Zonation of benthic communities in a tropical tidal flat of north-east Australia

Abstract

Tropical tidal flats are relatively less well-known marine ecosystems. Therefore, the distribution and abundance of infaunal organisms were surveyed in a tidal flat in the Haughton estuary, north-east Australia, testing several hypotheses on characteristics of intertidal faunal distributions. Using a stratified random sampling design, macrofauna, small macrofauna (mesofauna) and meiofauna were sampled at five sites along a transect from the high to the low intertidal in April and September 1991. In total, 77 macrobenthic species were recorded during this study, with polychaetes and crustaceans richest in species. While this species record was low compared to other tropical tidal flats, the low individual densities confirmed previous findings of lower abundances in tropical than temperate tidal flats. Along the transect, species densities were highest in the mid-intertidal muddy sand and sandflats, with values ranging from 2.9 to 7.6 species 177 cm −2 for macrofauna and from 2.2 to 3.8 species 10 cm −2 for mesofauna. At the Callianassa site in the mid-intertidal 35 species were recorded, while the lower sandflat site had the highest diversity ( H′=2.60). Macro- and mesofauna abundances were highest at the sandflat site (median values for macrofauna: 65 and 69 ind. 177 cm −2 in September and April, respectively, and 37 and 48 ind. 10 cm −2 for mesofauna). There was little variation between the two sampling dates, although single taxa occurred with significantly higher abundances in one of the two months. Polychaeta and Amphipoda were abundant at the sandflat and Callianassa site, juvenile bivalves were most frequent in the sandflat after a spatfall in September. There was no pronounced increase of suspension feeders in the lower intertidal, and deposit feeders dominated the fauna. Meiofauna was abundant throughout the intertidal with median values up to 310 ind. 5 cm −2. Their densities were highest in the lower intertidal and lowest at the transect site with Avicennia mangroves. Nematoda dominated the meiofauna at all transect sites, and only in April were copepodes more abundant at the Callianassa and sandflat site. Altogether, the benthic fauna in this tropical tidal flat showed a zoned distribution between the tide marks. While no meiofauna assemblages could be distinguished with multivariate analysis at the phylum level, defined groups were found for the macrofauna, corresponding to a zonation of distinct assemblages at the high intertidal mudflat, the mid-intertidal Callianassa and sandflat sites, and the lower intertidal sandflat. Based on this analysis and further information from qualitative mapping and literature recordings, benthic communities in tropical tidal flats of north-east Australia are described.