Short term response of plankton community to nutrient enrichment in central eastern Arabian Sea: Elucidation through mesocosm experiments

Area of Science:

• Marine Ecology
• Oceanography
• Microbial Ecology

Background:

• Oligotrophic waters (OW) typically support longer food chains driven by the microbial loop.
• Physical mixing events like upwelling inject nutrients and propagules, potentially altering plankton communities and food chains.

Purpose of the Study:

• To investigate the impact of deep-water nutrient enrichment on surface plankton communities in the Arabian Sea's oligotrophic waters.
• To assess the response of different plankton groups to nutrient addition and the role of microbial communities.

Main Methods:

• Mesocosm experiments using custom-designed enclosures in the central-eastern Arabian Sea (CEAS).
• Nutrient enrichment of surface waters with deep-water (100-150 m) content.
• Monitoring of plankton abundance (picophytoplankton, bacteria, protists, larger plankton) and chlorophyll-a levels.

Main Results:

• Surface waters were nutrient-poor with low chlorophyll-a (<0.2 μg m⁻³).
• Deep water contained higher concentrations of picophytoplankton, bacteria, and protists.
• Inoculation of surface waters with deep water significantly enhanced picophytoplankton, bacteria, and protist populations.
• Larger plankton abundance showed no significant change within three days.
• Despite positive picophytoplankton response, chlorophyll-a did not increase significantly, likely due to insufficient microphytoplankton propagules.

Conclusions:

• Microbial communities dominate the response to physical mixing in oligotrophic Arabian Sea regions.
• Propagule diversity is crucial for significant shifts in plankton communities, particularly for larger phytoplankton.
• Findings inform ocean modeling, forecasting, and global environmental management strategies.