E-mail: j.tweedley@murdoch.edu.au 2Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth, PLI 3DH, United Kingdom

Tidal range is a master factor governing the differences in physico-chemical and biological characteristics between microtidal (<2 m) and macro tidal (>2 m) estuaries, which, for convenience, thus include mesotidal estuaries (2-4 m). Microtidal estuaries differ from macro tidal estuaries in geomorphology, tidal water movements, salinity regimes, residence times, turbidity, sedimentology, and intertidal area. Consequently, their phytoplankton, microphytobenthos, and macrophyte communities differ in biomass and production, areal extent, distribution patterns, and composition. Mesozooplankton comprise predominantly autochthonous species in microtidal estuaries and allochthonous species in macro tidal estuaries. Meiobenthos in microtidal estuaries have greater densities in subtidal than intertidal areas, and species persist along the estuary. Macrobenthos is dominated by small deposit-feeding species in microtidal estuaries, whereas macro tidal estuaries contain some larger species and suspension-feeders. Species richness and abundance of estuarineresident sh species and the contributions of diving piscivorous birds and wading invertebratefeeding birds are greater in microtidal estuaries. As paradigms regarding estuarine ecology have been based mainly on Northern Hemisphere macro tidal systems, this review redresses this imbalance by detailing the extent of differences between microtidal and macro tidal estuaries. In particular, it uses data and case studies for Southern Hemisphere microtidal systems to demonstrate that the physico-chemical characteristics and ecology of the main ora and fauna of microtidal estuaries are frequently not consistent with those paradigms.