LONG-TERM DECLINE IN PICOPHYTOPLANKTON ABUNDANCE AND CARBON BIOMASS IN THE ARABIAN SEA: EVIDENCE FROM 1995 AND 2019

Picophytoplankton play a critical role in marine primary production, particularly in oligotrophic regions such as the Arabian Sea. In this study, we assessed changes in the picophytoplankton abundance and carbon biomass of Prochlorococcus and Synechococcus between 1995 (US-Joint Global Ocean Flux Study-US-JGOFS) and 2019, revealing a significant decline in both groups. Picophytoplankton abundance declined by 23% for Prochlorococcus and 37% for Synechococcus in the basin. Estimated picophytoplankton carbon stocks in the Arabian Sea decreased from 8.8 Tg C (range: 4.0–20.16 Tg C) in 1995 to 2.8 Tg C (range: 0.81–13.75 Tg C) in 2019. These declines are consistent with broader reductions in chlorophyll a concentrations and net primary production, likely driven by increased ocean stratification and reduced nutrient supply under warming conditions. While uncertainties remain regarding long-term chlorophyll a trend, recent studies support the conclusion of declining productivity in oligotrophic oceans. Our findings highlight the vulnerability of picophytoplankton communities to climate-driven changes and suggest significant implications for carbon cycling and food web dynamics in the Indian Ocean.