Seagrass (Thalassia testudinum) Nutrient Recycling in Florida Bay, a Carbonate-Dominated Shallow Estuary

Nitrogen (N) and phosphorus (P) senescent leaf retention, as a measure of
resorption, and the subsequent decay of senescent leaves, roots and rhizomes were
examined for the dominant tropical seagrass species, Thalassia testudinum, across a
nutrient gradient in Florida Bay. Leaf nutrient loss while still attached to the short
shoot, from both resorption and decay, was highest at the nutrient-rich (88% P; 51%
N) compared to nutrient-poor site ( 49% P; 25% N). High nutrient loss rates by
attached leaves (0.37-2.5 mg P and 6.5-34 mg N m^-2 d^-1) represented significant
recycling (46-87% P and 17-48% N) oftotal estimated external nutrient loads to the
bay. In contrast, senescent leaf, rhizome and root nutrient loss rates were > 1 00-fold
lower than intact leaves. In tropical /subtropical estuaries dominated by T testudinum,
seagrasses may be acting as nutrient pumps, translocating high porewater nutrients to
the overlying waters and promoting phytoplankton blooms in the bay.