Benthic macroinvertebrate faunas of microtidal estuaries in Albany, south-western Australia

Stephanie Fourie

Estuaries are unique and productive ecosystems that provide key habitats and refuges for many aquatic species including benthic macroinvertebrates. These species are an integral component of estuaries, supporting ecosystem services, such as nutrient cycling, and are a food source for higher-order consumers. Despite their crucial role, benthic fauna are often under-represented in assessments of conservation status and overlooked in aquatic management efforts. This study investigated whether the benthic macroinvertebrate faunas of eight estuaries in the Albany region of south-western Australia differed from each other, and assessed the influence of sandbar regime, water physico-chemistry and sediment granulometry. Among the estuaries, one is typically closed to the ocean (Beaufort Inlet), five open at least annually (Torbay Inlet, Taylor Inlet, Normans Inlet, Cordinup River and Cheyne Inlet) and two permanently open (Oyster Harbour and Waychinicup Estuary). Sites in three regions along the longitudinal axis were sampled during the austral summer (February), autumn (May), winter (July) and spring (November) of 2020 using a cylindrical sediment corer. Each estuary was found to have a unique set of environmental conditions and supported a significantly different suite of species. A total of 21,354 benthic macroinvertebrates representing 151 taxa from eight phyla were recorded. Overall, diversity and community composition was largely influenced by salinity, sandbar regime and coarse sediment grain sizes. In annually-open estuaries, however, grain size and particulate organic matter was found to influence benthic community structure the most. Permanently-open estuaries contained the greatest number of species and diversity. These estuaries had a consistent longitudinal salinity gradient, in which salinities were around full-strength seawater at the estuary mouth (35 ppt) and declined upstream towards the riverine reaches. This was reflected in the community composition, in which a higher abundance of stenohaline species were found at the mouth, replaced by euryhaline taxa in the middle and upper estuarine areas. In contrast, annually-open estuaries had a greater abundance overall, and their assemblages were dominated by truly-estuarine, euryhaline species. The normally-closed estuary was hyper-saline in nature and thus was depauperate and exhibited the lowest overall level of species richness, abundance and diversity. The data compiled in this thesis are among the first quantitative data on the benthic macroinvertebrate faunas of estuaries in the Albany region. These data serve as a baseline against which future changes may be measured and may be used in space-for-time substitutions to estimate the effects of climate and land-use change.

Benthic macroinvertebrate faunas of microtidal estuaries in Albany, south-western Australia

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