhanced when the overlying winds are colder, drier and stronger, but conversely, weakened when they become warmer and more humid as witnessed recently in the AS (Fig. S1a-c).
The temperature of winter winds and their dryness is governed in large part by the extent of winter snow cover over the HTP [10][11][12] , which also impacts land-ocean temperature and pressure gradients that modulate the strength of the winter monsoonal winds 15,16,[18][19][20] . Since 1998, the HTP has experienced a persistent year-on-year decline (~5 ×10 3 yr −1 ) in snow cover extent 26 (Fig. 1b), attributable to the warming of the Eurasian continent ( Fig. S2) 3 , a trend that has possibly been exacerbated by the deposition of soot and dust on the HTP snow surface 27 . Regression analysis of HTP snow cover extent versus average mixed layer depths indicates that the loss of snow accounts for around 51% of the recent shallowing trends in the mixed layer of the AS (Fig. S3, r 2 = 0.51, p < 0.0001), through generation of warmer, weaker and more humid offshore-blowing winds that result in an increase in net heat flux ( Fig. S1a-d). Cumulatively the major consequence of these changes is the undermining of convective mixing responsible for nutrient entrainment into the euphotic zone and the fertilization of large winter phytoplankton blooms 4 .
Although a decline in winter biological productivity as forecast previously 20,22 under the current scenario of weaker convective mixing and enhanced stratification seems like a realistic conjecture, trends in satellite-derived chlorophyll a (Chl a) concentrations present a different picture (Figs. 1c, 2). Since the early 2000s, winter-time Chl a concentrations averaged for the AS have been on the rise, increasing over 3-fold in recent years, particularly in the northwestern and central AS (Fig. 2), where winter Chl a concentrations now supersede those measured during the more productive summer monsoon season (Fig. S4). What is different, however, is that this increase in winter monsoon Chl a concentrations is not being fuelled by diatoms; the ubiquitous, trophically important, siliceous photosynthetic organisms, which dominated winter phytoplankton communities in the AS during the 1960s International Indian Ocean Expeditions (IIOE) and the mid-1990's Joint Global Ocean Flux Studies (JGOFS) 4,28 . Rather, this is due to blooms of the mixotrophic green dinoflagellate Noctiluca scintillans Suriray (synonym Noctiluca scintillans Macartney) [29][30][31][32][33][34] . Since they were first detected in the early 2000s 29,31,32,35,36 green Noctiluca blooms have become increasingly more pervasive over diatoms (Fig. S5) and more widespread, occurring every winter with predictable regularity 33,[35][36][37][38][39][40] .
So how is a changing AS ecosystem, whose upper water column is becoming increasingly stratified and bordering nutrient limitation, benefiting Noctiluca? Here, we contend that changes in physico-chemical conditions in the euphotic water column caused by stratification, as well as Noctiluca's mixotrophic mode of nutrition, act in tandem to give this organism a tremendous competitive advantage.
Firstly, as a mixotroph 31,35,41 , Noctiluca can meet its metabolic requirements via autotrophic CO 2 fixation by thousands of free-swimming prasinophyte symbionts: Protoeuglena noctilucae 42 within its central vacuole (symbiosome) (Fig. S6), and also via heterotrophic feeding on a wide range of external prey including phytoplankton, micro-and mesozooplankton and zooplankton eggs 36,41,43 . This mixotrophic mode of feeding gives green www.nature.com/scientificreports www.nature.com/scientificreports/ Noctiluca a trophic advantage over both autotrophic and heterotrophic plankton and makes it different from the globally ubiqutous red Noctiluca species, which are devoid of endosymbionts and exclusively heterotrophic 44 .
Secondly, our earlier work had ascribed the advent of Noctiluca blooms to the upshoaling of hypoxic waters in the euphotic zone 35 , caused by a possible expansion of the AS's permanent oxygen minimum zone (OMZ) 45 . In this study 35 , we showed using shipboard experiments in which natural populations were exposed to suboxic seawater 36 that endosymbionts in Noctiluca cells photosynthesized more efficiently under suboxic conditions. More recent studies 33,34,39 , have attributed the advent of Noctiluca blooms to acute "silicate stress", a situation that would prevent diatom populations from attaining bloom proportions, particularly in the northwestern AS where they have been the predominant algal group in the past 28,29,32,46 . Our examination of winter-time nutrient concentrations coincident with Noctiluca blooms provides no evidence of silicate stress 31 , but instead raises the intriguing possibility that autotrophic phytoplankton in the contemporary AS ecosystem may in fact be experiencing acute "nitrate stress". 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 OMZ 45 . Datasets of nutrient measurements from the early 1960s to date provide no evidence of any increases in inorganic phosphate in the AS, as would be expected from enhanced weathering of continental rocks in a warmer and more humid environment 47 . The decline in euphotic column nitrate concentrations and the profound departure in both NO 3 :PO 4 and NO 3 :SiO 4 ratios from traditional Redfield ratios (Fig. 3b,c) is unparalleled for any open ocean ecosystem. In this nutrient-poor scenario, we contend that as with other mixotrophs 48 , Noctiluca's dual mode of obtaining nutrients 36 , confers upon it a significant competitive advantage over autotrophic phytoplankton especially when nutrients are limiting. When feeding on external prey, Noctiluca accumulates large amounts of nitrogen as ammonium (0.003-0.012 μM NH 4 + cell −1 ) within its www.nature.com/scientificreports www.nature.com/scientificreports/ central cytoplasm 36 , alleviating its dependence on extraneous NO 3 . Feeding on external phytoplankton prey also reduces the standing stock of autotrophs competing for seawater nutrients, providing an additional advantage for Noctiluca. Additionally, in laboratory cultures illuminated on a diel cycle, we have consistently observed that Noctiluca grows best in medium enriched with ammonium. Laboratory-grown Noctiluca can also survive in the absence of nutrients and external prey for prolonged periods of time (~1 year), suggesting an internal, tight nutrient recycling mechanism that ensures its survival under conditions that would be considered hostile for most autotrophic phytoplankton.
