Seasonal and interannual cycles of total phytoplankton phenology metrics in the Persian Gulf using ocean color remote sensing

Abstract

Spatial and temporal patterns of climatological seasonality, interannual variability, and phytoplankton phenology were estimated using satellite-derived ocean color chlorophyll-a data (Chl-a) 1998 to 2020 in the Persian Gulf from. Biogeography of phytoplankton seasonal and interannual climatology was determined using k-means multivariate clustering analysis applied on the Chl-a time-series data. As a result, two distinct regions were identified: one in the deep north and middle area (DZC) with a minimum value of Chl-a in April–July (0.62–0.76 mg m−3) and maximum in December–February (1.07–1.59 mg m−3), and the other in the north–west coastal areas and along the southwest-southern area (SZC) with a minimum value of Chl-a in March (0.84 mg m−3) and maximum in September–October (1.35–1.42 mg m−3). More than 90% of both DZC and SZC regions remained spatially unchanged during the study period. Spatial shifts from SZC to DZC and DZC to SZC were located along the boundaries between two clusters and covered 5.37% and 1.95% of the whole study area, respectively. Phytoplankton phenological metrics (timings of initiation, peak, termination, and growth duration) were estimated from Chl-a data in the stable regions during the 23-years of the study period. Interannual and seasonal variations of phenological metrics on stable pixels showed unparalleled episodic shifts in phytoplankton growth in the SZC and DZC in December–January and May–September, respectively. The initiation of phytoplankton growth shifts to earlier months by 0.58 days y−1 in the DZC and 0.56 days y−1 in the SZC, while the timings of maximum amplitude of phytoplankton growth become later by 0.59 days y−1 in the DZC and 0.47 days y−1 in the SZC, according to interannual linear trends of phenological metrics. Likewise, a relatively stable trend in phytoplankton growth duration and termination was observed over the whole study area. Geographically, a gradient of the growing period detected from northwest (90–110 days) to the east (175 days). However, the spatial analysis showed that the correlation between depth and phytoplankton phenology was not significant, although depth-dependent patterns were observed in both DZC and SZC regions. Moreover, the spatio-temporal correlation between interannual and seasonal cycles of phytoplankton phenology and wind stress, solar radiation, and Sea Surface Temperature (SST) are discussed in the context of the ecosystem state assessment. The results show that the stratification of water layers in summer and wind mixings are the primary seasonal factors affecting the total phytoplankton phenology in the Persian Gulf.