MEPS 570:57-70 (2017)  -  DOI: https://doi.org/10.3354/meps12112

ABSTRACT: Key metrics of Thalassia testudinum aboveground morphology vary along a spatial gradient in concentrations of total phosphorus in the water column that has persisted for over a decade off the central Gulf of Mexico coast of peninsular Florida. Leaf area shoot^–1, shoot density, shoot height, and areal biomass of T. testudinum, the dominant seagrass, were measured at sites along this gradient. The roles of differential production and nutrient supply were assessed as potential drivers of these relationships via growth rates, quantities of carbon (C), nitrogen (N) and phosphorus (P), and stable isotope ratios of leaves (δ¹³C, δ¹⁵N). Leaf area shoot^–1, shoot height, and shoot production were positively correlated with total P (TP), whereas shoot density and leaf N:P and leaf C:P ratios were negatively correlated with TP. There were no significant relationships with TP for epiphyte load, areal biomass, areal production rate, or leaf C:N ratio. The results also suggested that T. testudinum maintained a relatively uniform aboveground biomass and specific growth rate across the range of P concentrations observed in the water column during this study. The findings highlight the extent of morphological plasticity of T. testudinum. Collectively, the morphology, growth rate, elemental content, and stable isotope findings offer compelling evidence that T. testudinum along the Gulf coast of peninsular Florida balances shoot morphology and density to maintain growth and survival across a wide range of nutrient supply.

KEY WORDS: Seagrass · Nutrients · Environmental gradients · Stable isotopes · Gulf of Mexico · Phenotypic plasticity · Phosphorus