Spatial Variability and Long‐Term Trends in Winter Phytoplankton of the Warming Eastern Mediterranean Sea

Abstract

The Eastern Mediterranean Sea (EMS) is among the fastest‐warming ocean regions, but the impacts of this warming on phytoplankton are unclear. Here, wintertime field observations were conducted across the EMS. These observations showed that small photosynthetic eukaryotes (<20 μm) dominated phytoplankton communities, collectively accounting for >60% of total chlorophyll a (Chl‐a) despite severe nitrate‐ and phosphate‐depleted conditions. Among the environmental variables examined, surface Chl‐a concentration and phytoplankton community structure were most strongly correlated with sea surface temperature (SST; Mantel's r  = 0.46 and 0.41, respectively), which itself was significantly correlated with surface nitrate (Spearman's ρ  = −0.49), phosphate ( ρ  = −0.48), silicic acid ( ρ  = −0.38), and nitracline depth ( ρ  = 0.63), and thus integrated a broader hydrographic gradient associated with nutrient availability. Warmer, more stratified eastern waters had lower Chl‐a concentrations, greater cyanobacterial contributions to total Chl‐a, and reduced contributions from eukaryotic phytoplankton. Quantile‐specific analysis of 20‐year satellite records showed widespread surface warming. Warming was strongest at the upper SST quantiles (spatial average 0.06°C year ⁻¹ ), which occurred mainly during summer and early autumn and was seasonally aligned with limited changes in lower Chl‐a quantiles. In contrast, lower SST quantiles warmed more slowly (0.04°C year ⁻¹ ) and occurred mainly during winter and early spring, but coincided with the most spatially extensive declines in upper Chl‐a quantiles. Together, these results suggest that regional warming is accompanied by disproportionate Chl‐a losses, which occurred during the period of higher Chl‐a concentrations in winter–spring.