The Remane model, derived from the Baltic region, has been consistently portrayed as the textbook model for estuarine diversity trends (Attrill 2002). However, the model was criticized for having too many limitations for general use in estuaries. This model has been extended and modified over the years to provide an alternative generalized model for describing diversity trends in estuaries (e.g. Hedgpeth 1967; Odum 1988; Barnes 1989; Hudson 1990; Attrill 2002). Whitfield et al. (2012) presented a revised review, believed as an appropriate model for describing faunal biodiversity patterns along a salinity gradient in estuaries. The revised model shows how the relative proportions of freshwater, estuarine and marine fauna change along a salinity gradient (Whitfield et al. 2012).
Whitfield et al. (2012) presented the following features in their revised model:
1) Freshwater taxa have fewer species than marine taxa in the same estuary;
2) The vast majority of freshwater species are restricted to freshwater habitats, with only a few taxa extending into mesohaline, polyhaline, and euhaline zones;
3) Marine taxa predominate in estuarine waters that are mesohaline, polyhaline, or hyperhaline.
4) Although in small numbers, marine species can be found in oligohaline estuaries and even freshwaters.
5) Estuarine species are more diverse in mesohaline and polyhaline waters, but they can also be found in oligohaline, euhaline, and hyperhaline waters.
6) A reduction in species diversity from high saline to low saline waters. Also, diversity starts to decline above a salinity of about 40.
In contrast to the ORE, the system is distinct in that it has high productivity but low diversity in comparison to other estuaries in the cool-temperate biogeographical region. Furthermore, the system has a limited number of estuarine species (Brown 1959). Low species diversity corresponded to the South African coast's biogeographical trend of decreasing species richness from east to west (Lamberth 2003). The ORE is dominated by freshwater biota rather than marine and estuarine species, with species diversity declining along a salinity continuum. Based on the estuarine association categories the distribution of freshwater, estuarine and marine species differed significantly among salinity zones but not guilds.
The solely estuarine G. aestuaria was present among all salinity zones along the river-estuarine continuum. Freshwater stragglers dominated the freshwater zone and progressively declined from the oligohaline water toward more saline water up to a salinity of 20.70 (polyhaline), while estuarine species progressed into the euhaline water. Three T. sparrmanii were recorded in mesohaline/polyhaline water during the high-flow season. As expected, Tilapias are known as tolerant of higher salinity stress (Popma and Lovshin 1994). Three species of freshwater estuarine-opportunists Clarias gariepinus, Pseudocrenilabrus philander and Oreochromis mossambicus were present in fresh and oligohaline but the latter species penetrated further into mesohaline but not in the polyhaline and euhaline reaches of the Orange River Estuary. Mozambique tilapia O. mossambicus has been recorded in salinities ranging from 0–100 (Whitfield 1998; Næsje et al. 2007), despite its highly euryhaline characteristics, it was absent in polyhaline and euhaline reaches of the Orange River Estuary. Oreochromis mossambicus may be avoiding estuarine reaches associated with an open mouth.
Estuarine & marine species were the second dominant species (i.e. Syngnathus temminckii, Caffrogobius nudiceps, Caffrogobius saldanha, Clinus superciliosus and Atherina breviceps) in freshwater and declined gradually toward polyhaline (10.0–29.9) and thereafter it increased into euhaline water. The presence of marine estuarine-dependent species M. cephalus, L. lithognathus and L. amia in fresh and oligohaline water are not surprising as they are known to penetrate the freshwater (especially if trapped) for one or two years and mature before moving out to sea to breed (Lamberth 2003; Næsje et al. 2007). The presence of piscivorous predators such as L. amia and P. saltatrix can indicate feeding usage of the ORE by the two species. Overall, the low number of marine fish species diversity in the Orange River Estuary may be related to the high dominance of riverine influences throughout the estuary. However, C. richardsonii is dominant throughout the ORE.
This study concludes that the ideas presented in both the Remane (Remane 1934) and the revised model (Whitfield et al. 2012) do not accurately reflect the biodiversity situation for ORE, taking into account the plasticity observed in species that use the river-estuarine system. As a result, this study developed a new concept suitable for the River-Estuarine Continuum (REC) in the ORE. The following features are included in the proposed new REC:
1) Both marine and estuarine species are present in all salinity zones (i.e. fresh, oligohaline, mesohaline, polyhaline, and euhaline waters);
2) Freshwater species outnumber marine and estuarine species in terms of taxa;
3) The majority of freshwater species are confined to freshwater, oligohaline, and mesohaline waters, with only a few taxa extending into polyhaline waters. No freshwater species were found in euhaline waters;
4) Estuarine species are more diverse in euhaline waters than in freshwater, oligohaline, mesohaline, and polyhaline waters;
5) The ORE is dominated by a single marine species (C. richardsonii).
6) There is a decrease in species diversity as one moves from low to high salinity waters.
In comparison to the original and revised models (Remane 1934; Whitfield et al. 2012), it is clear that fish diversity in the ORE is distinct, as more saline water lacked the substantial amount of marine taxa as hypothesized by pre-existing models. Nonetheless, fish assemblages that utilize the river-estuarine environment, particularly the dominant ones, use a wide salinity range (i.e. plasticity) along the ORE (i.e. C. richardsonii and G. aestuaria). Overall, the observed patterns were influenced by the biogeographical trend of decreasing species richness on the South African coast from east to west (Lamberth 2003). Furthermore, differences in species tolerance and preference for changes in salinity along the ORE can dictate the presence of fish species along the river system.