The presence of hydrophobic organic contaminant (HOC) mixtures in marine environments threatens aquatic life and ecosystem processes. With thousands of chemicals present in the environment, accurately estimating their potential effects remains a major challenge. Here, chemical activity is employed as a unified metric to link baseline toxicity with the overall chemical load of a polycyclic aromatic hydrocarbon mixture, which serve as model compounds for HOCs. In Paper I, exposure to the chemical mixture resulted in growth inhibition in the cryptophyte Rhodomonas salina, following a dose-response curve with an effective activity (Ea₅₀) of 0.078. Notably, chlorophyll a concentrations exhibited hormesis. Baseline toxicity impacted photosynthesis at the cellular level, which led to more pronounced effects at the population level. In Paper II, five phytoplankton species showed varying levels of vulnerability, with chemical activity explaining at least 74% of the growth inhibition. Adaptive mechanisms (e.g., increases in lipid content, Chl a hormesis) and demographic traits (e.g., species-specific growth rates) likely contributed to the unexplained variance. Natural variations in lipid content and profile, along with alterations in lipid composition due to stress, provided insights into distinct patterns for energy utilization and their connection to chemical stress. The diatom Phaeodactylum tricornutum (Ea₅₀ = 0.184) was the least affected by chemical exposure, exhibiting low lipid content and a higher growth rate. In contrast, populations of Prymnesium parvum (Ea₅₀ = 0.072) and R. salina, both with high lipid content and low growth rates, were more vulnerable. In Paper III, a natural phytoplankton and bacterioplankton community was exposed to the PAH mixture. Exposure to a chemical activity of 0.1, which caused approximately 50% growth inhibition in monocultured laboratory populations (Paper II), resulted in significant reductions in phytoplankton diversity (Paper III). Sensitive taxa, including the chlorophyte Pseudoscourfieldia marina, cryptophytes, and picocyanobacteria, declined by 40-94% (Paper III). Bacterial communities also showed reductions in both α- and ꞵ-diversity, with a shift toward dominance by tolerant Proteobacteria taxa (98% in exposed samples). To assess chemical exposure under more realistic environmental conditions, Paper IV experimentally demonstrated that passive samplers can be used to assess the uptake and toxicity of the PAH mixture in the red macroalgal species Ceramium tenuicorne. By combining passive sampler uptake data with water turbidity, a predictive model was developed to estimate the chemical activity in C. tenuicorne, providing a basis for estimating photosynthesis inhibition in the alga. This thesis advances the understanding of algal sensitivity to HOC mixtures by using chemical activity to link toxicity with chemical load. It also demonstrates the potential of passive samplers for estimating the chemical activity of HOC mixtures and assessing their ecological risks in ecologically relevant settings. The findings highlight the key physicochemical processes governing algal uptake, baseline toxicity, and the resulting effects on photosynthetic efficiency, population vulnerability, and community structure.