Impact of rice grass spartina anglica, and the effect of treating rice grass with the herbicide Fusilade forte¬¨vÜ on benthic macro-invertebrate communities in a northern Tasmanian estuary

The deliberate introduction of Spartina anglica into Tasmania has resulted in several estuaries becoming infested with S. anglica, with growing concerns that this highly invasive species is causing adverse impacts on estuarine ecosystems. A strategy for the management of rice grass in Tasmania was developed in 1995, determining that the most cost effective and environmentally least damaging treatment for the control of S. anglica was the use of the herbicide Fusilade¬¨vÜ, which was believed to not affect native saltmarsh species or seagrasses, is rapidly degraded and appeared to have low toxicity to estuarine fauna. The strategy was largely based on two pilot studies, both of which acknowledged limitations because of their short-term sampling designs. Consequently, this study was designed to examine the possible impacts on benthic macro-invertebrate communities of using Fusilade¬¨vÜ over a longer time period. During the course of my research, Fusilade¬¨vÜ was replaced by Fusilade Forte¬¨vÜ. As a result of this change this study involved the impact of Fusilade Forte¬¨vÜ. This study aimed to 1) investigate the biological differences between benthic macro-invertebrate assemblages that inhabit rice grass and mudflat communities, and 2) examine the potential acute and chronic impacts on benthic macro-invertebrate communities of rice grass treated with the herbicide Fusilade Forte¬¨vÜ, and the potential residence time of its chemical constituents post spraying. Multivariate analyses (nMDS and PERMANOVA) showed that colonisation by rice grass changes the benthic macro-invertebrate community structure. Univariate ANOVA indicated mudflats generally exhibited lower total faunal abundances and diversity compared to rice grass habitats, and tended to be dominated by filter feeders and opportunistic scavengers rather than grazers. Differences in community structure correlated with differences in sediment size structure and organic matter content. No residues of the active constituent or any breakdown products of Fusilade Forte¬¨vÜ were detectable in oysters or water after 1 day post spraying, but Fluazifop-P (acid) is detectable in sediments up to 30 days post spraying. Multivariate analyses showed that after spraying the community was significantly different to both the rice grass community pre-spray and the mudflat communities. Univariate analyses indicated that this difference was largely driven by an explosion of grazing gastropods within the sprayed rice grass communities. By the last round of sampling (12 months post-spraying) the community structure in the sprayed rice grass area began to resemble that which occurs in mudflats. These findings support previous studies that have shown that rice grass colonisation changes the community structure of benthic macro-invertebrate assemblages. Following spraying with Fusilade Forte¬¨vÜ, the benthic macro-invertebrate community structure reverts to a pre-rice grass condition that is analogous with a mudflat community structure. The work showed that spraying rice grass with Fusilade Forte¬¨vÜ appeared to result in acute toxic impacts to the benthic macro-invertebrate communities but within months these communities appear to recover with limited detectable long-term impacts. As this work was entirely field-based and no laboratory experiments were specifically conducted on targeted macro-invertebrate taxa, direct toxicity to in situ organisms was not explicitly established. Nevertheless, there is a weight of evidence from the research, to suggest that it is indeed possible.