Leaf production, shoot demography, and flowering of Thalassodendron ciliatum along the east African coast

Several characteristics of Thalassodendron ciliatum populations were evaluated along the coasts of Kenya and Zanzibar Island, with the aim to study spatial variability in this species. A reconstruction technique, using scars left by abscised leaves and flowers, was employed to determine leaf production, shoot demography, and flowering frequency. Eight subtidal sites in different back-reef lagoons were sampled along with a subtidal site that was not protected by a reef, a site with intertidal rack pools, and a subtidal site in a mangrove bay. Leaf-production rates were lowest for the population of the unprotected subtidal site and for the population from the intertidal rock pools (30 leaves per shoot per year). At these sites, leaf life-span was almost twice as long (94 days). Low leaf-formation rates seem to be compensated by long leaf life-spans to maintain similar numbers of standing leaves per shoot. Highest leaf-production rates were found at the mangrove-bay site (53 leaves per shoot per year). The mangrove-bay population showed internodal lengths of almost 7 mm and stem lengths of almost 90 cm. The stems were two- to four-fold longer than those at the other sites. A short leaf life-span of 51 days at the mangrove site indicates that the investment in stem growth occurs at the expense of leaf maintenance. Median ages of the populations varied almost four-fold from 0.5 to 1.8 years. All meadows showed shoot-recruitment rates that were either the same as or larger than shoot mortality rates, suggesting that the environmental quality in this region is still suitable for sustaining vigorous seagrass vegetation. Flowering frequencies were generally low and seedlings were not found. (C) 2001 Elsevier Science B.V. All rights reserved.