Posts Tagged 'Impacts'

Lake Munson: A Case Study of the Impact of Exotic Apple Snails on Aquatic Vegetation

Munson June 30, 2008The old saying about Lake Munson is that “it is a beautiful lake, if you don’t look down.” Lake Munson’s beauty lies in the dense cypress strand, largely in public ownership, that surrounds it. Looking down, however, reveals that nutrient pollution and sedimentation have plagued this lake for over 70 years. This 255-acre system has been then receiving water body for treated waste water from 1934 to 1988, and for most of the City of Tallahassee’s stormwater at present. Though 25% of the watershed is protected, 50% is urbanized and home to 100,000 people. Little wonder that aquatic plants have historically been highly prolific in this fertile lake. In fact, this lake was usually “topped-out” in submersed vegetation during the growing season . . . that is, until a prolific, voracious herbivore arrived.

From 1982 to 2007, I monitored the aquatic plant communities of Northwest Florida, as DEP’s Regional Biologist.  Matt Phillips (FWCC), who filled this responsibility after I retired, recently provided me the vegetation surveys for 2008 and 2009. Lake Munson is one of the sixty public water bodies we have collectively monitored for 27 years. It is also the lake where the Island Apple Snail (Pomacea insularum) put on quite a show in 2002, painting all of the cypress trees along the shoreline with pink egg clusters. I had never seen anything like it!

The resulting population of exotic, apple snails dramatically changed the plant community of Lake Munson. Historically, submersed vegetation blanketed 80% of the lake during our surveys, necessitating the use of an airboat. In the early 90’s, the dominant submersed species were Coontail (Ceratophyllum demersum) and Southern Naiad (Najas guadalupensis).  However, in 1993, the invasive, exotic, plant  Hydrilla (Hydrilla verticillata) found its way into the lake. By 1995, hydrilla reached the surface in 200 acres of Lake Munson and that density became the norm. When the exotic snails first appeared in 2002, I noted 150 acres of Hydrilla. In two years, the Hydrilla was gone – – completely gone. In fact, no submersed, vascular plants have been found in Lake Munson, since 2004.  

Emersed and floating vegetation has fared little better. A sixty-acre stand of the beautiful, native, American Lotus (Nelumbo lutea) vanished between 2003 and 2004. The invasive, exotic, Water Hyacinth (Eichhornia crassipes) has nearly disappeared. From 2002 to 2006, a total of 330 acres of hyacinths were controlled in Lake Munson. Such control is no longer necessary. Only a trace of Water Hyacinths remains in Lake Munson. A favorite food for Pomacea insularum, Wild Taro (Colocasia esculenta) declined but rebounded on the east side of the lake in 2009, while Pomacea insularum egg deposition was concentrated on the west side (Another behavioral mystery!) .

There are some lessons here: First, the Island Apple Snail can completely strip a heavily-vegetated lake of its submersed plants and most of its emersed vegetation. Secondly, some hardy species will remain, such as Soft-Stem Rush (Juncus effusus) and Smartweed (Polygonum densiflorum). These and other apple snail resistant species should be considered first in lake mitigation, though there is still much to be learned on the subject.  Research on the food preferences of Pomacea insularum is badly needed.

So what is left in Lake Munson in terms of plants? There are plenty of blue-green algae (cyanobacteria). The lake is dominated by a dense phytoplankton bloom, consisting mainly of Microcystis spp. and Anabaena spp. The only submersed “plant” is there now Lyngbya – – a black filamentous algae. Sounds inviting, doesn’t it? Leon County continues to make great progress in improving the quality of water entering the lake, but Lake Munson remains “a beautiful lake, if you don’t look down” and, by destroying the vascular plants, the Island Apple Snail certainly isn’t helping any. By Jess Van Dyke

For more information on Lake Munson, contact:

 Matt Phillips, Florida Fish and Wildlife Conservation Commission, Invasive Plant Management Section, 3900 Commonwealth Blvd.  MS 705, Tallahassee, Florida 32399, (850) 245-2831, mattv.phillips@myfwc.com

“Lake Munson: Past, Present, and Future” by Johnnie Richardson, Water Quality Scientist, Leon County Public Works

http://www.leoncountyfl.gov/PUBWORKS/Engineering/Stormwater_Management/Lake Munson Update Short 6_25_09.pdf

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About Snail Busters

The Snail Busters Blog was created to facilitate communication between aquatic resource managers who are fighting the spread of invasive, South American apple snails, specifically Pomacea maculata (formerly P. insularum) and P. canaliculata, in the U.S.

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