What limits the distribution of subtidal macrobenthos in permanently open and temporarily open/closed South African estuaries? Salinity vs. sediment particle size

Thirteen estuaries in the Eastern Cape Province, South Africa, were assigned to one of four types of estuaries (river-dominated permanently open, marine-dominated permanently open, medium-sized temporarily open/closed and small temporarily open/closed), and macrobenthic zonation patterns were compared using multivariate statistics. Three major habitat zones were identified: two relating to substrate (a sand zone and a mud zone), and a third zone characterised by water of low salinity (≤2.2). These zones are inhabited by four major groups of macrobenthos. The distribution of two of these (estuarine endemic sand fauna and estuarine endemic mud fauna) is limited by the nature of the substratum, whereas the distribution of the other two (fauna originating from the marine habitat and oligohaline fauna) is limited by salinity. Estuarine endemics were present in all four types of estuaries. Marine fauna was numerically important only in permanently open systems, while oligohaline fauna was abundant only in the upper reaches of river dominated open systems. Medium-sized temporarily open/closed estuaries lacked both marine and oligohaline forms, but these were present to a limited extent in small temporarily open/closed estuaries. Hence, salinity is not the primary environmental variable determining zonation patterns in Eastern Cape estuaries, and particularly in temporarily open/closed estuaries, it is of minor importance.