Biotic hotspots in mangrove-dominated estuaries: macro-invertebrate aggregation in unvegetated lower intertidal flats

Macro-benthic invertebrates are important components of estuarine communities providing links between primary producers and higher trophic levels. In temperate regions, abundance and biomass peak in the mid-intertidal, giving this zone substantial functional importance. In tropical estuaries, the higher intertidal is generally occupied by mangrove forests, potentially influencing the way intertidal invertebrate assemblages are structured. We investigated the distribution and richness of benthic invertebrates across intertidal sand-and mud-flats seaward of mangrove forest to quantify the extent and location of areas of high benthic richness, abundance and biomass. Four estuaries in tropical north Queensland and 2 habitat types, sand and mud, were sampled using a van Veen grab. Macro-invertebrate richness, abundance and biomass peaked with a 'hotspot' in the lower low-intertidal, with patterns of distribution similar among substrate types. However, substantial differences in biotic and abiotic contexts between tropical estuaries and the temperate systems, where models of intertidal macro-invertebrate distribution were developed, mean that the variables influencing tropical macro-invertebrate hotspots are likely to differ from those that operate in temperate regions. The high concentration of benthic biomass in the lower low-intertidal means this zone probably regulates many of the ecological processes occurring in the intertidal regions of tropical estuaries. Consequently, any damage or modification to this zone is likely to severely impact estuarine function.