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The tidal rise and fall of the sea has a great impact o n the vertical distribution of plants and animals on the shore. Different abilitie s to tolerate varying levels of desiccation and hig h temperature s determin e tha t par t o f th e shor e which organisms can occupy. These tolerances define bands or zones of plants and animal s extending roughl y paralle l to th e shorelin e an d whic h experienc e broadl y differen t conditions. Zones can, therefore, be defined b y the sessil e organisms which dominate them . Three major zone s may usually be identified: th e Littoral fringe which i s high o n the shor e an d i s dominated b y lichens , encrustin g alga e and littorini d snails ; the Eulittoral zone , the home of th e great diversit y o f shore life , suc h a s mussels, limpet s an d barnacles and the Sub4ittoral fringe which is exposed only at th e lowes t tide s and i s characterized b y brown an d re d algae an d a n eve n greate r diversit y o f animal life . Belo w the leve l of the lowes t tid e i s the Sub4ittora l zone . Th e littoral fringe i s also often referre d t o a s the Splas h zon e since it is mostly only ever wetted by wave spray. All levels of a hard shor e ar e occupie d b y a group o f marine plant s which we commonly refer to as algae (Hodgkiss 1984 ; Ho 1986). Suc h plant s ar e importan t i n tha t the y for m th e basis fo r muc h o f th e shore' s productivity ; tha t is , the y constitute th e principa l die t o f man y grazin g herbivore s such a s limpets , crabs , se a urchins , eve n fish . Man y macroalgae, with either green, brown or red fleshy or calcareous thalli , occup y lowe r level s o f th e shore . A t th e highest levels, however, the shore can be variously coloured with encrusting lichens. Between macroalgae and lichens, there ar e blue-gree n bacteria l film s (cyanobacteria ) an d encrusting brow n an d re d alga e (Kaehle r 1994) . Thoug h seemingly unimportan t i n compariso n t o thei r flesh y cousins, such bacterial films and epilithic algae are nevertheless the basic food for the tiny littorine snails that characterize this area of the shore . w฀Seasonalit y฀of฀intertidal฀algae฀ The spatial and temporal distributions of Hong Kong's intertidal alga e sho w a strong elemen t o f seasonality . I n summer (Jun e t o September ) th e intertida l roc k surface s seem t o b e bar e o f all alga l cove r wherea s i n winte r an d spring th e shor e i s rich i n plan t life . Suc h observation s were investigate d b y Hodgkis s (1984 ) who , i n winter , recorded som e 5 8 specie s o f macroalga e (1 7 green , 2 0 brown 1 9 re d an d 2 blue-green ) fro m a wid e variet y o f sites around Hong Kong, including Cape d'Aguilar. In the summer, however , thi s numbe r droppe d t o onl y nin e species. Th e alga l coun t a t expose d localities , includin g Cape d'Aguilar , wa s les s reduced tha n a t sheltere d sites , leading Hodgkis s t o conclud e tha t i t wa s exposur e t o intense summer heat, particularly during early afternoo n low tides , whic h cause d th e disappearanc e o f th e majority o f plants , throug h th e deleteriou s effect s o f water los s an d extrem e temperatur e an d salinit y changes, the latter being experienced, agai n at low tide, following heav y rain . This seasonality i n algal distribution i s very marke d and i s mos t obviou s wit h th e wid e ban d o f Sargassum hemiphyllum tha t cover s th e sub-littora l...

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