Importance of location and habitat structure in determining nearshore fish and macroinvertebrate assemblages within Botany Bay, Australia
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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- Estuarine habitats are particularly susceptible to anthropogenic disturbance, however the consequences of habitat loss are not predictable because processes determining distributions of nearshore small fish and macroinvertebrates are not well understood. In this study the importance of location and habitat structure in determining nearshore faunal assemblages within Botany Bay, Australia, was investigated over four years in a series of mensurative and manipulative experiments, incorporating different spatial scales (metres and kilometres). In addition to natural habitat, artificial seagrass beds and patch reefs were used as standardised habitats in field and aquarium experiments. Emphasis was placed on recruitment of small fish and macroinvertebrates to seagrass (Zostera capricorni) beds in the bay's north - an area that has a history of abundant recruitment of several economically valuable fishes but has also been altered by development, such as construction of an airport runway. The location of Zostera beds within Botany Bay was found to influence assemblages of small fish and macroinvertebrates. Consistent patterns in macrofaunal assemblages in seagrass beds were observed for several sites. In particular, the pattern of abundant recruitment of several economically valuable fishes to North Botany during winter and spring was still evident after the airport runway construction. This finding highlights the importance of this area for recruitment of small fishes within the bay. In contrast to seagrass location, seagrass complexity (shoot height and density, bed area) did not explain the spatial patterns in recruit abundance in Zostera beds or artificial seagrass beds at North Botany and Kurnell. Nevertheless, seagrass structure was still important in determining nearshore faunal assemblages when compared to other available habitat types. There were considerable differences in macrofaunal assemblages in artificial seagrass beds and patch reefs over different spatial scales (metres and kilometres). In particular, several small fishes and macroinvertebrates in seagrass beds did not recruit to patch reefs, irrespective of equivalent volumes of shelter available in the two habitats. The species that contributed to differences between seagrass beds and patch reefs varied between sites but there was still a noteworthy absence on reefs of taxa associated with seagrass beds. Reef-associated taxa were more frequent on patch reefs at sites near to the estuary mouth, where natural rocky reef occurs. However, spatial patterns for the entire macrofaunal assemblage on patch reefs were not explained by distance into Botany Bay. Instead, assemblages of small fish and macroinvertebrates were more localised. This finding was consistent with that observed for assemblages in natural and artificial seagrass beds. In addition to habitat structure, aspects of natural habitat such as chemical or biological cues influenced macrofaunal assemblages in seagrass and on patch reefs over small spatial scales less than 200 m. Apparent preference of three fishes (Achoerodus viridis, Rhabdosargus sarba, and Pelates sexlineatus) for habitat that had been soaked in seawater indicates that factors associated with the natural habitat other than structure may influence distribution of these fishes in seagrass and on patch reefs in shallow water. It is likely that a combination of location, habitat structure and characteristics of the natural habitat influence assemblages of small fish and macroinvertebrates in nearshore habitats within estuaries. It is concluded that species within Zostera beds will not recruit to any available nearshore, habitat and that management of seagrasses within Botany Bay should occur at a fine spatial scale less than 1 km.
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