Thalassia

The invertebrate infauna of four Thalassia testudinum beds and
adjacent open sand areas in northern Biscayne Bay, Florida, were
sampled in July and December 1976. A total of 109 species, including
5804 individuals, were collected. Thalassia beds were found to harbor
a denser and more diverse fauna than open sand sites. This may be
attributed to increased food availability in the form of detrital
Thalassia and concealment from predators provided by the plant blades.
The most abundant species in the Thalassia beds included the polychaete
Cirriformia filigera, the amphipod Lembos smithii, and the isopod
Janira minuta. The most abundant species in the open sand areas were
the polychaetes Exogone dispar and Hesionura elongata, and the isopod
Xenanthura brevitelson. High within -site (grass or sand) faunal
similarity values and low between-site similarity values indicate the
presence of distinct communities associated with Thalassia and
adjacent open sand areas.

Recent evidence indicates P-limitation in tropical seagrass beds, but to date no data are available on P kinetics of the dominant tropical seagrass Thalassia testudinum. In this study, T. testudinum leaves and roots exhibited Michaelis-Menten saturation kinetics under high (0.5 to 25 muM SRP) and low (0.5 to 5 muM SRP) P concentrations. Leaf Vmax was similar under light and dark conditions. Root Vmax under the high range was slightly lower (30%) in the dark and 2 to 3-fold lower than leaves in the light and dark. Leaf and root P uptake rates were similar at low substrate concentrations. A two-phase kinetic system appears to be functioning with lower values of leaf Vmax and Km at low P levels, indicating a higher affinity for P by leaves at lower P concentrations. In conclusion, T. testudinum leaf and root tissues may contribute similarly to plant P uptake at low concentrations.

Reconstructive aging techniques were used to construct the age frequency distributions of T. testudinum short shoots in the Salt Ponds of Key West, FL. Median short shoot age was approximately two years. No evidence of sexual reproductive effort was found. A site-specific leaf emergence rate was not used, which may have contributed error in the reconstructive technique. Gradients in salinity, phosphorous availability, community composition, and leaf width were observed along the water flow pathway through the ponds. Populations of short shoots more distal from the tidal source exhibited an older age structure reflecting higher salinity, temperature, and dissolved oxygen stresses. Populations closest to apparent sources of nutrient enrichment were skewed toward young individuals. A significant positive relationship between increasing short shoot mortality and recruitment was identified. A significant correlation between low phosphorous availability and narrow leaf width was observed, and a positive relationship between phosphorous availability and recruitment was suggested.

Tropical carbonate estuaries are P-limited due to high carbonate sorption of DIP, therefore alternative P sources may represent an important contribution to the nutrient requirements of submerged aquatic vegetation. Alkaline phosphatase (AP'ase) is a cell-surface enzyme that allows aquatic plants to utilize the organic-P pool, and its activity (APA) may generate a significant portion of plant P-requirements in oligotrophic environments. Seasonal APA of the seagrass Thalassia testudinum and its epiphytic community was measured in NE Florida Bay. APA was higher for T. testudinum leaves with intact epiphytes (TTE) than for T. testudinum leaves alone (TT) in all seasons, and the spring growing season elicited the highest activities. Based on in situ PME levels (0.117--0.387muM), AP'ase P-remineralization rates were 0.326--2.181 for TTE and 0.237--0.823 for TT (mumol g-1 AFDW day-1), representing 26% and 4% of P-uptake, respectively. Therefore, APA may be an important P source to the seagrass-epiphyte consortium in tropical carbonate estuaries.