Subsurface chlorophyll maxima in the North Eastern Arabian sea: Simulation on impact of warming

Highlights

• Density structure regulates settling depth of phytoplankton rather than light factors.

• Stratification regulates settling rate rather than cellular density/light adaptation.

• The concentration of SCM is increasing significantly for the NEAS.

• Numerical expression for SCM based on Chl distribution and environmental linkage.

Abstract

Stratified tropical oceanic systems are in general observed with Subsurface Chlorophyll Maxima (SCM), which was identified as light adaptation of shade-loving picophytoplankton groups. Apart from the light adaptation strategies of phytoplankton, the physical properties of water masses have significant role to hold the phytoplankton in particular layers. The present study gives theoretical explanation on the influence of fluid properties of water on the settling velocities of micro-nano phytoplankton groups, which contribute the SCM. The data and samples were collected from North Eastern Arabian Sea (NEAS) during March (Spring Inter Monsoon), by means of Bio-Argo floats, CTD and remote sensing. The analysis gives indications to the possible strengthening of SCM in the scenario of warming and enhanced stratification. The study conducted simulations using basic stoke’s equations on most abundant species of the area and found that the fluid density has a significant role in the settling of non-motile, suspended phytoplankton groups, irrespective of their cellular density. The simulations show strong decelerations at the same depths, in the upper part of the pycnocline but with varying settling velocity. A numerical expression derived based on curve fitting and multiple regression analysis substantiates the influence of vertical density on SCM. The sensitivity analysis (Global sensitivity Analysis) indicates warming trend in NEAS strengthening the stratification, which in turn influences the concentration in SCM and is capable of altering the primary production.

Introduction

Subsurface chlorophyll maxima (SCM) are a ubiquitous feature of the ocean and are very common in tropical and subtropical waters. This submerged biological feature contributes significantly to the column primary production with distinct variations in time and space. SCM get influenced by the prevailing physical processes, mixing, stratification (density), and the phytoplankton group that contributes to the maxima layer. Globally, many studies have attempted to explain the occurrence, dynamics, influence on the SCM due to the predominant ecosystem processes and the ecological implications due to this enhanced concentration of phytoplankton. Among these, the most notable explanation is the preference of certain shade-loving organisms resulting high abundance in the subsurface (Falkowski, 1980, Wang et al., 2016) or photo-acclimation (Fennel and Boss, 2003). Another accepted version is the adaptation of phytoplankton with low cell settling velocity to utilize diapycnal nutrient fluxes in these low light levels (Moore and Villareal, 1996). Biologically, maxima of phytoplankton biomass occur where the growth rate is balanced by respiration (loss) and the variation in sinking velocity. Thus, buoyancy of the phytoplankton cells is a major factor regulating the depth of their distribution, which is a function of water density and cell volume (Macías et al., 2013, Moore and Villareal, 1996).

A number of studies have been carried out on SCM of the Arabian Sea; the explanation of the role of tiny organisms, viz., the pico (Jochem, 1995) and nano components. Derivation of embedded maxima in vertical chlorophyll from satellite data (Matondkar et al., 2006), and time-space variation in the SCM (Ravichandran et al., 2012) at the EAS based on bio-Argo float’s profiles, have been the major contributions in the recent decade. Observations from various sources explain the occurrence of chlorophyll maxima at a 40–100 m depth column with significant seasonal variations. Accordingly, the level of occurrence of the maxima associated with the euphotic depth and variation in biomass was reported to be related to the vertical oscillations in Rossby waves owing to its association with thermocline (Ravichandran et al., 2012). Density stratification ascribed to warming acts as a barrier, which limits the vertical exchange of dissolved inorganic substances resulting in the retention of nutrient-rich waters within the subsurface. The microbial community appears to be more versatile in these subsurface low light (LL) ecotypes (Buitenhuis et al., 2012, Jochem, 1995) and the regenerated nutrient regimes, resulting in their abundance. During inter-monsoon, light is available for photosynthesis in the Arabian Sea in abundant even to the deep thermocline (Hay et al., 1993, Krey, 1976) (Brock et al., 1993). The higher light penetration during the month of March (Hay et al., 1993, Krey, 1976) resulted in abundance of Synechococcus and pico-eukaryotes (Jochem, 1995), whereas Prochlorococcus is ubiquitous with minor fluctuations. Seasonal processes of the region, especially the degree of vertical mixing is relatively weak during Spring Inter Monsoon (SIM), and it is evident with a strong and well-defined SCM. Also, the season is associated with widespread blooming of Noctiluca at surface (Gomes et al., 2008). According to recent studies, (Lotliker et al., 2018, Rosario Gomes et al., 2009) the bloom intensity and its spread, are intensifying due to warming. Therefore, the warming has a significant impact on bio-geochemical processes in NEAS, which even extends to the deep column due to biological pumping, influencing the perennial OMZ, with possible impact on the regional mesopelagic ecosystem.

At present, major studies on SCM are based on two assumptions, (1) Shade-loving - picophytoplankton and picoeukaroytes, which have a distribution throughout the water column, but contributes to SCM on their optimal light and (2) Photo-acclimation, which seems to be similar to shade-loving, but these organisms have a wide range of light adaptation and forced to withstand at low light intensity. Diatoms and less motile dinoflagellates contributed more to this category. The first category mostly docked below 100 m, and commonly called as Deep Chlorophyll Maxima (DCM). The second category related to the upper layers of ocean, and closely related to Mixed Layer Depth (MLD). In these, the photo-acclimation of surface phytoplankton to subsurface layers is less studied in NEAS. To describe the stratified system of NEAS, the work was conducted during March, the initial month of SIM.

The present study explores the possible role of the physical structure of water masses for photo-acclimation resulting in the formation of SCM. The phytoplankton groups such as diatoms and dinoflagellates have a wide range of light adaptability to adjust in response to the physical settings. A similar study by Fennel and Boss (2003), on Crater Lake, where the pycnocline is weak, suggests that small cells in less stratified waters do not contribute to SCM by settling; instead it occurs at the depth where growth and respiration compensate. It is assumed that the fluid dynamics of the regional water mass regulates the settling of phytoplankton, over the adaptation strategies of various phytoplankton groups to form SCM. SCM are more associated with stratified waters, which is getting stronger due to warming. Specifically, this study is an attempt to explain how the fluid dynamics influence the depth of settlement of surface phytoplankton groups in stratified waters thus explaining the potential impact of warming in biological systems.