Seagrass growth, reproductive, and morphological plasticity across environmental gradients over a large spatial scale

• Seagrasses can be subjected to a wide range of environmental variability in the coastal environment.
• Phenotypic variation can be a highly effective strategy for seagrass survival in sub-optimal environments.
• We examine phenotypic variation of separate Thalassia testudinum populations on a large spatial scale.
• Northern Gulf environments are more variable, while the Florida Keys environment is more suitable.
• Northern Gulf shoots were smaller, exhibited shorter lifespans, and higher flowering intensity.

Phenotypic variability is a valuable adaptive mechanism for seagrass species that exist in a dynamic environment and can lead to significant intraspecific regional distinctions in life history. Research is lacking in studies examining the significance of within-species phenotypic variation in relation to gradients in environmental condition at a large spatial scale. These studies are essential to better understanding the potential for acclimatization and tolerance capabilities of seagrasses in declining coastal environments. Thalassia testudinum (turtlegrass) is a ubiquitous keystone seagrass species across the Caribbean and Gulf of Mexico (GoM) that populates both environmentally dynamic estuaries and stable coastal environments. In order to elucidate environmentally driven distinctions in spatially separated populations, we examined characteristics of shoots exposed to widely separated distinct coastal environments with varying degrees of environmental stability and suitability. In our comparison, three sampling locations vary considerably in ambient water temperature, salinity, and water column clarity along a gradient from oscillating, higher stress conditions to stable, more favorable conditions. Shoots tended to have larger leaves with more biomass in the stable environment and also exhibited an older shoot age structure and higher horizontal expansion rate. However, shoots in the more variable, higher stress environment exhibited greater evidence of flowering and first flowered at an earlier age. The results elucidate large spatially distinct and environmentally relevant differences in morphology, growth, and life history highlighting the need for more studies regarding phenotypic variability of seagrass populations across environmental gradients.