Author

Robert Dixon

Year

2017

Degree Name

BEnviSc Hons

Department

School of Earth & Environmental Science

Advisor(s)

Kerrylee Rogers

Abstract

Mangrove and saltmarsh contribute extensively to health of estuarine systems. Their extent and distribution is dependent on hydrological regime, in particular, tidal inundation. The hydrological regime of Intermittently Closed and Opened Lakes and Lagoons (ICOLLs) usually limit mangrove growth, primarily during entrance closure when anoxic shock occurs from the prolonged inundation of pneumatophores. Saltmarsh however, can be extensive in these systems. Entrance training alters the hydrological regime of ICOLLs by preventing closure, which ensures a constant, and increasing tidal exchange promoting mangrove expansion. This has occurred at Lake Illawarra, particularly on the southern shoreline following recent (2007) entrance training. Increasing mangrove coverage could lead to declines in saltmarsh which is classified as an endangered ecological community following significant losses elsewhere in southeast Australia.

This study aimed to investigate mangrove dynamics in Lake Illawarra, focusing on the response to entrance training. Mangrove response was defined on the basis of the ecological niche that it occupies, which was quantified using spatial and field based techniques. The term accommodation space was used to conceptualise mangroves niche and was defined as portion of the intertidal zone where the majority of post-entrance training recruitment occurred. Saltmarsh accommodation space was assigned above mangrove to the tidal limit. The current accommodation space, as well as its future extent under increased sea-levels and tidal amplitude at 2050 was also quantified.

The majority of mangrove increase between 1977 and 2016 occurred post-entrance training. Growth was concentrated in entrance back channel (Zone 1). The intertidal zone between Mean High Water (MHW) level and slightly below Mean Sea Level (MSL) supported the greatest proportion mangrove establishment and was defined as the accommodation space. If mangrove could occupy all of the current accommodation space, considerable areas of saltmarsh could be lost. The areas at 2050 suitable for mangrove and saltmarsh were found to be dependent on local topography, with retreat possible in some areas and exclusion in others.

This research suggests that entrance training allows for mangrove expansion which has the potential to impact on saltmarsh. As increased mangrove coverage may have a range of ecological benefits, it is recommended that management strategies be applied that focus on saltmarsh preservation rather than removing mangrove. Strategies could include, allowing for managed retreat under sea-level rise and controlling for other factors which favour mangrove expansion in saltmarsh areas.

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