Can an aquatic weed control outcome be too good? And is “collateral damage” Ok for long term gain?

Entering into a program to control Cabomba caroliniana using an aquatic herbicide brings with it a range of questions: will this herbicide be effective at controlling Cabomba in this water body? Will we waste large amounts of funding on poor weed control outcomes? Will there be impacts on the aquatic ecosystem? Well…in Glenbrook Lagoon in NSW the answers were Yes, No, Yes!

Until recently Glenbrook Lagoon was densely infested with Cabomba (Cabomba caroliniana), an invasive aquatic weed listed as a Weed of National Significance (WoNS). This infestation had been prioritised in the top ten national for control due to it's outlier status, threat of spread to significant ecological and economic aquatic assets (including Sydney drinking water reservoires, Hawkesbury-Nepean river system, World Heritage National Park waterways), and its feasibility to be controlled.

In December 2012 the Cabomba infestation was treated using ‘Shark Aquatic Herbicide’ (active ingredient: ‘carfentrazone-ethyl’) or ‘Shark’ (as per label directions). The application was intended to treat 50% of the infestation, but was effective in controlling Cabomba across the entire water body.

Dissolved oxygen depletion had been identified as a potential impact of the aquatic weed control program, thus a water quality monitoring program was implemented to observe dissolved oxygen levels. Following the rapid dieback and decomposition of Cabomba and other aquatic weeds, together with other contributing factors (including stratification; warm, still prevailing weather conditions; and high water temperatures), severe oxygen depletion was observed across the entire lagoon. Signs of distress in some fish species were observed and a fish kill of stocked Australian Bass followed a prolonged period of extremely low oxygen levels (<0.6 mg/L (6%) DO recorded at 10 am on the day of the fish kill). Attempts to re-oxygenate the 8 ha lagoon were largely unsuccessful.
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