Trait Plasticity and Warming Vulnerability in a Structurally Diverse Seagrass Ecosystem

Seagrass ecosystems deliver critical ecological functions but are increasingly threatened by climate change and local stressors. In temperate lagoons, thermal stress, turbidity and tidal exposure influence the structure and persistence of seagrass meadows. We investigated spatial and seasonal variability in Zostera capensis morphology and density in Langebaan Lagoon, South Africa. Field surveys and environmental monitoring revealed strong spatial contrasts: shallow, exposed sites supported small-leaved morphotypes with high shoot densities but low biomass, while deeper, cooler sites harboured sparse large-leaved morphotypes with greater canopy height and biomass. Seasonal diebacks were pronounced in late summer, coinciding with elevated temperature and irradiance, followed by regrowth in autumn. Generalised additive mixed models identified temperature, turbidity and tidal exposure as the strongest predictors of seagrass density, jointly explaining over 80% of variation. Experimental investigations confirmed thermal thresholds, with both morphotypes declining at > 26°C, indicating vulnerability to future warming. Morphological plasticity enabled some acclimation, but small-leaved morphotypes did not outperform large-leaved morphotypes under heat stress. Epiphyte loads and faunal associations were greater in large-leaved beds, suggesting potential trophic consequences if these morphotypes decline. Historical imagery indicates long-term Zostera contraction near the lagoon mouth, where large-leaved morphotypes dominate, raising concerns about habitat simplification. With ongoing climate warming, large-leaved morphotypes may persist in the short term, but further loss of structurally complex beds is likely, with cascading effects on biodiversity and ecosystem function. Conservation efforts should prioritise mitigating local stressors and preserving thermal refugia to support seagrass resilience.