Phosphorus uptake kinetics of a dominant tropical seagrass Thalassia testudinum

Although nitrogen is primarily the dominant nutrient limiting seagrass production in temperate estuaries, phosphorus (P) limitation can be important in tropical carbonate-dominated seagrass systems. While nitrogen uptake kinetics of seagrasses are moderately well established, very limited data exist on the dynamics of P-uptake. In this study, we determined the kinetics of dissolved inorganic phosphorus (Pi) uptake for a dominant tropical seagrass Thalassia testudinum across a range of Pi levels (0.5–25 μM). Under this broad range, leaf Pi-uptake (μmol g−1 dw h−1) rates were similar under light (Vmax=1.90) and dark (Vmax=2.10) conditions, while root Pi-uptake rates declined 30% in the dark, and were significantly lower than leaves under both light (Vmax=0.57) and dark (Vmax=0.38) conditions. At lower Pi concentrations (0.5–5.0 μM), leaf Vmax was 2–3-fold lower (0.50–0.77), while root Vmax was the same at high and low Pi ranges. Based on linear and non-linear models of Pi-uptake kinetics for T. testudinum, leaves can contribute a majority of the P sequestered by the plant when surface and porewater Pi levels are equally low (0.05–0.5 μM). Based on the calculated P-demand of T. testudinum in South Florida, solely root or leaf uptake can account for the P requirements of T. testudinum when porewater or surface water Pi levels are 0.5 μM. However, when surface and porewater Pi levels are extremely low (<0.10 μM), such as in Florida Bay and other carbonate seagrass systems where Pi sequestration by the sediment is highly efficient, even root + leaf Pi-uptake rates do not meet the P requirements for growth and P-limitation may occur.