Resilience to Shading Influenced by Differential Allocation of Biomass in Thalassia Testudinum

Seagrasses are marine plants with fully developed leaves, roots, and rhizomes and a high degree of phenotypic plasticity. In the shallow waters along Florida's central Gulf of Mexico coast, leaf morphology of the dominant seagrass, Thalassia testudinum, varies along a spatial gradient in concentrations of total phosphorus (TP) in the water column. We examined ratios of aboveground to belowground biomass (AG : BG) for T. testudinum along this gradient to determine if they varied consistently with TP. Ratios were positively correlated with TP, indicating T. testudinum allocated more carbon to leaf biomass relative to belowground biomass as TP increased. To determine if this variation in AG : BG influenced resilience to shading, we carried out an 8-week, comparative shading experiment in three T. testudinum meadows that spanned the range of recorded ratios. The experiment showed that seagrasses employing a range of AG : BG strategies persisted for 5 weeks with ambient light reduced by similar to 93%. Thalassia testudinum with intermediate AG : BG exhibited the least severe impacts and strongest recovery when compared to T. testudinum with either high or low AG : BG. Seagrasses with high AG : BG ratios showed the most severe responses and weakest recovery. These results suggest T. testudinum allocates biomass such that growth and survival are maximized under the local, long-term nutrient regime, which affects the direction and magnitude of a response to a short-term reduction in light availability. In addition, we suggest that AG : BG is an important metric to monitor in T. testudinum meadows because of its potential to identify areas of high and low resilience.