Impact of 2015-2016 El Nino on phytoplankton size class distribution during the summer and winter monsoon in the Arabian Sea

Phytoplankton community response to extreme climate variability has an important effect on the regional marine ecosystem. The 2015-2016 El Nino event has been the strongest El Nino event on record of the century, which has a high chance of impacting the most productive ecological zone like the Arabian Sea. In this study, we investigated the influence of the 2015-2016 El Nino on the seasonal pattern and its impact on the phytoplankton size class (PSC) distribution in the Arabian Sea based on monthly satellite PSC observations and physical observations from various sources from 2003 to 2019. Results showed that the 2015-2016 El Nino event influenced the physical drivers and consequently weakened the monsoonal pattern that led to the enhancement or inhibition of specific PSC in the Arabian Sea. During the 2015 summer monsoon, the intensity of summer upwelling was weaker than normal, which caused a reduction in the abundance of microplankton to similar to 8% and an increase in the abundance of picoplankton to similar to 12%. Similarly, during the 2016 winter monsoon, the weakening of convective mixing and the formation of warm core eddy led to a reduction in the abundance of microplankton to similar to 6% and an increase in the abundance of picoplankton to similar to 9%. However, the nanoplankton distribution was observed stable with no significant response to the extreme physical condition due to its ubiquitous nature. The effect of the 2015-2016 El Nino event altered the physical conditions and associated specific PSC distribution in the Arabian Sea. Notably, the distinct response of each PSC to the changing climate and their environmental adaptation was identified in this study. Our results indicated that the phytoplankton community structure and associated marine ecosystem are sensitive to the effects of climate change.