The aim of the present study was to investigate lvhether trop~cal intertidal seagrasses were better adapted to tolerate des~ccatlon than subt~dally growing seagrasses To do t h ~s , the photosynthetic performance of 8 seagrass species, gro1nng from the upper intertidal to the shallow s u b t ~d a l m Zanzibar East Africa was studied during the event of air exposure and the subsequent rehydration Photosynthet~c eff~ciencies were measured by pulse amplitude modulated (PAM) fluorometry as effective electron quantum y ~e l d s of photosystem I1 (Y) since it had recently been shown that t h ~s measure parallels rates of O2 evolutlon tor several specles under a d e f ~n e d ~r r a d ~a n c e Contrary to our expectabons it was found that the shallow intertidal specles wele in genelal more sensitive to desiccation than the deeper species Thls was expressed both as a faster decllne In Y at decreasing water contents and as an l n a b ~l ~t y to regain full photosynthetic rates d u r ~n g reh\ dratlon follow~ng even mild d e s ~c c a t ~o n as compared with the deeper-glowing speclcs One exception was the subtldally growlng S y n n g o d ~u m ~soetlfoiium, w h ~c h was very sensitive to deslccatlon The 2 specles whlch grow highest up in the Intert ~d a l zone Haioph~ia ovalis and Halodule wnghtu may not desiccate much in s ~t u during low tide because the leaves lie flat on the molst sand and for the latter specles, overlap one another so as to m n l m s e water loss Thus ~t seems that d e s ~c c a t ~o n tolerance is not a trait whlch determnes the vertical zonatlon of trop~cal seagrasses Rather, it IS hypothesised that the ablllty to t o l e ~a t e h ~g h ~~r a d ~a n c e s , as well as to benefit from h ~g h nutnent lnputs f ~o r n the shore, allows the shallow specles to occupy the uppermost lntert~dal zone KEY WORDS Desiccat~on .
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Björk et al. (1999) studied this question.