Seasonal Indian Ocean primary productivity and key drivers

doi:10.21203/rs.3.rs-2526014/v1

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Seasonal Indian Ocean primary productivity and key drivers

https://doi.org/10.21203/rs.3.rs-2526014/v1

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Oceanic Net primary production (NPP) results from the photosynthesis of marine plankton, which accounts for half of the global primary production and influences the carbon cycle, and transfers organic matter and energy to marine ecosystems. Anthropogenic activities severely stress the ocean ecosystem through warming and acidification and have significantly altered NPP. In this context, we assess the long-term changes in NPP in the Indian Ocean (IO) with respect to the changes in physical processes and nutrient input to the oceans. Under the extreme warming scenario, the western AS shows a positive trend (0.7–0.9 °C/yr) in summer, where the basin-wide seasonal maximum in NPP is observed (400–500 mg/m₃). Similarly, the rise in SST and decline in DNO₃ in the upwelling-prone regions like western AS, Seychelles-Chagos Thermocline Ridge (SCTR), and southwest AS have adversely affected the NPP in IO. Contrary to this, cooling is observed in the northern AS during winter (-0.1–0.2 °C/yr), combined with the rise in DFe concentration, favour the NPP there. A decline in NPP in the IO (-25.31 mg/yr) will adversely affect the marine food chain and biogeochemical cycles.

Climate Change

Food Security

Primary Productivity

Biogeochemistry

ocean warming

carbon cycle

Long-term changes in seasonal net primary productivity in Indian Ocean are estimated
Indian Ocean warming results in an abrupt decline of productivity
Changes in nutrients availability and their transport influence the productivity
Cooling of northern AS and rise of DFe concentration in winter strongly affect productivity

The tiny organisms, called phytoplankton, play a key role in life in the ocean, which provides food and breath for marine life (Richardson and Schoeman, 2004). They constitute half of the global biosphere productivity (Righetti et al., 2019), through photosynthesis by which absorbed photon energy is converted to organic molecules (Lee et al., 2015), and contribute carbon and oxygen to terrestrial life. Additionally, oceans act as a sink and source of CO₂, and long-term changes in Net primary production (NPP) have a significant role in regulating the global carbon cycle (Gregg et al., 2003). About 45 gigatons of organic carbon are produced per year by phytoplankton through the fixation of photosynthetic carbon, and 16 gigatons of which are exported to the interior of oceans (Falkowski et al., 1998). NPP is a very complex process because it involves interactions among the cellular physiology, surrounding environment, irradiance field, and other oceanic and climate factors (Gregg and Rousseaux, 2019).

Plankton growth rate depends on the stoichiometric ratios of carbon to nitrogen to phosphorus (106:16:1), known as the Redfield ratios (Redfield, 1958). Nevertheless, phytoplankton growth rates are limited by light and a number of nutrients. The most limiting nutrient is employed when more than one nutrient is taken into account since the fractional limits of temperature-dependent maximum growth rates are based either on external or cellular nutrient supply (Tagliabue et al., 2021). Seasonality also modifies plankton growth, e.g., summer offers a direct benefit for photosynthesis due to the increased day duration and area of sunlight. Plankton cell exposure to photosynthetic active radiation (PAR) is controlled by mixing and the vertical displacement of the cells (Comesaña et al., 2021; Paul et al., 2022). In addition, the shoaling of the mixed layer limits the plankton within the photic zone and aids in its growth (Sallée et al., 2021; Hou et al., 2022).

There is substantial evidence showing the changes in plankton abundance and community structure over the past few decades in various regions of the global oceans (Hays et al., 2005). Despite the ongoing search for mechanical connections between climate and plankton, the strengthening of ocean mixing or stratification is possibly the core of the connection (Hays, et al. 2005). Apart from these, negative feedback (cloud albedo feedback) caused by Dimethyl Sulphide (DMS) produced by phytoplankton is still one of the urgent concerns in studying the effects of anthropogenic climate change (Siegel et al., 2016; Vogt and Liss, 2009). In addition, the strengthening of summertime pycnocline stratification and mixed-layer shoaling in the global oceans arise from ocean warming due to climate change (Sallée et al., 2021).

