Community-level impacts of low-level diesel exposure on a natural cold-temperate/subarctic phytoplankton assemblage

Phytoplankton play central roles in marine food webs and carbon cycling, yet their responses to recurrent low-level inputs of refined oils remain poorly understood. We used a 16-day mesocosm experiment to assess the response of a natural cold-temperate/subarctic phytoplankton assemblage from the Lower St. Lawrence Estuary to repeated maritime diesel exposure. Triplicate control mesocosms were compared with triplicate diesel-amended mesocosms receiving two additions during the pre-bloom and bloom phases, equivalent to a nominal final concentration of 5.6 mg L^-1 and a simulated slick thickness of approximately 13 μm. Phytoplankton biomass, cell density, size structure, taxonomic composition, and pigments were monitored using flow cytometry, image analysis, microscopy, and HPLC. Diesel exposure did not inhibit initial exponential growth but was associated with a delayed post-bloom decline in phytoplankton biomass and cell density after Day 11, strongest in the microeukaryotic fraction and bloom-forming diatoms. Grazing dilution experiments and nutrient-amended assays indicated that biomass loss was unlikely to be primarily driven by microzooplankton grazing or nutrient limitation, while viral particle density patterns showed no clear treatment-related response. Instead, the decline was consistent with diesel-associated inhibition of phytoplankton growth and biomass accumulation, potentially amplified under late-successional conditions. Although the transition toward smaller phytoplankton groups was consistent with natural post-bloom succession, it was more pronounced under diesel exposure and accompanied by stronger losses of diatom-associated biomass and pigments. These findings suggest that recurrent low-level diesel inputs can alter phytoplankton succession, reduce diatom-associated carbon biomass, and potentially modify carbon-transfer pathways in cold coastal ecosystems.