Fluctuating high temperature hinders long-term phytoplankton acclimation to Saharan dust.

Saharan dust aerosols are increasingly transported and deposited into Mediterranean ecosystems, with dust events doubling in recent decades. While dust supplies nutrients, it also limits light available to phytoplankton, with significant impacts on aquatic communities. Its interaction with other stressors, particularly temperature fluctuations, remains unclear but may impair phytoplankton growth, and trigger protective stress responses. The short, mid, and long-term interactive effects of dust deposition (five levels, 0-320 mg/L) and temperature (three levels, TC: control; TCH: constant high; TFH: fluctuating high) on phytoplankton metabolism were experimentally assessed in Lake La Caldera (Sierra Nevada, Spain) using in situ mesocosms and paired laboratory microcosms. Photochemical yield (ΦPSII), primary production (PP), and phytoplankton efficiency (PE) were regulated by the light:nutrient balance, peaking at high dust loads (>160 mg/L) in the short term (1 week) and stabilizing across the gradient over time. High temperature regimes induced contrasting metabolic responses: while TCH had a moderate short-term impact and promoted long-term acclimation (8 weeks), TFH had a strong short-term effect but hindered long-term acclimation. The percentage of excretion of organic carbon (%EOC), a stress indicator, doubled under TFH relative to TCH at low dust loads. These differences also appeared in the nature of the interactive Dust×Temperature effects on ΦPSII and PP. Our findings suggest that under future scenarios of increased temperature fluctuations and more frequent dust storms, these combined stressors could hinder phytoplankton metabolism in the long term. This study addresses critical knowledge gaps on phytoplankton adaptive mechanisms in a changing world.