The phytoplankton in the equatorial regions is mainly controlled by the El Niño Southern Oscillation (ENSO) in the past few decades. The in situ chlorophyll-a measurements (a proxy for NPP) suggest that the North Indian Ocean (0.0018 ± 0.0015 mg/m^₃/yr) and South Indian Ocean (0.020 ± 0.0011 mg/m^₃/yr) have an increasing trend the periods 1899–2008 Boyce et al., 2010). The rapid warming in the tropical Indian Ocean (0.15°C/dec) directly affects PP, and acidification is putting calcifying plankton and marine life under a great threat (Krishnan et al., 2020; Nagelkerken and Connell, 2015). Recent studies suggest that tropical and subtropical ocean warming highly reduces phytoplankton growth in nutrient-limited regions (Fernández-González et al., 2022; Gittings et al., 2018). The rising ocean temperature can affect the intracellular transport process, pathway, and enzymatic turnover rates, and play an essential role in regulating phytoplankton life cycle and physiology (Jabre et al., 2021). These physicochemical transformations shift the distribution, phenology, abundance, and composition of major phytoplankton species (IPCC, 2019). The growth of phytoplankton is mainly supported by nutrient recycling or by the mixing of nutrients from deeper waters. Generally, a small portion of PP is sustained by 'external' or 'new' nutrients, and these macronutrients control how much carbon can be permanently stored in the deep ocean. The Indian Ocean receives a countable volume of nutrients, such as phosphorus, iron, and silica through aerial deposition and from riverine input, whereas their primary sink is the sedimentation of particulate matter (Bristow et al., 2017). Thermal stratification and strengthening of the barrier layer can accelerate the warming and impede the vertical transport of deep ocean minerals (Li et al., 2020). Behrenfeld et al. (2006) suggested the prolonged decline in global oceanic NPP in the period of 1997–2006 is driven by stratification in low-latitude oceans and is related to climate variability. Therefore, as there are not many regional analyses examining the NPP changes, we assess the decadal changes in NPP and its drivers in the Indian Ocean and its primary drivers for the period 1998–2019.

We use the Copernicus Marine Environment Monitoring Service (CMEMS) Chlorophyll-a (Chl-a), photosynthetically available radiation (PAR), sea surface temperature (SST) and mixed layer depth (MLD) data with 0.25° × 0.25° spatial resolution (GLOBAL_REANALYSIS_BIO_001_029) for the period 1998–2019. The CMEMS Multi-Observation global gap-free Level-4 (MULTIOBS-GLO-PHYS-SURFACE-MYNRT-015-013) data (resolution) are used to examine salinity and Sea Surface Density (SSD) in the Indian Ocean. The CMEMS salinity reanalysis is based on the combined Soil Moisture and Ocean Salinity (SMOS) satellite data and in situ salinity measurements, but AVHRR satellite and in situ measurements from Argo for temperature. The SSD data are calculated from these temperature and salinity datasets. We also used CMEMS Multi-Observation Global Ocean ARMOR3D (MULTIOBS-GLO-PHY-TSUV-3D-MYNRT-015-012) Level-4 temperature data with a horizontal resolution of 0.25°×0.25°. We also took the CMEMS biogeochemical reanalysis with 0.25°×0.25° spatial resolution (GLOBAL_REANALYSIS_BIO_001_029) to find the changes in the distribution of major drivers of PP, such as dissolved iron, nitrate, silicate, and phosphate. This reanalysis procedure uses the NEMO-3.6_STABL ocean model with the Pelagic Interactions Scheme for Carbon and Ecosystem Studies (PISCES) module for the simulation of biogeochemistry. This model was forced with FREEGLORYS2V4 (for ocean) and ERA-Interim reanalysis data. These data have good agreement with the World Ocean Atlas (WOA) in situ measurements. For geostrophic currents, multi-observation (MULTIOBS-GLO-PHY-TSUV-3D-MYNRT-015-012) Level-4 CMEMS data with 0.25°×0.25° spatial resolution are considered. We also use the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) 2 m wind components data to calculate the upwelling and downwelling. We took the drag coefficient as 1.25 x 10^− 3 and sea water density as 1025 kg/m³. All datasets considered for the analyses are for the period 1998–2019.

