Subsurface phytoplankton community dynamics in the North Eastern Arabian Sea and their response to elevated light...

Phytoplankton are essential components of the earth's ecosystem, forming the basis of the marine food chain. Variations in environmental conditions across the vertical scale in the ocean profoundly impact phytoplankton distribution, making it essential to delineate the vertical structure of phytoplankton communities in the ocean and to comprehend how shifts in environmental conditions, such as increasing sea temperatures, could influence it. This study aimed to investigate differences in phytoplankton communities across the upper (surface to subsurface chlorophyll maximum (SCM) depth) and lower (SCM to euphotic depth) euphotic zones within the temperature gradient present in the northeastern Arabian Sea during the winter monsoon. Furthermore, with global warming simulations predicting increased light intensity and enhanced light penetration into the deep ocean, controlled deck incubation experiments were conducted for the first time to explore the response of subsurface phytoplankton communities to elevated light intensities. The findings revealed a clear vertical distribution pattern of different phytoplankton groups in the ocean with cyanobacteria (40%), chlorophytes (50-70%), and dinoflagellates (2%) favouring the upper layers, while prasinophytes (70%) and diatoms (5%) thrived in the deeper layers of the euphotic zone. However, these preferences shifted in response to increasing sea temperatures, with cyanobacteria (chlorophytes) anticipated to become more prevalent (60%, 80% respectively) in the upper (lower) euphotic zone in future warming scenarios. Additionally, when subsurface communities adapted to limited light conditions were exposed to higher light intensities, diatoms outcompeted other phytoplankton groups, emerging as the dominant group (60-90%). Among diatoms, smaller cells (< 200 µm) exhibited higher growth rates and greater resistance to stressful high light conditions compared to their larger counterparts. This study provides an ecological framework for understanding the potential response of subsurface phytoplankton communities to the effects of global warming. Moreover, it sets the foundation for future research endeavours aimed at exploring the physiological mechanisms that drive these responses.