Sediment depth-dependent trait expression controls species contributions to nutrient cycling

Global biodiversity loss is restructuring ecological communities and compromising ecosystem functioning. Yet, we still lack an empirical understanding of how specific trait combinations regulate ecosystem processes or how their functional expression is organised within the immediate environment. Here, we investigated trait-expression across 17 functionally contrasting sediment-dwelling marine invertebrate species. We show that while sediment reworking and burrow ventilation are depth dependent, their influence on nutrient concentrations are highly nutrient-specific. Our analyses reveal that nutrient concentrations are better explained by continuous, spatially explicit trait combinations than by categorical bioturbation modes. Surface modification was associated with higher ammonium concentrations, moderated by ventilation, while deeper sediment reworking and biomass were associated with higher nitrite concentrations. Phosphate relationships were weak, and nitrate was not explained by the measured traits. These results establish a mechanistic baseline for species-specific trait-nutrient relationships under controlled conditions. This shift from categorical trait assignments to a spatially explicit, empirically resolved trait-interaction framework provides a mechanistic foundation for predicting how changes in species composition may alter benthic nutrient cycling.