3.1 Net Primary Productivity

Figure 1a shows the average seasonal NPP in the Indian Ocean (IO) for the period 1998–2019. The highest production is found in the Arabian Sea (AS) ranging from 400 to 550 mg/m³ and the lowest in the South Indian Ocean (SIO) (180–220 mg/m³) during summer and spring and, Bay of Bengal (BoB) and Equatorial IO (EQIO) show similar NPP estimates (350–450 mg/m³). However, BoB shows higher production than EQIO (about 40–50 mg/m³) in spring, but the highest in summer, as also estimated for AS. In EQIO, NPP is similar in summer and winter, with an average value of 420 mg/m³, and is the seasonal maximum there. There is high seasonal variability in NPP across the regions, which is mostly driven by the seasonally reversing winds and monsoon currents in the North Indian Ocean (NIO). We find that EQIO is less productive than NIO, which might be due to the equatorial warm pool (Beaufort et al., 1997). The spatial distribution of NPP in IO shows that the western AS is highly productive in summer and autumn, but the northern AS in winter and spring. Additionally, 70% of the coastal ocean (0–5 km from the shoreline) is highly productive (700–800 mg/m³) due to the high nutrient availability and coastal upwelling. In BoB, winter shows basin-wide high NPP, though the regionally averaged value is slightly higher in summer, which might be due to the highly productive coastal waters.

Figure 2 shows that NPP has a positive trend of about 80–90 mg/m³/yr, mostly in the northern regions of AS in spring and winter. Conversely, the NPP trend is negative in summer and autumn, where a significant basin-wide negative trend is observed in summer. In BoB, a weak positive trend is observed in the southern region in autumn and winter (-20 mg/m³/yr). The EQIO and SIO regions show an increase in NPP, which is more prominent in summer and winter. Though a negative trend is observed in EQIO for all seasons, the coastal waters of Sumatra are an exception, where a high positive trend is observed in autumn (~ 80 mg/m³/yr).

3.2 Major drivers of oceanic primary productivity

3.2.1 Temperature and salinity

To understand the control of the physical and chemical factors on the changes in NPP, we examine the temporal evolution of relevant processes in IO. On average, the mean temperature of AS is about 27°C, 28.5°C in BoB, 29.5°C in EQIO, and 25°C in SIO (Fig. 3a). There are also strong regional and seasonal differences in temperature distribution in each basin. The highest SST is found in spring (29°–30°C) in the central Indian Ocean and the lowest in autumn and winter. In SIO, SST is comparatively high in spring and winter (22°–23°C) and low in summer and autumn (20°–21°C). In BoB, temperature varies between 28° and 29°C, but the variability is high in the northern region due to extreme river discharge there (Yaremchuk, et al., 2005). In AS, lower SST is found in Oman and Yemen coasts (25°–26°C), which is prominent in winter. Generally, the high SST regions show low productivity in NIO, and the peak productivity regions have temperatures in the 25°–27°C range; showing the importance of optimum temperature for primary productivity. Even though SIO shows 8°–10°C lower temperature than that in NIO, limited nutrient supply and gyre effects make these regions less productive there (Gupta et al., 2022).

The Indian Ocean is one of the warmest oceans, and its rate of warming is about twice that of other oceans (Roxy et al., 2015). For example, SST trends show about 0.25°C/yr rise in AS, 0.20°C/yr in BoB, 0.30°C/yr in EQIO, and 0.5°C/yr in SIO across the seasons for the study period (Fig. 3c). The highest trend in NIO is observed in the northwestern AS in summer, about 0.7°–0.9°C/yr. SIO also exhibits intense warming in the western regions (0.4°–0.5°C/yr). Unlike NIO, the positive trend in SIO is mainly restricted to the western regions and the largest trend is observed in autumn (austral spring). On the contrary, the eastern region of SIO shows cooling, particularly in summer and autumn (austral winter and spring). The trend analyses show that the warmer regions (Equatorial) have low NPP and the high-temperature trend regions have a high reduction in NPP. Generally, phytoplankton is expected to grow more slowly in warmer ocean conditions, which in turn reduces NPP and seriously affects the ocean carbon budget (Huertas Emma et al., 2011; Trombetta et al., 2019).

Seasonal climatology (Fig. 3b) shows that salinity is higher than 35 psu in AS and SIO, and basin-wide high saline waters in the northern AS (> 36 psu). However, BoB has comparatively less saline waters (29–33 psu) due to the high amount of river discharge (Dandapat et al., 2020). The salinity drops by 2–3 psu in BoB and the seasonal lowest is found in the northern region during autumn–winter, which is due to high river influx in summer and seasonal stratification (Das et al., 2020; Sridevi et al., 2015). The highest positive trend of salinity in IO is observed in the eastern AS (0.15 psu/yr) during summer and autumn (Fig. 3d). Similar to temperature, salinity trend shows positive values in the west and negative in the east, particularly in SIO. The negative trend in eastern IO is strong (-0.15 to -0.20 psu/yr) and its spatial spread is larger in spring and summer. Additionally, the southern tip of Indian peninsula also exhibits intense freshening, about − 0.2 psu/yr, in winter. In BoB, the northern region shows a positive trend (0.19–0.20 psu/yr) in all seasons, except in autumn. The reduction in salinity in the northern BoB due to Extreme River input to the strengthening of stratification, which is favorable for NPP there. Even though the changes in SST are in accordance with the salinity trend, a notable change in NPP is not observed with the changes in salinity.

