Author: Laura Schroder
Schroder, Laura, 2025 Ecosystem functioning of polychaete reefs (Ficopomatus enigmaticus) in the Coorong, South Australia, Flinders University, College of Science and Engineering
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Biogenic reefs are created by calcifying invertebrates, which are important ecosystem engineers of shallow water environments. The provision of a living three-dimensional hard structure supports biodiversity by increasing the habitat heterogeneity in coastal ecosystems. Biogenic reefs are not only created by coral or shellfish, but also by polychaete worms such as those from the family Serpulidae. The ecosystem engineering role of reef-building polychaetes has received less scientific attention than typical biogenic reef builders. Ecosystem functions from biogenic reefs can include improvements to water quality through filtration, stabilisation of sediment, and provision of habitat for fish and invertebrates. For temperate estuaries in the Southern Hemisphere, polychaete reefs can be the only provider of biogenic reefs, yet the ecological interactions and ecosystem functions of polychaete reefs are not well known. There is limited insight into the value of polychaete reefs in the absence of other biogenic reefs (e.g. coral or shellfish) as primary habitat for both invertebrates and fish. This project filled knowledge gaps on the ecological interactions of polychaete reefs, focusing on their positive effects for associated fauna in an otherwise soft-sediment environment lacking physical structure and other biogenic reefs.
The cosmopolitan polychaete species Ficopomatus enigmaticus (Fauvel 1923) is one of the most prominent providers of serpulid reefs across brackish to hypersaline conditions in sheltered coastal lagoons and estuaries. It was proposed to have an Australian origin yet, molecular evidence for a true origin remains unclear. Biogenic reefs created by F. enigmaticus vary with the environment, and growth can be prolific under eutrophic conditions. Chapter 2 presents a classification of polychaete reef morphologies, and a spatial analysis on the density and distribution of reefs in a large temperate estuary in southern Australia; The Coorong. A remote sensing and ground truth approach identified irregular, halo and platform reefs in the estuary which reached high densities, compared to circular, irregular and platform types in the lagoon which reached lower densities. Average reef size increased from the estuary to lagoon, correlating with a gradient from low to high eutrophic conditions (i.e., reduced flushing). Chapter 3 used a radiocarbon dating approach to determine the history of F. enigmaticus reefs in the Coorong. Reef cores collected were modern in radiocarbon age, providing some evidence that the establishment of F. enigmaticus in the estuary and lagoon likely coincided with major ecological changes in the Coorong over the last two to three decades.
The effect of ecosystem engineers on associated fauna can vary with reef size and spatial extent. The different habitats of polychaete reefs in the Coorong provided a unique opportunity to test the effect of reef habitats on macroinvertebrate and fish diversity and abundance in the estuary and lagoon (Chapters 4 and 5).Chapter 4 describes the native fish diversity, abundance and community associated with polychaete reefs in the estuary and lagoon of the Coorong and compares these to non-reef habitats in similar and shallow depths. Fish surveys included a total of 192 fyke net deployments in spring, summer and autumn over two years. Small bodied and estuarine species are associated with polychaete reefs in the Coorong, particularly for Gobiidae. Spatial and temporal patterns of reef communities indicate fish at different life stages from juvenile to adult are associated with reefs for sheltering, feeding or breeding behaviours. To further elucidate interactions and foraging habitat value of polychaete reefs for fish, in Chapter 5 the macroinvertebrates associated with reef, reef sediments and non-reef sediments, were compared. This project provides novel insight into the ecosystem functions of temperate polychaete reefs, and their habitat value for associated fish and macroinvertebrates.
Keywords: Polychaete reefs, ecosystem engineers, estuaries, fish, macroinvertebrates
Subject: Biodiversity and Conservation thesis
Thesis type: Doctor of Philosophy
Completed: 2025
School: College of Science and Engineering
Supervisor: Sabine Dittmann