Bund removal to re-establish tidal flow, remove aquatic weeds and restore coastal wetland services—North Queensland, Australia

Abstract

The shallow tidal and freshwater coastal wetlands adjacent to the Great Barrier Reef lagoon provide a vital nursery and feeding complex that supports the life cycles of marine and freshwater fish, important native vegetation and vital bird habitat. Urban and agricultural development threaten these wetlands, with many of the coastal wetlands becoming lost or changed due to the construction of artificial barriers (e.g. bunds, roads, culverts and floodgates). Infestation by weeds has become a major issue within many of the wetlands modified (bunded) for ponded pasture growth last century. A range of expensive chemical and mechanical control methods have been used in an attempt to restore some of these coastal wetlands, with limited success. This study describes an alternative approach to those methods, investigating the impact of tidal reinstatement after bund removal on weed infestation, associated changes in water quality, and fish biodiversity, in the Boolgooroo lagoon region of the Mungalla wetlands, East of Ingham in North Queensland. High resolution remote sensing, electrofishing and in-water logging was used to track changes over time-1 year before and 4 years after removal of an earth bund. With tides only penetrating the wetland a few times yearly, gross changes towards a more natural system occurred within a relatively short timeframe, leading to a major reduction in infestation of olive hymenachne, water hyacinth and salvina, reappearance of native vegetation, improvements in water quality, and a tripling of fish diversity. Weed abundance and water quality does appear to oscillate however, dependent on summer rainfall, as changes in hydraulic pressure stops or allows tidal ingress (fresh/saline cycling). With an estimated 30% of coastal wetlands bunded in the Great Barrier Reef region, a passive remediation method such as reintroduction of tidal flow by removal of an earth bund or levee could provide a more cost effective and sustainable means of controlling freshwater weeds and improving coastal water quality into the future.

Current and future carbon stocks in coastal wetlands within the Great Barrier Reef catchments

“…Such restoration opportunities are most likely over agricultural and grazing land that are non-productive because they are low-lying and frequently waterlogged, and where the market price for carbon outweighs farming profitability (Waltham et al, 2021). A promising activity that can provide high levels of abatement in Queensland is the reinstatement of tidal inundation in areas previously occupied by mangroves, such as in drains and ponded pastures (Abbott et al, 2020). These projects have the potential to decrease greenhouse gas emissions, while protecting and enhancing mangrove conditions and are likely to be eligible for carbon credits under the Australian Emissions Reduction Fund scheme in the near future (Kelleway et al, 2020).…”

Mechanical harvester removes invasive aquatic weeds to restore water quality and fish habitat values on the Burdekin floodplain

“…This allowed the formation of seasonally ponded pastures (Abbott et al . 2020), and protected coastal freshwater resources, including shallow ground water aquifers, from tidal intrusion. Most of these bunded pastures are still in place today and are perennial due to the near‐continual discharge of regulated tailwater (excess water that flows directly from cane paddocks untreated and unfiltered into adjacent watercourses).…”

“…One strategy receiving interest for helping to reduce agricultural run‐off into receiving waters is to convert low‐lying marginal cane land back to saline wetlands through removing bunds from low‐lying cane paddock (Abbott et al . 2020; Waltham et al . 2020b).…”

Improving outcomes and delivering multiple benefits in ecological restoration

“…There are examples of landholders willing to restore wetlands in north Queensland, for example restoration of degraded wetlands on a former pastoral property, Mungalla Station, on the Herbert River near Ingham has been initiated. Removal of an earth wall, reinstatement of tidal flows, and working with the property's traditional owners to control weeds and modify grazing management has been undertaken to improve water quality (Abbott et al, 2019). Alternatively, the feasibility of restoration may depend on the agricultural productivity of the current land use.…”

“…The purpose of our study was to assess the costs, benefits and feasibility of Manders et al, 2009). We therefore focus on land that has likely been historically influenced by tidal flows and hydrologically modified, and where natural recovery of coastal wetlands can be encouraged by reinstatement of tidal flows (Abbott et al, 2019). We also included abandoned aquaculture ponds, where reinstatement of tidal flows in ponds has facilitated mangrove regeneration and sediment accumulation in other tropical regions, e.g.…”