Seaweed functional traits predict species' roles in nutrient cycling and primary productivity

Functional traits - organisms' attributes that are quantifiable at the individual level - have emerged as a framework for predicting species' roles as mediators of ecosystem functions and services in terrestrial, freshwater, and pelagic marine systems. However, despite the high productivity and diversity of seaweeds in coastal habitats, we currently lack a trait-based framework for evaluating the effects of macroalgae on ecosystem functions such as nutrient cycling and productivity. Based on previous research, we predicted that resource acquisition by seaweeds, including nitrate uptake and photosynthetic rates, would be associated with species' surface-area-to-volume ratios (SA:V). However, given the potential for other traits (e.g., thallus dry matter content, specific thallus area) to influence resource acquisition we also investigated a broader range of potentially linked traits that captured the diverse morphology and ecological strategies of seaweeds.

We filled this knowledge gap by measuring eight morphological traits of 15 seaweed species and using Principal Component Analysis to evaluate multivariate trait space and explore relationships between traits (i.e., Principal Component axes) and Michaelis-Menten parameters of nitrate uptake and photosynthetic rates.

We found that nitrate uptake parameters were most closely related to the first Principal Component axis (PC1), which was associated with traits such as SA:V and the specific thallus area (STA). In contrast - and contrary to our predictions - photosynthetic parameters were related to PC3, which was associated with thallus dry matter content (TDMC), linking photosynthetic performance to metabolically active (i.e., low TDMC) structural composition.

The fact that parameters associated with acquisition of different resources (i.e., nitrate uptake and photosynthesis) were underpinned by different, orthogonal functional traits was unexpected and highlights the utility of trait-based approaches in providing new insights into the links between seaweed form and function, with implications for ecosystem functioning and services.