3.2.2 Water mass transport and mixing

The circulation in NIO is seasonally reversing with Summer Monsoon Current (SMC) flows eastward during summer and Northeast Monsoon Current (NMC) flows westward in winter (Fig. 4a). On the other hand, currents in SIO are characterized by westward flowing South Equatorial Current (SEC) in the equatorial region, which bifurcates near the Madagascar coast. The central AS shows a comparatively small surface current velocity (0.05 m/s), but the western region shows a high current velocity of about 0.3 m/s in summer. This indicates that SMC is strong in the western EQIO, which weakens as it moves eastward. In the case of coastal currents in NIO, the current speed is comparatively high on the west coast of both AS and BoB (0.35 m/s). Similarly, the surface current velocity is high in the western region of SIO. Another notable feature is the high current velocity (0.4 m/s) in the Indonesian throughflow region, which carry Pacific waters to IO. Similar to summer, winter monsoon also shows high velocity in surface currents in the western AS (yamen and Somalia coastal currents) 0.35 m/s and BoB (coastal currents; east coast of India) 0.25 m/s with small intensity in central AS and eastern part of BOB (0.15 m/s). However, the surface current velocity is higher than that of the summer monsoon (0.4 m/s) in EQIO; indicating strong NMC in the western region. The long-term current trend shows the strengthening of equatorial currents in both monsoon seasons with an average rate of 0.08 m/s/yr. In addition, a positive trend is observed in the western AS and BoB during the summer monsoon.

The prevailing consensus is that nutrients are brought to the surface from deeper waters by vertical diffusion or advection, and horizontal circulations redistribute them (Williams and Follows, 1998). The wind-driven Ekman transport controls nutrient dynamics, but the water density controls the NPP rate (Tandule et al., 2022). The major upwelling systems in IO are the western Arabian Sea (WAS), southwest coast of India, Sri Lanka Dome (SLD), Sumatra coast (SC), Seychelles-Chagos Thermocline Ridge (SCTR) (Fig. 4e–h), and they have a major role in nutrient transport. The southwest tropical Indian Ocean (SWTIO) is a key upwelling area in the Indian Ocean due to the Seychelles-Chagos Thermocline Ridge (SCTR) system due to prevalent upward Ekman pumping there. The seasonal monsoon wind systems control the atmospheric and oceanographic conditions (Ravichandran et al., 2021) and force the upwelling processes, which induce the variability in SST and Chl-a and thus, NPP in SWTIO (Horii et al., 2022).

Similarly, the western and eastern coasts of AS experience positive Ekman pumping and upwelling during summer. This results in high Chl-a and NPP in these regions, and are regionally modified by the coastal currents. The high sub-surface stratification restricts vertical mixing and thereby reducing the nutrient flux to the well-lit euphotic zone (Chen et al., 2021). Limiting the nutrient supply to the surface waters decreases NPP in the upwelling zones.

3.2.1 Nutrients

Phytoplankton needs a suite of chemical compositions in water and nutrients. Nutrients like iron (Fe), nitrogen (N), phosphorus (P), and silicon (Si) are essential for phytoplankton growth (Bristow et al., 2017). Fe is an important nutrient for photosynthesis (Schoffman et al., 2016) and the aerial deposition of biogenic iron dust can accelerate primary production (Emerson, 2019; Tandule et al., 2022), though ocean contains iron in its dissolved form (DFe). In IO, DFe is high in AS and BoB, with its highest concentration in BoB (0.0018 mmol/m³) during summer and autumn, and the lowest in spring (Fig. 5a), as also discussed in Sharada et al. (2020). The eastern IO has comparatively high DFe (> 0.001 mmol/m³) though the western IO is close to the arid land regions. In AS as found for BoB, the highest DFe is observed during summer–autumn, which is limited to the eastern AS, but the basin-wide high concentration is in summer. The positive trend in DFe during these seasons are favorable for Chl-a. Positive trend for DFe is observed in all season, which is mostly limited to northern region in AS. Even though, DFe distribution shows high values in the western IO, the highest positive trend is found in the eastern EQIO (0.0002 mmol/m³/yr) in spring. The DSi trend shows high positive values in the eastern EQIO in all seasons, and is highest in spring (1 mmol/m³/yr). The concentration of nitrate in the ocean is a proxy for phytoplankton biomass studies (Trommer et al., 2020).

