Date of Award

5-2007

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Aram J.K. Calhoun

Second Committee Member

Ann C. Dieffenbacher-Krall

Third Committee Member

Katherine E. Webster

Abstract

Native to the southeastern United States, variable-leaf watermilfoil (Myriophyllum heterophyllum) is an invasive species in the Northeast and has been documented in Maine lakes for twenty years. Variable-leaf watermilfoil is targeted by the Maine Department of Environmental Protection as a species of grave concern as it has aggressively colonized twenty-six water bodies in Maine. This aquatic invasive plant grows in dense mats and outcompetes native vegetation. It is causing both ecological and economic disruption to Maine's lakes and ponds. The plants clog boat motors and deter people from swimming and other water related activities. Allofragmentation and autofragmentation occur quite extensively in this species, and contribute to its ease of dispersal. During implementation of management techniques, further fragmentation of the plants can occur. Although natural resource managers commonly assume a 2.5 cm fragment size as being the smallest size that can regenerate, we were unable to find any research documenting this assertion. My research focused on two key areas for variable-leaf watermilfoil: (1) determining which control methods are most effective for removing variable-leaf watermilfoil from lakes and (2) vegetative regeneration. We looked at three management techniques for variable-leaf watermilfoil, hand removal, cutting, and benthic mats, to determine the most effective management strategy. Our study showed that all three methods reduced plant growth significantly. However there were no significant differences among the three management methods. Differences were present in time and cost required to implement the strategies between benthic mats and hand removal, as well as benthic mats and cutting. Although less expensive than benthic mats, cutting was found to be unrealistic to implement in practice because of difficulties in implementation. Determining the most effective management technique for an area depends on the extent and density of the infestation. Benthic mats provided an excellent option for thick, large infestations, whereas hand removal was more efficient for lighter infestations. Hand removal is best used in areas with small, high density infestations or for selective removal in sparsely infested stands of mostly native macrophytes. This method would also be useful during management surveys when individual plants or small clusters of variable-leaf watermilfoil are detected. Based on our study we suggest that the benthic barrier and hand removal methods are the most effective non-mechanical management techniques for lake associations and state agencies to incorporate into their management plans. In a twenty-two week greenhouse investigation, variable-leaf watermilfoil vegetative fragments were observed to determine smallest size for regeneration. Four fragment sizes were collected from the plants (a leaf, a single whorl, 2.5-cm stem with whorls, and 5-cm stem with whorls) and two substrates, sand and top soil, were used. All fragment sizes regenerated buds with the exception of the individual leaves. Evidence that fragment regeneration from any plant fragment containing a stem node is very useful for developing long-term management strategies for Myriophyllum heterophyllwn. Managers need to emphasize the removal of fragments generated during removal processes as well as after heavy recreational use of an infested lake in order to reduce the potential spread of M. heterophyllum.

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