Tweed River Estuarine Vegetation Monitoring
Tweed River Estuarine Monitoring Survey - November 2012
The Tweed River Entrance Sand Bypassing Project (TRESBP) is a permanent sand bypassing system that collects sand from Letitia Spit, just south of the Tweed River entrance and pumps it to beaches to the north via a series of pipes.The system is also supplemented, as required, by occasional dredging at the river entrance. The scheme thereby maintains navigability through the Tweed River entrance while bypassing the natural coastal sand drift to the southern Gold Coast beaches.
To minimise the potential impacts of TRESBP operations on the tidal regime and subsequent health of wetland vegetation in the Tweed River, strict controls were exercised during the course of dredging operations by the placement of limitations on both the maximum dredge depths and the quantity of sand to be removed. To ensure the effectiveness of these controls, wetland vegetation within the lower Tweed estuary has been routinely monitored since commencement of the project for changes in the distribution of mangrove, saltmarsh and seagrass communities.
The most recent assessment of the condition of estuarine vegetation in the lower Tweed River was undertaken by Pacific Wetlands Environmental Consultants and released in a report dated November 2012 (Click here to download the report - 7.85mb). The study primarily used aerial photographic data from 13 May 2000, 19 April 2002, 8 June 2003, 5 March 2005, 12 June 2007, 5 April 2010 and 3 April 2012 to map changes in the aerial extents of estuarine vegetation communities over time.This data was supported by on-ground sampling that provided information on temporal changes in the structural characteristics of the estuarine vegetation and by assessments of sedimentation and surface elevation that provided information on changes in water depth.
The results of the Pacific Wetlands' study reveal that between 2000 and 2012 mangrove extent in the lower Tweed River estuary increased by 11.68% . This increase in mangrove area is typical of most other estuaries in the region and is probably the result of estuarine sedimentation and elevated water levels. At the same time, saltmarsh extent in the lower estuary decreased by 25.93%. Although potentially concerning, this rate of decline is once again consistent with other estuaries in the region and corresponds with mangrove expansion into the saltmarsh zone.
Seagrass vegetation was found to have increased in extent over the 2000-2012 period by approximately 20.08%. The majority of this increase was observed over the 2005-2007 period, with a slight decline from 2007-2010 and an increase from 2010-2012. Seagrass increases typically occur under stable climatic, hydrologic and geomorphic conditions. Thus, the increase in seagrass extent in the lower Tweed River estuary is most likely a result of stability following several years of reduced rainfall and storminess in the catchment. Indeed, even minor flood events in June 2005 and January 2008 did not cause significant declines in seagrass extent in the lower Tweed River estuary.
Analyses of vegetation quadrats revealed no significant changes in mangrove structural characteristics over the period of study. The cover of the saltmarsh species Sporobolus virginicus, however, was quite variable, declining between 2000 and 2001 (possibly as a consequence of drought), but recovering by December 2002 before declining again between 2007 and 2012. The cause of this recent thinning is unknown, although it corresponds with a ground surface erosion trend that was observed at the saltmarsh monitoring sites in the estuary.
Comparisons between the study periods of 2000-2002 (prior to any potential impacts associated with the operation of the TRESBP) and 2003-2012 revealed no statistical difference between patterns of vegetation change. This suggests that the pumping and dredging of sand by the TRESBP have not had a major impact on the distributional trends observed in the vegetation of the lower Tweed River. This is supported by the observation that the trends in vegetation distribution observed in the lower Tweed River estuary are consistent with trends in other estuaries in the region (that are not affected by TRESBP operations). In addition, assessments of tidal changes in the lower Tweed River estuary suggest that while the tidal range increased in the first few years following the initiation of sand pumping by the TRESBP, the range now approximates the pre-TRESBP condition. Ongoing monitoring of estuarine vegetation in the lower Tweed River will continue to be undertaken by the Project to assess for potential impacts associated with its operation.