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The previously reported rapid spread of mangroves at Port Gawler on the northeastern shore of Gulf St Vincent, South Australia has continued. This is a tidal-dominated coastline where reduced incoming tidal currents have led to increased sedimentation on the tidal flat and the development of an area of samphire and mangroves that have split the tidal flat in two. We predict that the remaining sections of the tidal flat will eventually become a mature mangrove forest, although human activity may delay this process. [ABSTRACT FROM AUTHOR]

Urbanised coastlines are affected by cumulative impacts from a variety of anthropogenic stressors, but spatial information on the distribution of these stressors at the local scale is scarce, hindering the ability of managers to prioritise mitigation options. This work investigated the spatial footprint of land-based nitrogen discharges to a metropolitan coastline and assessed the potential role of this stressor alone on seagrass dynamics at the scale of the ecosystem. The macroalga Caulocystis cephalornithos was used as a time-integrative sampler of nitrogen in the water column over 202 sites monitored across an area of ∼800 km 2 . The stable isotopic signature of nitrogen in tissues (δ 15 N) was used to map plumes of anthropogenic origin. The surface area of these plumes was found to be proportional to nitrogen loads from land. The largest plume was associated with discharges from an industrialised estuary and a wastewater treatment plant, where a monthly nitrogen load in excess of 110 tonnes affected an area >80 km 2 . The location and size of the plumes changed with seasons as a result of wind forcing and rainfall/wastewater reuse. The location of the plumes was compared to published seagrass distribution obtained from video transects. Dense seagrass meadows only occurred in areas unimpacted by plumes throughout the year, mostly in shallow (<5 m) regions for Amphibolis antarctica , and deeper (5–10 m) for Posidonia sp., possibly as a result of this species higher tolerance of low light conditions. This higher tolerance might also explain why Posidonia sp. is observed to preferentially recolonise areas of previous loss in the region. While a decrease in the spatial footprint of nutrient plumes has created conditions for natural seagrass recolonisation in some areas, it did not halt seagrass loss in others, suggesting the influence of additional stressors such as wave dynamics and light attenuation due to turbid/coloured stormwater. [ABSTRACT FROM AUTHOR]

The northeastern shore of Gulf St Vincent, South Australia is a mesotidal, low-wave-energy coastline, characterised by tidal-dominated bioclastic carbonate sedimentation. Distinctive assemblages of subtidal, intertidal and supratidal marine plants and invertebrate animals have resulted in well defined, adjacent bio- and litho-facies that lie parallel to each other across broad areas of coastal sedimentation. At Port Gawler, the intertidal zone is bounded by a landward high-tide beach and a subtidal Posidonia australis seagrass meadow seaward. Thirty years ago, within the immediate study area, this intertidal zone comprised an inner sandflat and an outer muddy seagrass meadow dominated by Zostera muelleri, the two areas separated by a shallow tidal distributary. Avicennia marina is a species of mangrove that has colonised much of the protected, tidal-dominated coasts of South Australia. Propagules of this tree readily take root in intertidal mud, with a natural ecological succession of Z. muelleri followed by A. marina. Over the time of observation, juvenile mangroves have progressively colonised a narrow zone along the landward side of the muddy seagrass meadow; they appear unable to colonise a sandy substrate. Juveniles advanced rapidly southward from mature woodland in the north towards equally mature woodland in the south. From the initial tap root, juvenile plants send out radial roots from which project, snorkel-like, an array of vertical pneumatophores. Redistributed algal, seagrass and other organic flotsam collect around these vertically protruding pneumatophores, promoting accumulation of organic sediment. Within the newly forming mangrove woodland, there has been pronounced sediment aggradation of >0.5 m, greatly altering the dynamics of tidal flow. Tidal flow is now largely constrained to a channel between the mature southern woodland and that formed most recently. Higher velocity flows through this channel have exhumed populations of infaunal bivalves, and floodtide and ebbtide deltas of coarse bioclastic sediments are a consequence. The aggraded sediment of the new woodland shelters the area landward. Consequently, more muddy sediments are now covering what was formerly bare sandflat, thus providing new areas for mangrove colonisation. Migration of mangroves towards the high-tide beach is inevitable and will be limited only by their need for regular flushing of accumulated salt from their roots by seawater at high tide. Thus, A. marina drives rapid geomorphic change in the tidal-dominated coasts of the South Australian gulfs. In similar coastal areas of southeast Asia, where intertidal muddy sediments predominate, this species appears to be an ideal candidate for reforestation projects. Juvenile plants should grow successfully and rapidly, so preparing coastal environments for the establishment of more ecologically diverse mangrove communities. [ABSTRACT FROM AUTHOR]