Thirdly, Noctiluca's ecological success and range expansion in the AS also appear to be tied to the lack of predatory pressure 35 . Noctiluca's only known consumers are salps (Fig. S7) and jellyfish, which appear off the Omani coast in January, and in the central AS by March, long after large blooms of Noctiluca are established.
Finally, dinoflagellates, the functional group that Noctiluca belongs to, prefer less turbulent waters 49 and typically reach their greatest abundance at the surface during relatively quiescent and nutrient-deplete periods 50 . This stands in contrast to diatoms, which with a few exceptions 47 are generally restricted to more turbulent and nutrient-rich conditions 50,51 . In field studies, we have consistently noticed that a highly stratified and stable water column is favorable to the formation of Noctiluca blooms. This is evident along the coast of Oman, where although coastal upwelling shoals Noctiluca-favorable hypoxic waters into the upper euphotic zone as early as Aug. (Fig. S8), accumulation of Noctiluca to bloom proportions takes place only in sheltered coastal, embayments, when the summer monsoon winds ease 29 and the water column becomes more stable than offshore waters. Offshore in the AS, prior to their appearance as surface blooms in Dec-Jan, Noctiluca are found at depth 31 , often close to the oxycline and where photosynthetically available radiation (PAR) is <100 μmol (photons) m −2 s −131 . Surface Noctiluca blooms appear by early Jan. 31 , when the water column begins to stratify, under high light conditions that are typical at that time of the year. Then in the presence of extraneous prey, they transition to a greater dependence on heterotrophy to attain the high growth rates of ~1.2 cells day −1 35,36 necessary for bloom formation.
Besides the build-up of ammonium 31,36 mentioned earlier, Noctiluca can accumulate significant amounts of lipids in its central cytoplasm when feeding on extraneous prey 31 , which makes individual cells increasingly buoyant and thus easily prone to dispersal by surface currents, filaments and streamers associated with mesoscale eddies 52 , allowing it to become more widespread (Fig. 2). www.nature.com/scientificreports www.nature.com/scientificreports/ In summary, we contend that while Noctiluca outbreaks are triggered each summer by the intrusion of hypoxic waters into the upper layers of the euphotic column 35 , conditions under which Noctiluca's endosymbionts have shown to have higher rates of photosynthesis than free-living autotrophs. Stratification and a stable water column aid in enhancing their growth to large bloom proportions. In the latter situation, Noctiluca's unique mixotrophic mode of obtaining nutrition offers it considerable advantages over autotrophs, which are severely nutrient limited by a weaker convective mixing due to the loss of snow cover in the HTP, and changes in nutrient stoichiometries caused by the expansion of the OMZ. Under these conditions, Noctiluca's unique symbiotic system provides it with a tight nutrient recycling mechanism; wherein nitrogenous nutrients, accumulated during digestion of ingested prey, help alleviate nutrient limitation in the external environment.
A recent study 53 has drawn attention to an increase in anthropogenic organic carbon exiting the northern Arabian (Persian) Gulf that may be contributing significantly to the expansion of the OMZ in the northwestern AS, where Noctiluca blooms have been particularly more intense (Fig. 2).
Recent observations of Noctiluca blooms, in association with the shallowing of hypoxic waters along the coast of Oman during the summer upwelling season 30 , have raised the specter that this organism may also be expanding its temporal range to include the highly productive summer period when diatom blooms support Oman's large coastal artisanal fisheries. In under-developed countries like Somalia and Yemen, which are currently being challenged by unrest, poverty and deprivation 54,55 , and where fisheries are the primary source of protein and income, any further loss of fishery resources, due to the spread of hypoxia (Fig. S8), has the potential to further exacerbate socio-economic turmoil in the region, including piracy on international shipping 56 .
The inability of large zooplankton, except salps and jellyfish to feed on Noctiluca, is indicative of the capacity of Noctiluca blooms to short-circuit the trophic food chain. Thus, their annual reoccurrence and growing dominance in winter each year will require a revision of our fundamental understanding of the AS food web garnered from the AS-JGOFS era 6 , when autotrophs dominated during productive seasons and alternated with diazotrophs during the transition to nutrient limiting conditions of the inter-monsoon seasons 28 . The emergence www.nature.com/scientificreports www.nature.com/scientificreports/ of a mixotroph as a dominant organism will also necessitate revisions to the traditional manner in which AS carbon cycling and biogeochemical rate processes are modeled and studied. Current models compartmentalize organisms at the base of the food chain into autotrophic phytoplankton and heterotrophic zooplankton, but mixotrophy blurs the strict boundary between producers and consumers 48,57,58 . Recent model simulations 58 that include mixotrophs have indicated that mixotrophy greatly enhances the transfer of biomass to larger size classes further up the food chain, which is consistent with our observations of large amounts of salps, jellyfish, and squid in the AS, particularly along the coast of Oman in association with Noctiluca blooms.

Methods
Mixed layer depth anomalies were calculated using area averaged (60°E-70°E, 14°N-25°N All datasets have been processed by the Ocean Biology Processing group at NASA Goddard Space Flight Centre (GSFC). These daily products have been corrected for atmospheric light scattering and for sun angles differing from the nadir. In addition, the influence of clouds has been substantially reduced. To account for sensor degradation over time, the instrument is calibrated using internal lamps, solar diffuser observations, and lunar images, as well as vicarious methods.
Seawater nutrient data for estimating decadal changes in winter time (Dec-Jan-Feb-Mar) NO 3 :PO 4 and NO 3