Attenuation of climatic signals limits prediction of phytoplankton blooms in a temperate shelf sea

Phytoplankton blooms link climatic conditions to marine food webs and carbon cycling, yet relationships between climate forcing and bloom magnitude remain weak and inconsistent in shelf seas. Using a multi-decadal, high-frequency observational record from the western Irish Sea gyre, we investigated how climate variability influences nutrient dynamics and spring phytoplankton bloom development. Sea surface temperature, marine heatwave intensity and the North Atlantic Oscillation exhibited weak and inconsistent relationships with bloom intensity. However, climatic influences were more evident in nutrient dynamics, particularly the timing of seasonal nutrient drawdown, whereas the magnitude of nutrient utilisation was strongly constrained by winter nutrient inventories. Translation of nutrient drawdown into phytoplankton biomass showed marked attenuation: nitrate consumption explained little variability in bloom intensity, whereas silicate drawdown exhibited a moderate, non‑deterministic association consistent with functional constraints on diatom growth and community structure. Our results reveal relationships between climate variability and ecosystem function weaken across successive ecosystem levels. While nutrient inventories provide strong constraints on nutrient use, additional ecological and stoichiometric processes influence how nutrient uptake is translated into phytoplankton biomass. Our findings suggest that weak climate-bloom relationships may arise from hierarchical ecological filtering and highlight the importance of biological constraints when assessing ecosystem responses to climate variability in shelf seas.