Leaf production, shoot demography, and flowering frequency of the seagrass Thalassodendron ciliatum (Cymodoceaceae) along the East African coast | Anthropogenically induced changes in groundwater outflow and quality, and the functioning of Eastern African nearshore ecosystems (GROFLO)- INCO-DC: International Cooperation with Developing Countries (1994-1998) Contract number: ERBIC18CT960065 - GROFLO Final Report Annexes -- I. Meeting Reports, II. Copies of Publications, III. Questionnaires

Several characteristics of Thalassondendron ciliatum populations were evaluated by a large scale sampling effort along the Kenyan coast and Zanzibar Island, with the aim to study spatial variability. A reconstruction technique, which makes use of scars left by abscised leaves and flowers, was employed to determine leaf production, shoot demography and flowering frequency of a number of T. ciliatum populations. Eight subtidal back-reef lagoons were sampled. Furthermore, samples were collected in an exposed subtidal site, intertidal rock pools, and a subtidal mangrove bay. Leaf production rates ranged from 33 to 57 leaves shoot'1 year1. Differences could not be related to habitat type. Median ages of the populations varied almost six fold from 0.34 year to 1.93 years. The population of the intertidal rock pools showed the lowest shoot age and a small shoot size. The rock pools may experience high temperatures and light intensities. Thus, they presumably provide a more stressful environment for T. ciliatum than the lagoons or the bay. The mangrove-bay population showed internodal lengths of almost 7 cm, stem lengths of almost 90 cm and leaf life spans of ca. 45 days. The stem values were two to four fold longer, and leaf life spans were 10 to 50 % shorter than those at the other sites. This investment in vertical rhizome growth at the expense of leaf maintenance is probably caused by inferior light conditions in the mangrove bay compared to the other locations. Shoot recruitment and mortality rates differed significantly among sites. Both rates showed a decline with increasing median age, suggesting a relation with time after colonisation. All meadows in the present study were either expanding or in steady state. This suggests that, even though some sites were located near accumulations of beach hotels, all populations appeared to be healthy. Flowering frequencies were generally low. In addition, seedlings were not found in our study. These results indicate that sexual reproduction is of minor importance for the permanently submerged T. ciliatum populations, which reduces the ability to adapt to changes.

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