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.