Micro-estuaries and micro-outlets as incipient estuarine systems - Does size and coastal connectivity count?

Micro-estuaries and micro-outlets are very small coastal systems that share some biotic and abiotic characteristics with larger temporarily closed estuaries, but differ in that the former have small localized catchments and limited connectivity with the marine environment. This multidisciplinary study reviews the first comprehensive investigation into the ecological structure of a subset of southern African micro-estuaries and micro-outlets, elucidating the biotic and abiotic components that characterize these systems. Furthermore, the microsystems (which have an open water area of less than 5 ha and a depth of less than 1 m) are compared with larger estuarine systems in the region, with the former representing one end of a continuum ranging from micro-outlets to large estuarine lakes and bays. In terms of the abiotic regime, the micro-outlets were dominated by oligohaline or freshwater conditions, while the micro-estuaries were mostly mesohaline. Relatively few freshwater-associated microalgae, zooplankton, macrozoobenthos and fish taxa were present in the micro-outlets, while a richer combination of freshwater and estuary-associated taxa dominated the micro-estuaries. Due to their small size and limited periods of abiotic stability, microsystems only serve as intermediate nursery areas for 0+ juveniles of estuary-associated marine fish which, after less than a year, migrate to nearby larger estuarine systems in order to complete their juvenile life stages. When compared to temporarily closed estuaries, predominantly open estuaries, estuarine lakes and estuarine bays, microsystems are depauperate in terms of typical estuarine and marine biota. However, further research is recommended on microsystems in southern Africa and globally, in order to define in greater detail their physico-chemical and biological characteristics.