Eurasian watermilfoil (Myriophyllum spicatum L.) has become a
nuisance plant species in North America. In recent years, it has
become increasingly abundant in the Upper Mississippi River System
(UMRS) and at some locations has regularly colonized areas which
had previously hosted wild celery (Vallisneria americana Michx.),
an important waterfowl food species. In summer 1993, initial
efforts were undertaken to understand the environmental requisites
of Myriophyllum in the UMRS. To accomplish this, a sampling grid
was established within a Myriophyllum bed in Pool 7 near La
Crosse, Wisconsin. Transects were spaced at 18.3-m (60-ft)
intervals, with sample sites 9.15 m (30 ft) apart on transects.
In total, 164 sites were sampled, and at each site information was
collected on plant biomass, water depth, sediment moisture
content, and sediment organic content. In this study, the data
were examined via geographic information systems (GIS) software.
Goals of the work were to examine the functionality of using GIS
to visualize and analyze riverine data and to determine whether
additional understanding relative to Myriophyllum's growth
requisites could be elucidated by use of these technologies. The
point data, collected at 164 sites, were interpolated to create
continuous surfaces. This was done via five different methods
available in ARC/INFO Version 6.1: trend, inverse distance
weighted (IDW), spline, krige, and Triangular Irregular Network.
Numerical and graphical comparisons were done for each method.
Generally, trend and IDW were considered to be unacceptable, with
other methods offering varying strengths and weaknesses. Analysis
of field data indicated that a spatial representation of plant
biomass that replicated the known patchiness of the bed could be
interpolated as well as a spatial representation of related
physical parameters. Analysis also suggested that the presence or
absence of Myriophyllum is not easily related to the depth,
sediment moisture content, or sediment organic content of the area
in which it is growing. For each physical parameter, a general
zone of mean or preferred conditions existed, but the plant was
tolerant and also occurred in quite different conditions as well.
