Busting myths on water weeds

Abstract

Troublesome aquatic weeds have been an issue for the Rotorua Te Arawa Lakes for decades, but what do we really know about what drives water weed problems? Some common held beliefs about water weeds are ‘one weed is as bad as another’, ‘waterfowl move water weeds around’ and ’nutrient enrichment drives weed invasion’. However, these assumptions are incorrect or represent an oversimplified view of the problem. Exploring these popular misconceptions on water weeds will help us get a better understanding of weed problems, the role of humans and what we can do about it. This talk will draw evidence from New Zealand and international research findings and information specific to the Rotorua Te Arawa Lakes. Weed issues for the Rotorua Te Arawa Lakes involve just a few species of alien plant species that share characteristics including a high ‘standing crop’ (bulk biomass present at any one time), dense canopy at, or close to the water surface and generation of numerous fragments that can accumulate onshore. Nevertheless, these few species can be further distinguished and ranked in terms of ‘weediness’ and impact. Recognition that some weeds are worse than others allows for more effective and proactive management of weed threats. Water fowl are often implicated in spreading the most problematic water weeds that we have in New Zealand and yet there little real evidence that this is the case. Instead there is ample evidence that human activities are primarily responsible for the spread of water weeds between lake catchments. This means that it should be possible to intercept the routes and mechanisms (pathways and vectors) by which these water weeds can enter lakes. Weed invasions in freshwater systems are suggested to be linked with nutrient enrichment. However, the presence of our worst water weeds and the development of weed-related problems are not limited to eutrophied lakes, but can be equally problematic in oligotrophic New Zealand lakes. Indeed, the most enriched lake systems have very limited submerged vegetation development. Therefore improving water quality does not necessarily flow on to the anticipated improvements in weed problems and there may even be increased weed development as lakes become less enriched.