The Redfield ratio suggests that the ideal proportion of oceanic particulates carbon, nitrogen and phosphorus for phytoplankton growth through photosynthesis is 106:16:1 (Sharoni and Halevy, 2020). High concentrations of nitrate and phosphate lead to more nitrogen-fixing organisms (Gruber, 2016). The northern BoB shows high dissolved nitrate (10–50 mmol/m³) in all seasons, but is very small in the southern BoB (< 1 mmol/m³). Compared to the coastal regions, which show high DNO₃ (0.1–1 mmol/m³ in the western AS and more than 10 mmol/m³ in the northern BoB), the open-ocean waters have low nitrate concentrations (0.01–0.5 mmol/m³), except at SCTR, where high values are found in all seasons. Coastal upwelling during summer monsoon contributes to high DNO₃ in the western AS and convective mixing results in the deepening of the mixed layer in the northern AS and thereby high concentrations in winter. On the other hand, the riverine input might have produced elevated levels of DNO₃ in the northern BoB. The distribution of DPO₄ shows high values in the western AS, with high values in all seasons (~ 0.5 mmol/m³). Contrary to other nutrients DPO₄ shows comparatively high values in the open ocean, although spring is an exception. Additionally, low values are found in the eastern BoB, with basin-wide low values (< 0.01 mmol/m³) in spring. We find a considerable reduction in the nitrate concentration in the western AS during the summer monsoon and northern AS in winter. A decline in DNO₃ concentration in these regions during the productive seasons is a major threat to NPP in IO, as nitrate concentration is essential for phytoplankton bloom. Additionally, SCTR also exhibits a decline in DNO₃ and is highest in summer (-0.15 mmol/m³/yr), which is the most productive season. Amidst this decline in AS and SCTR, a positive trend is observed in BoB, which is prominent in spring, autumn and winter. In the case of DPO₄, a positive trend is observed in the eastern EQIO, in summer and autumn. However, a basin-wide negative trend is observed in AS in all seasons.

The NPP in IO exhibits high seasonal variation, driven by seasonal reversing wind and current system. The basin-wide highest NPP is observed in AS during summer (400–500 mg/m³) and the lowest is in SIO during spring (180–220 mg/m³). The NPP distribution is almost similar in BoB and EQIO, ranging between 350 and 450 mg/m³. Generally, EQIO and SIO waters show comparatively low NPP (< 300 mg/m³) in all seasons, though the eastern EQIO region and western SIO waters are an exception to this. Amidst the rise in SST due to the ongoing global warming, we find an increase in NPP in the northern AS during winter–spring, which is in accordance with the negative SST trend there (-0.1–0.2°C/yr). Additionally, the increase in SST (0.7–0.9°C/yr) during summer adversely affects NPP in the western AS. Besides physical drivers, nutrient concentrations also play a vital role in long-term change in NPP. The positive trend in DFe concentration in the northern AS during winter might have favored the rise in NPP there. Similarly, the negative trend in DNO₃ during summer could have diminished NPP in the western AS. The warmer regions in IO have low NPP and further warming in the basin, particularly in the upwelling-prone regions like western AS, SCTR, and the southwest coast of India, might strengthen stratification and limit the nutrient supply to the euphotic zone. The decline in NPP in these hotspots will severely affect the marine food chain, global carbon, and biogeochemical cycles.

Acknowledgments

We acknowledge the Indian Institute of Technology Kharagpur for facilitating the study. We also appreciate all the data managers and the scientists who made available those data for this study.

Author Contributions

SM: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization. JK and AP: Writing - Review & Editing, Validation, Formal analysis, Investigation.

Conflicts of interest

The authors confirm no known conflicts of interest associated with this article.

Funding

This study received no funding

Data availability

All the data used for this study are publicly available. The CMEMS ocean temperature, salinity, density, currents, and nutrient data are available at https://data.marine.copernicus.eu/products. ERA-5 wind data are available at https://cds.climate.copernicus.eu.

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Seasonal Indian Ocean primary productivity and key drivers

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Abstract

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HIGHLIGHTS

1. Introduction

2. Materials And Methods

3. Results And Discussion

3.1 Net Primary Productivity

3.2 Major drivers of oceanic primary productivity

3.2.1 Temperature and salinity

3.2.2 Water mass transport and mixing

3.2.1 Nutrients

4. Conclusions

Declarations

References

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