Friday, August 6, 2010
Final Project
Hydrilla (hydrilla verticillata) is considered to be one of the most problematic aquatic plants in the United States. Hydrilla is considered a noxious pest because it grows so rapidly, out competing and eliminating native species. Hydrilla is native to Europe and Asia and was probably brought to the Tampa and Miami, Florida areas as an aquarium plant in the late 1950s; by the 1970s, it was established throughout Florida. Hydrilla verticillata continues to be sold through aquarium supply dealers and over the Internet, even though the plant is on the U.S. Federal Noxious Weed List (IFAS). Hydrilla has been found in several locations in Lake Juliette and represents a threat of increasing cost of treatment and possible take over of the lake. The costs of not managing it early in its growth curve are extreme. Hydros and cooling water intakes can be shut down. Fish kills have occurred, and recreational use could be eliminated. Management methods include herbicides, grass carp (Ctenopharyngodon idella Val.), and mechanical removal (IFAS). The herbicide active ingredients, copper, diquat, endothall, and fluridone can be used to selectively control hydrilla to some extent, but do not destroy the tubers. Grass carp is a herbivorous fish that is effective for controlling hydrilla (Van Dyke et al. 1984). Possession of this fish is illegal in most states because of the potential environmental damage that could result if escaped fish establish a breeding population. Sterile, triploid grass carp (Malone 1984) are also effective (Cassani and Caton 1986) and are now available and legal by permit in some states in the U.S. In small ponds or lakes and canal systems, with adequate control structures, and where total removal of vegetation is acceptable, triploid grass carp stocking is highly recommended. The decision to be made is the cost effectiveness of continued chemical treatment versus the use of sterile grass carp as an alternate means of control based on the areas of hydrilla covered in the past 10 years, as well as, the hydrilla density level trend to be used as an indicator for treatment effectiveness.
Data was collected in late August, early September for the years 2000 – 2005, and 2007 – 2009 in the form of uncorrected GPS data and survey sheets (excluding 2006 due to GPS data mismanagement but data sheets and survey points were located) containing data point numbers and percentage of hydrilla coverage at each survey point. Survey points will be input using the corrected GPS coordinates taken at each survey location during each survey.
Other useful data sets are:
County - Georgia GIS Clearinghouse
Aerial Image – ESRI
Roads – Georgia GIS Clearinghouse
Rivers/Ponds/Lakes data – Georgia GIS Clearinghouse
Topography – USGS
The survey data was input into excels files using station numbers as a guide to match to the GPS data. Once this was done the points containing hydrilla were selected and removed into separate shape files and labeled by year. Next a polygon shape file was made to use as an extent for the spatial analysis and as a clip range to isolate the lake polygon. A reverse clip was then made to allow for the lake area to be seen as a void and the rest of the areas blanked out. A topographic map was then placed below this map. This allowed for the elevations for the lake area to be estimated with a new polygon named hydrilla range. All areas with a depth of 40 feet or less were included in the polygon allowing for deeper areas and islands to be removed from the growing range. The hydrilla range polygon was used as the mask for spatial analysis so that a more accurate depiction of the growing areas could be depicted. Next each year’s point data was placed on the map, one at a time, and the spatial analysis technique ‘kernel density’ was used as an estimate to extrapolate the data to the entire population over an area based on the density found and number of points in the vicinity. Once this was completed for each year the base map from above was used to make a map for each year and a group animation to illustrate the changes over the 10 year span.
Regular treatment has taken place since 2001 around boat ramps and the intake and from the results you can see a baseline plant levels present from year to year with the exception to the explosions of growth in 2004 and 2007. In 2000 the lake was allowed a draw-down in October and the water elevation receded almost to the point were Plant Scherer could no longer operate. This could account for the low levels that first year but each year after treatment took place at each of the 3 boat ramps and the plant intake thrice a year in the beginning, middle and end of the growing season. It is possible that regular treatment has retarded the growth of hydrilla each year and kept it at a manageable level but the explosions in growth make this seem unlikely. Also as the spread has continued almost the entire perimeter of the lake has been surrounded by the plant and most likely its tubers. . It is possible to continue with chemical treatment and allow for regular draw-downs to control hydrilla but the with the risk of a clogged intake and loss of valuable recreational areas of the lake, as well as, mounting expenses other options could be explored. With the extent of the spread and the risk of damage due to an imminent explosion of growth it would be wise to treat this reservoir with sterile grass carp to allow for regular and longer term control. This measure, while initially expensive, would allow for a savings in man-hours and chemical treatment cost after a few years and with the majority of water being pumped from the nearby Ocmulgee River only minimal reservoir control would need to take place to control the spread of the grass carp.
References
Cassani, J. R. and W. E. Caton. 1986. Growth comparisons of diploid and triploid grass carp under varying conditions. Progr. Fish-Cult. 48:184-187.
Van Dyke, J. M., A. J. Leslie, Jr. and L. E. Nall. 1984. The effects of grass carp on the aquatic macrophytes of four Florida lakes. J. Aquat. Plant Manage. 22:87-95.
http://plants.ifas.ufl.edu/node/183. IFAS Extension, University of Florida. Center for Aquatic and Invasive Plants. 2009.
Thanks to Georgia Power Environmental Affairs for access to data. Other data sources included by map.
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