Seagrasses--Florida--Florida Bay

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.

Seagrass meadows in Florida Bay have rapidly declined due to synergistic stresses. The microalgal communities present on the leaves of Thalssia testudinum were studied in the Snake Bight and Whipray Basin areas of Florida Bay. These areas have been the sites of recurrent phytoplankton blooms and this study was aimed at exploring similar effects within the microalgal epiphytic communities. Epiphytometers, or surrogate seagrass, were used to provide a time zero (T0) point which is impossible in natural samples. Epiphyte samples were analyzed using HPLC/PDA which provided measurements of standing crop and community structure (>90% diatom). Epiphytometers were found to be excellent tool for monitoring epiphyte productivity on Thalassia. The monitoring of productivity, standing crop and community structure should allow insight into positive and negative effects of water supply changes.