The Wetlands Initiative is seeing very positive signs that the invasive common carp has finally been eradicated in Hennepin & Hopper Lakes, which provided a window of opportunity this spring to reestablish higher-quality native lake vegetation.
TWI has been battling the invasive common carp in Hennepin & Hopper Lakes at the Dixon Waterfowl Refuge for several years, and our 2012 efforts appear to have been successful. Among the indicators is the depth of light penetration, or water clarity. When carp are present the water clarity is low, as they disturb the lake bottom when they feed. When Hennepin & Hopper were dominated by carp in recent years, the light penetration was less than 3 inches. Current readings are more than 12 feet in depth—by far the best the water clarity has been since TWI began restoration at the site.
The carp did tremendous damage to the lake vegetation, but TWI found the silver lining to that cloud: The lack of competing vegetation in the open water this spring created a unique window of opportunity to plant high-quality native vegetation. The lake vegetation restoration plan is focused on hemi-marsh, a 50-50 mix of emergent vegetation and open water; in Hennepin & Hopper Lakes it can provide the perfect habitat for a range of waterfowl and also for juvenile fish and many insect species.
Hemi-marsh had been a significant part of the Dixon Refuge's biodiversity before the carp population exploded. TWI's senior ecologist, Dr. Gary Sullivan, is introducing or augmenting several aquatic plants that will actually increase the quality of the hemi-marsh over what it was before. These plants will also be extremely beneficial for ducks and other waterfowl, securing the Refuge's unique position along the Mississippi flyway.
The priority native perennial for creating high-quality hemi-marsh is wild celery (Vallisneria americana). With ribbon-like leaves below the water surface, wild celery is a preferred food for waterfowl, particularly diving ducks, and it also attracts marsh birds and shorebirds; it was once very common in our region. This spring we planted 9,500 wild celery tubers throughout the lakes.
To complement the wild celery, Dr. Sullivan chose a range of emergent and submersed vegetation for the hemi-marsh, including two types of arrowhead (Sagittaria rigida and Sagittaria latifolia), sago pondweed (Stuckenia pectinata), pickerelweed (Pontederia cordata), and several other native perennials. These aquatic plants will provide food and cover for waterfowl as well.
The Dixon Refuge already has a strong reputation among birdwatchers and, thanks to the generosity of several individual TWI donors, the hemi-marsh restoration will make it even more of a destination. With just one growing season behind us, the 2013 fall migration should be a great one at the Refuge.