Hong Kong is within the tropics and has a seasonal climate. In winter, shores support patches of ephemeral macroalgae and areas of seemingly bare rock close to crevices where molluscan herbivores are abundant. Using a factorial design of herbivore exclusions in areas far and close to crevices, the development of algal assemblages was monitored in mid-shore, cleared areas, in winter. To estimate the role of herbivore mucus deposition, half the treatments received a mucus application. Algal development was estimated from macroalgal and biofilm development and chlorophyll a levels. In all areas, biofilms (diatoms, unicellular cyanobacteria) developed rapidly in herbivore exclusions followed by ephemeral macroalgae (Enteromorpha spp. and Porphyra suborbiculata). In herbivore access treatments, however, the algal assemblage was influenced by treatment location; few macroalgae developed in areas close to crevices, and the rock was dominated by cyanobacteria. A negative relationship between macroalgae and biofilms suggested that ephemeral algae were competitively dominant. In areas distant from herbivore refuges, ephemeral macroalgae did develop, illustrating that the effectiveness of molluscan herbivores was limited to 50 to 100 cm from these refuges. The absence of large herbivorous fish, and the sparse numbers of herbivorous crabs at this site, means that algae can achieve a spatial escape from consumption, and where this occurs competition between producers is important in assemblage development. Mucus appeared to play a limited role, only sometimes stimulating initial stages of unicellular cyanobacteria and macroalgae. With the onset of summer, macroalgae died back, and rock space became available for colonization. Unicellular cyanobacteria developed rapidly but were replaced in all treatments by the encrusting macroalga, Hapalospongidion gelatinosum, which dominated treatments until the end of the experiment. On seasonal, tropical shores processes influencing community structure can, therefore, be temporally variable and their relative importance, even at the same shore level, can change with season.

The effect of substrate surface roughness on small-scale patchiness and the ability of molluscan grazers to feed on intertidal biofilms was examined in a factorial experiment. Granite slabs were treated to create 4 different levels of surface roughness, and biofilm and macroalgae were allowed to recruit. Biofilm cover varied greatly with slab roughness, and was spatially patchy at a scale of millimetres. Diatoms dominated the biofilm, but were less abundant on surfaces with the smallest pits. Cover of diatoms and cyanobacteria was affected by surface roughness, with increased abundance around surface features. Different species of grazer varied in their success at removing certain diatoms and cyanobacteria from slabs of varying roughness, due to either the morphology of the different food types or grazer radula structure. Cover of macroalgal species on the slabs of different roughness also varied, and one species, Hypnea sp., did not recruit on smooth slabs. Rock roughness, therefore, affects both the biofilm and algal species that recruit and their abundance. Grazers were able to remove algae from slabs of all roughness with no apparent species-specific differences in their ability. However, grazer species appear to be more or less efficient at feeding according to the level of roughness, and this combination of variation in rock roughness and grazer efficiency may explain the observed small-scale patchiness on rocky shores in Hong Kong.

ABSTRACT-The epilithic biofilm of troplcal shores is dominated by cyanobacteria with only a sparse cover of diatoms. Geographical comparison of biofilms is difficult, however, due to variation in enumeration techniques. This paper describes techniques to quantify the biofilm in an attempt to standardize comparative studies. Spec1c.s rlchness and relative abundance were best enumerated using scanning electron microscope (SEM) images of air d r~e d rock chips, as compared to light, epifluorescence and confocal laser mlcroscopy. SEM preparations provided cleal-lmages and allowed identification of most specles Alr d r~e d and Cryo-stage preparations prov~ded the least damaged images of species whereas the harsh dehydration process in crit~cal-point dried specimens damaged morphology and removed loosely dttached species resulting in unclerest~mation of specles richness and abundance A combination of SEh4 techniques (e.g. Cryo-stage, air d r~e d and critical-po~nt dried san~ples) is reconimended for initial species identificdtion. Chlorophyll a extraction from rock chips, as an indirect estlmate of biomass, was more eff~cient using methanol or ethanol a s compared to acetone. Methanol heated for 2 min and cooled for 12 h ylelded 100% chlorophyll a extraction while cold methanol, methanol heated for 2 min and cooled for 3 and 6 h, and ethanol heated for 5 min, extracted greater than 95% chlorophyll a. No significant loss of chlorophyll a was recorded in rock samples stored in moist or dry conditions at 4°C or room temperature for 24 h or at -20°C for a month, but in samples stored at 4°C or room temperature for a week, 60 to 75% loss occurred. Chlorophyll a, once extracted. can b e stored for a week in the dark prior to measurement without significant loss. To standardize techniques for tropical, cyanobacteria-rich epilithic biofilms, chlorophyll a extraction of rock chips using cold methanol 1s recommended.

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