Spatial surfacing of point data for Myriophyllum investigations McConville, D. 1995. GIs application: Spatial surfacing of point data for Myriophyllum investigations. National Biological Service, Environmental Management Technical Center, Onalaska, Wisconsin, April 1995. LTRMP 95-P004. 21 pp. (NTIS #PB95-232641) ABSTRACT 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. KEYWORDS Eurasian watermilfoil, Myriophyllum spicatum, geographic information systems, GIS, spatial interpolation, ARC/INFO, Mississippi River