Spatio-temporal patterns of faunal distribution along a near-pristine estuarine edge environment

Estuarine edges are important areas of estuarine systems, with high levels of both economic and ecological value. However, variable conditions created by factors such as changing tides, runoff, changing levels of habitat connectivity, shifting bathymetric conditions and fluctuating amounts of submerged structure make these areas extremely dynamic. Despite these difficult conditions, estuarine edges are important habitats for many estuarine fauna. However, the way they are utilised is not well understood. The response to temporal changes and the way in which edge fauna react to spatial changes is unclear. The overall aim of the study was to gain a better understanding of the spatial and temporal changes in the distribution of nekton along the margins of estuaries and thus to obtain clearer insight into the way these organisms utilise these habitats and respond to their varying conditions. The specific approach was to investigate the temporal changes in the distribution of edge organisms over the tidal cycle and to determine how site-specific habitat characteristics, such as the presence of structure, algal growth, rugosity, depth and current flow, may be influencing faunal changes seen in distribution.\ud \ud First, a new approach for sampling estuaries was needed in order to give a new perspective on these systems and to investigate changes over a period of time. Underwater videography was such a technique and was used throughout the study. As this is a novel method for these systems, protocols were first developed to ensure it was a valid approach for this study and that biases were limited.\ud \ud Utilising this camera technique to sample along a tropical estuarine bank in Deluge Inlet, Queensland, Australia, temporal patterns of change were observed in estuarine nekton over the tidal cycle. Abundances were greatest at times of higher tides, just as water began to flood or drain from the fringing mangrove forest, suggesting that nekton were aggregating outside the forest at these times due to their utilisation of its resources when it is inundated with water at high tides. However, abundances decreased at low tides, suggesting that not all nekton that use the estuarine edge at higher water levels remain there at low tide when the mangrove forest is unavailable. Estuarine fauna thus utilise the connections available between habitats within the estuary, although where these fauna move to at low tide is still unclear.\ud \ud While there was an obvious temporal response to tidal changes, site-specific habitat characteristics also influenced nekton distribution along the bank. The greatest max. N values were observed at two hot spot areas of the bank, where linear flow was disrupted by incoming tributaries to create turbulent currents. These broad-scale current conditions probably substantially contributed to the high abundances in these areas. However, the examination of current as a separate variable along the bank found that it did not have a highly substantial influence, and so other factors probably also influenced distribution. Depth was also important in structuring distribution, especially within the hot spot areas, although depth preference varied between taxa. The remaining variables that were investigated (algal growth, presence of structure and rugosity) had little influence over nekton distributions in this study. Overall, results suggest that it is probably a complex interaction of many factors that determine distribution.\ud \ud This study was effective in determining the spatial and temporal distribution patterns of estuarine nekton in a dynamic edge habitat. The study emphasises the complex nature of these faunal movements and the utilisation of habitat connectivity, as well as the importance of gaining a detailed understanding of the changes within an estuarine edge and how they influence organisms. Gaining insight into these systems allows for improved management and conservation efforts in these important areas