Growth and population dynamics of Thalassodendron ciliatum in a Kenyan back-reef lagoon

Abstract

The size, rhizome growth, and demography of a Thalassodendron ciliatum (Forssk.) den Hartog population in a back-reef lagoon (Chale lagoon, Kenya), was examined using techniques based on age determinations. The results obtained reveal that vertical growth of the T. ciliatum shoots is very fast, involving the production of, on average, 42 internodes (i.e. 42 leaves) per year. Annual length increments declined from young shoots (17 cm year −1) to older shoots (6 cm year −1). Vertical growth is not constant over the year with internodal length sequences suggesting the presence of two adverse periods for vertical growth in a year. As a result, the T. ciliatum population allocated a significant fraction of their biomass production (31%) to upright stems, which can grow for up to 5 years (the maximum life span of vertical shoots). The vertical growth of this species is the fastest reported to date for an undisturbed seagrass population, whereas the horizontal growth rates (16 cm year −1) rank amongst the slowest. The turnover time of the shoots was about 1 year (i.e. the median shoot life span), with shoot mortality and recruitment maintaining a close balance (0.71 ± 0.04 natural log units year −1 and 0.65 natural log units year −1, respectively), indicating that the population is in a steady state condition. Because horizontal rhizome growth was slow (producing six shoots per rhizome apex year −1), shoot recruitment through branching of vertical shoots (20% of the vertical shoots bearing branches) is an important component of clonal growth of this population. These results demonstrate that vertical growth is an essential component of the production of T. ciliatum, to which it allocates considerable resources. This allocation is used to raise the photosynthetic tissues well above the sediments, thereby preventing competition with other benthic phototrophs. It also guarantees sufficient shoot recruitment for maintenance of the population.