Seasonal patterns of macroalgal and sessile invertebrate communities in a monsoonal marine ecosystem

Abstract

Climatic transition zones are characterized by large annual variation in environmental conditions which heavily influence seasonal abundance of organisms and their assemblage patterns. With climate change altering phenology and distribution of species globally, establishing baseline biological patterns allows us to understand trends and challenges local communities might face in adverse conditions. We surveyed the seasonal biomass and cover of subtidal macroalgal and sessile invertebrate communities at 12 rocky subtidal habitats in Hong Kong to establish the first baseline for these communities. Overall productivity and composition of shallow (∼1 m depth) assemblages were prone to dynamic influences from environmental drivers and exhibited seasonal patterns, while those of deeper (∼5 m) communities had less biomass but were more stable across seasons. Abundance of fleshy macroalgae was greatest during winter, dominated by canopy forming Sargassum hemiphyllum, low-laying turfs, and encrusting algae. S. hemiphyllum had the greatest seasonal productivity on shallow substrate, reaching up to 33.8 kg m−2 in winter. In contrast, macroalgae were almost absent in the hot and rainy season, with encrusting algae defining benthic communities. Sessile invertebrate communities were generally dominated by oysters, barnacles, and live corals at both depths, characterized by a shift towards oyster-dominant during summer in the shallow. With the temperature of coastal waters increasing under climate change and winter algal growing seasons shortening, this study forms a baseline from which to assess any changes in algal productivity and community composition in the future.