Lessons Learned at Christmas Lake
Many of you have probably already heard – and possibly felt dismayed by – the recent news that divers discovered ten zebra mussels in Christmas Lake after it was treated using Zequanox, copper, and potash last fall and winter.
The efforts at Christmas Lake are not part of a controlled experiment, rather a real – life rapid response attempt by lake managers to kill a newfound infestation of zebra mussels. MAISRC researcher Dr. Michael McCartney has been serving as an advisor to the management team led by Minnehaha Creek Watershed District and the Minnesota Department of Natural Resources.
“Participating in an open lake treatment such as this has been an extremely valuable opportunity,” says McCartney. “It’s more challenging to get the cause and effect data without controlled conditions, but we are still able to learn important lessons that will guide future attempts.” Some of what McCartney says he has learned from this experience includes the following:
- We now have a better estimate of the short-term mortality rates achievable with pesticide treatments. Under open-water conditions such as Christmas Lake, treatments showed that pesticides killed 100% of zebra mussels in the treatment area, and approximately 98% of zebra mussels in the lake. This is among the highest kill rates ever documented for an open-water treatment of zebra mussels and is overall a very positive outcome. The long-term impact this will have on the lake population of zebra mussels, however, is still unknown, and will require longer-term study of lakes, including Christmas.
- Removing the impermeable barrier in a timely fashion is important. Marrone Bio Innovations (the manufacturer of Zequanox®) recommends leaving an impermeable barrier in place around the treatment area for a maximum of 24 hours. When left longer, as happened at Christmas Lake, a decline in dissolved oxygen can occur for as long as the barrier is retained, which increased the risk of impact to non-target organisms. This also complicated the conclusion that zebra mussel mortality in the first treatment was due to Zequanox® and not also to the lack of available oxygen, and it confounds the evaluation of Zequanox® mortality from this open water treatment.
- There are challenges in maintaining proper pesticide concentrations in open water scenarios. The second treatment at Christmas Lake used the EarthTec QZ® brand of copper sulfate. Workers discovered that upon application, concentrations of the pesticide dropped rapidly – to below the lethal level for zebra mussels. This prompted the need for continued monitoring and adjustment of concentration of the pesticide to maintain the lethal dose. It is hypothesized that this drop is due to uptake of the copper sulfate by lake organisms – probably algae – however this remains to be further studied.
- Under-ice application of Potash is not a viable option at this time. Researchers discovered that it was too difficult to maintain evenly dispersed, toxic doses of potash when applied under the ice. It is not clear to what extent these problems related to cold water or to restricted water circulation. Potash may still be a viable option during warmer months if permitting and labeling regulations allow for it.
- Scientifically-based survey methods are needed to establish treatment areas and to evaluate treatment outcomes. When divers searched for signs of mussels in April, they came up empty, suggesting that no mussels remained in the lake after treatment. In May, workers searched again and found ten adult mussels just outside the treated area. Survey and census methods prior to treatment may be improved as a result, to give future workers a better chance of making the treatment area large enough, given uncertainty in the spatial extent of an infestation. Improved survey methods after treatment, developed in part from lessons learned on Christmas Lake, are expected to also give future workers a clearer view of the abundance and distribution of the lake zebra mussel population. This information is needed to better evaluate how the lake population responds to the treatment, and to plan any follow-up efforts.
- Prevention is still the best option. The scenarios being tested at Christmas and other lakes are those in which the infestation has been detected before the zebra mussel population has expanded or dispersed beyond a very small area (in Christmas Lake, infestation was initially believed to be confined to ~0.1 acres). Whole lake treatment by any of these three pesticides is not a viable option due to cost, non-target impacts, and technical challenges including those described above. Much work is needed to understand potential controls for zebra mussels. In the meantime, the best option for our lakes and rivers is to prevent these species from being introduced in the first place.
While McCartney continues his research on understanding pathways of zebra mussel spread in Minnesota, he is also working to expand his research portfolio by evaluating the long-term effectiveness of pesticide use to control lake populations of zebra mussels. As part of this additional effort, McCartney will focus on developing and refining sampling protocols to be used in future pesticide treatments. Not only will this potentially improve treatment results, it also will allow for the collection and sharing of data so other managers around the state can learn from these early efforts at Christmas– and other lakes as well.