Spatial and temporal variation of cyanobacteria-dominated epilithic communities on a tropical shore in Hong Kong

Abstract

Species abundance (percentage surface cover) and overall biomass (chlorophyll a) of cyanobacteria-dominated epilithic communities were assessed for 19 months on a semiexposed, rocky shore in Hong Kong. Thirty-three cyanobacterial species, as well as diatoms and macroalgae, were recorded. Cyanobacteria were found at all levels below the supralittoral zone, which was bare owing to the small splash zone. The upper eulittoral zone was dominated by coccoid species, especially a brown band of Gloeocapsa pleurocapsoides. A black band of Kyrtuthrix maculans dominated the lower part of the upper eulittoral. A variety of filamentous and coccoid cyanobacteria were recorded in the mid eulittoral, and a thin film of the encrusting red alga Hildenbrandia rubra dominated the lower eulittoral. Chlorophyll a levels peaked in the mid eulittoral and were lowest in the supralittoral. Broad zonation patterns were consistent throughout the year, but there was a clear seasonal variation in species abundance. In winter, macroalgae and diatoms dominated the shore, although the species composition varied year to year. Species richness of cyanobacteria was highest in winter, and nonheterocystous filamentous species (Lyngbya spp., Oscillatoria spp.) dominated. With increasing air temperatures, macroalgae, diatoms, and some cyanobacterial species died; however, species such as Kyrtuthrix and Gloeocapsa appeared to be high-temperature tolerant and increased in summer. Toward the end of summer, heterocystous cyanobacteria (e.g. Calothrix spp.) proliferated. Chlorophyll a levels were highest in winter in the mid and low eulittoral, and they greatly decreased in summer, especially in the high supralittoral. On Hong Kong shores, temporal variation in species richness, abundance, and biomass of cyanobacteria is strongly influenced by seasonal changes in environmental conditions.