Fishes and salinities of low-inflow e estuaries in the Fitzgerald Biosphere

James R. Tweedley; Kurt N. Krispyn

doi:10.29094/FiSHMED.2025.001

Estuaries are ephemeral ecosystems on geological timescales, with lifespans dictated by their degree of isolation from the ocean (Hodgkin & Hesp, 1998). The south coast of Western Australia has a Mediterranean climate, with rainfall declining and becoming more variable from west to east. Of the ~50 recognised estuaries in south-western Australia, those within the UNESCO-listed Fitzgerald Biosphere (Picture 1) receive amongst the lowest river flow and are often closed from the ocean by a sand bar at their mouths (Tweedley et al., 2016). Prolonged closure and evaporation lead to hypersalinity. Furthermore, catchment land clearing increases sedimentation, which reduces scouring and the duration of any sand bar breach. Combined with secondary salinsation, these impacts increase the magnitude of future hypersalinity (Hoeksema et al., 2018). We visited estuaries in the Fitzgerald Biosphere in November 2020, after three years of below-average rainfall. Hypersaline conditions, which are known to reduce fish species richness and faunal complexity (Hoeksema et al., 2023), were recorded in all estuaries (55-240 ppt). The prevalence of hypersalinity has presumably acted as a selection pressure, as most native fish and invertebrate species found were highly salt-tolerant (Krispyn et al., 2021). While few fish species globally can survive prolonged exposure to salinities >50 ppt, in south-western Australia five species have been recorded in salinities ≥100 ppt. The prolonged bar closures required to create these environmental conditions reduce the provision of ecosystem goods and services by these estuaries, as they cannot act as nursery areas for marine species, export nutrients to coastal waters, and the reduced faunal assemblage limits food resources for fish and birds (Cronin-O'Reilly et al., 2024). As water evaporates and salinities increase, fish attempt to move into upstream refuges. However, rock and/or sand bars can prevent this movement, generating mortality events. Climate predictions suggest reductions in winter rainfall and changes in the frequency of summer storms, which would influence bar breaching (Andrys et al., 2017). Sea levels will rise, but as the sand bars can be 5 m high and hundreds of meters long, the effect of wave overtopping may be limited. The combination of climate change and sedimentation may speed up the transition of some estuaries to salt lakes (Tweedley et al., 2024). Understanding the ecology of estuaries in the Fitzgerald Biosphere may inform the management of estuaries further west, and of low-inflow estuarine systems globally (Tweedley et al., 2019).

Fishes and salinities of low-inflow e estuaries in the Fitzgerald Biosphere

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