Understanding how nutrient limitation and plant traits influence carbon in mangrove‐seagrass coastal ecosystems

Mangrove forests and seagrass meadows provide critical ecosystem services, including the accumulation of "blue carbon." Plants' functional traits could influence this blue carbon accumulation. To test for interactions among functional traits and blue carbon accumulation, we conducted a study in connected mangrove-seagrass coastal ecosystems in southeast Florida (USA). We quantified how plants' above-ground traits correlated with sediment nutrient content, and how changes in traits along inland-to-coastal gradients influenced inorganic and organic carbon storage potential. Physical traits of Thalassia testudinum were higher at sites with higher sediment phosphorus (SP) and nitrogen (SN) concentrations. Sediment organic carbon concentrations were positively correlated to T. testudinum physical traits. Root density, pneumatophore abundance, specific leaf area, leaf toughness, leaf nitrogen, and phosphorus content were positively correlated with SN concentrations in the mangrove forest coastal fringe. Mangrove leaf thickness and root complexity index were negatively correlated with SP concentrations in the coastal fringe. Our results also indicate that seagrass above-ground traits and blue carbon were strongly correlated in areas with higher sediment nutrient concentrations. Moreover, mangrove root complexity is coupled with phosphorus limitation, whereby highly complex root systems develop with decreasing phosphorus concentrations. Distinct functional traits of plants drive variation in carbon retention capacity even in interconnected ecosystems.