Long-time-scale investigation of phytoplankton communities based on their size in the Arabian Sea

ABSTRACT

The spatial and temporal changes of phytoplankton biomass in the Arabian Sea (AS), though a well-researched topic, its variability according to the phytoplankton size classes (PSCs) were less analysed and explored. Concisely, the chlorophyll-a (chl-a) concentration is not only considered as a proxy of phytoplankton biomass but also an essential factor for identifying phytoplankton community structure. Hence, the synoptic relationship between chl-a concentration and phytoplankton size classes (PSCs) (micro- >20 μm, nano-2 – 20 μm and picoplankton – < 2 μm) provides the better understanding of phytoplankton communities at regional and global scales. In this study, Data INterpolating Empirical Orthogonal Function (DINEOF) is applied on the 16 years Moderate Resolution Imaging Spectroradiometer (MODIS)-Aqua Level-3 chl-a concentration of the AS. The spatial reconstruction is performed on eight-day composite time resolution to achieve considerably accurate gap-free chl-a data. Further ocean colour phytoplankton functional types (OCPFT) model is applied on gap-filled chl-a data of the time series 2003–2018 to study the spatial and temporal distribution of PSCs. The results show that microplankton is found highly distributed in north-west and north-east AS and south-west AS during summer monsoon (June–September) and winter monsoon (December–March), respectively, due to nutrient and light availability, constituting only 16.60% of the entire phytoplankton biomass in the mean-field of 2003–2018. Conversely, nanoplankton encounters ubiquitous nature throughout the AS, constituting 51.80% of chl-a. Whereas picoplankton appeared to be dominant in lower latitudes of AS (i.e., south-east AS) because of their survival capability (nutrient and light-independent) in oligotrophic conditions, but constitute only 31.6% in the AS. Moreover, comparing sea surface temperature (SST) and photosynthetically active radiation (PAR) with the PSCs shows the relationship and distribution of PSCs towards favourable environmental conditions. Thus, PSCs estimation using reconstructed gap-free chlorophyll-a concentration provides a better understanding of PSCs shift and their dynamics on seasonal and temporal scales in the AS.

Acknowledgements

The authors thank NASA Ocean Color (https://oceancolor.gsfc.nasa.gov/) and Ocean Color Climate Change Initiative (OC-CCI) for providing the open source chlorophyll-a concentration data and NASA- SeaBASS for providing open source in-situ data for validation. The authors are thankful to the editor and anonymous reviewers for their constructive comments that helped in improving the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.