Aerial Herbicide Spray to Control Invasive Water Hyacinth (Eichhornia crassipes): Water Quality Concerns Fronting Fish Occupying a Tropical Floodplain Wetland

Water hyacinth (Eichhornia crassipes) is an aquatic weed degrading tropical floodplains everywhere. On the Burdekin flood-plain, northern Australia, it is widespread and contributes to poor water quality, specifically hypoxia which contributes to voluminous wetland fish kills each summer. Removing weeds have focused on applying herbicides using aerial spraying, though restoration success is not monitored. Here, we investigated four aerial spray applications scheduled between November 2013 (Year 1, November 2013 to November 2014) and November 2015 (Year 2, November 2014 to November 2015) in Lochinvah wetland (35 ha wetland, Burdekin floodplain). Using high-frequency (20 min) loggers, dissolved oxygen (DO%) was tracked, which revealed that concentrations were similar before and several weeks after a spray application (independent t test, p > 0.01, except spray application 2, p = 0.06). More interestingly, aquatic weed coverage was low (5% of wetland) during Year 1 and DO had a typical diurnal cycle (20% to 130%). In contrast, low wetland flushing in Year 2 and high weed coverage (80% coverage) combined to increase DO hypoxia exposure risks for fish, with nearly 100% of the logging time failing acute and chronic values known for local fish. The Year 2 weed cover also increased water temperature exposure risk (twofold increase), which was unexpected and which means that fish probably could access cool, deeper, water refugia more frequently compared with Year 1. Controlling aquatic weeds using aerial spraying seems to have minimal risk for fish when cover is low; however, the proliferation of aquatic weeds and spraying has deleterious impact on available oxygen for fish.