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Description
ABSTRACT. During the growing season of 2004, floristic inventories were conducted on Silver Lake, Porter County, Indiana to quantitatively describe the aquatic flora. A modified point intercept technique was used to determine the frequency and relative frequency of each taxon from a total of 41 sampling points. Measurements of physical and chemical water parameters were taken from a random subsample of these points. Floristic inventories resulted in the documentation of 42 taxa in 32 genera from 23 families. Families exhibiting the greatest richness were the Lemnaceae and Potamogetonaceae. Nuphar advena had the highest frequency of occurrence (>75%). Chemical and physical evidence indicates that Silver Lake is a circumneutral water body prone to acidification because of its shallow depth, poor buffeting capacity, and the presence of remnant peat deposits. The floristic and natural area quality of the lake proper was evaluated using the Floristic Quality Assessment (FQA) methodology of Swink & Wilhelm (1994). The floristic quality index (I) and mean coefficient of conservatism ([C.sub.mean] were calculated from two sources of C values (i.e., Indiana and the Chicago region), which have overlapping geographical ranges. A comparison of C values for the Silver Lake aquatic vascular plant taxa indicated that Indiana C values were at least one unit lower than those from the Chicago region. Although the use of Indiana C values resulted in lower values of [C.sub.mean] and I, values of [C.sub.mean] calculated from both sets of C values indicated that Silver Lake was high in natural area quality. However, the presence of a shallow lake basin, coupled with anthropogenic disturbance and drought-induced drawdowns, has contributed to a frequent and substantial loss of littoral zone, which has negatively impacted the lake's ability to sustain floristically significant submerged aquatic taxa. We recognize that problems exist with standard FQA methodology when applied to lake ecosystems and suggest ways to improve the diagnostic resolution of the FQA metrics.
Keywords: Floristic quality assessment, Indiana flora, Silver Lake, aquatic macrophytes, C values, [I.sub.nni]
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Silver Lake is one of only two small natural lakes located inside the city limits of Valparaiso, Indiana. Much of the property surrounding the lake is under private ownership; however, the southern shore is owned by the City of Valparaiso and has been selected as a project site for a road extension. The road extension has been permitted by the United States Army Corps of Engineers and the Indiana Department of Environmental Management and will be constructed through two small wetland areas. It has been determined that this project will impact an area of <0.04 ha. Many local conservationists believe the future ecological health and floristic quality of the lake and its surrounding wetlands will be diminished, or at least compromised, by the construction and use of this thoroughfare. Unfortunately, there are limited baseline data on the quality of this lake, rendering any scientific comparison of before and after conditions almost futile. These genuine concerns provided the impetus for us to conduct this study.
The main objectives of this study were (1) to quantitatively document the aquatic macrophytes within Silver Lake, (2) to measure physical and chemical habitat parameters often used in the characterization of aquatic macrophyte communities, and (3) to determine and evaluate the floristic and natural quality of the lake proper by using the Floristic Quality Assessment (FQA) methodology developed by Swink & Wilhelm (1994). Data on the habitat parameters provided here and used in conjunction with results from the floristic analysis will serve as a benchmark for the future monitoring of Silver Lake. We also provide suggestions on how to maintain, if not enhance, the floristic quality of this Lake.
A secondary objective of the Silver Lake analysis was to compare the Indiana coefficients of conservatism (C values) with those of the Chicago region (Swink & Wilhelm 1994) for the aquatic vascular plants at this site and determine which source was more appropriate for the FQA of Silver Lake. Because of its uncomplicated methodology, FQA is one of the most appealing diagnostic tools for evaluating habitat, given that one possesses sufficient taxonomic skills to correctly identify vascular plants. However, there are problems with the application of FQA to lake ecosystems. Therefore, we also examine some of the current problems with the use of FQA in the context of a lake ecosystem and provide suggestions on how to improve the adequacy of the FQA metrics.
METHODS
Study site.--Silver Lake is located in the northern portion of the City of Valparaiso in Porter County, Indiana (41[degrees]29'43"N, 87[degrees]03'34"W) directly northeast of Valparaiso High School and is situated between Campbell Street and Valparaiso Street (Fig. 1). This body of water is extremely shallow (ca. mean depth 0.6 m) and small (ca. 10.5 ha) and is the southern most lake of 13 lakes found in the Valparaiso Lakes Area watershed, which expands across 1085 ha. Much of the surface area of Silver Lake is dominated by spatterdock (Nuphar advena) and the majority of its shoreline is composed of cat-tails, sedges, and smartweeds. Due to its small size, shallow depth, and extensive emergent zone, Silver Lake is more reminiscent of a basin marsh than a lake. Although much of its shoreline is privately owned and no public access currently exists on this lake, the property adjacent to its south shore is owned by the City of Valparaiso and has been selected as the site for the extension of Vale Park Road to Campbell Street (Fig. 1).
[FIGURE 1 OMITTED]
Silver Lake lies within the Valparaiso Moraine Section of the Northwestern Morainal Natural Region (Homoya et al. 1985). This natural region is known for its dunes, glacial ridges, and diversity of wetland types (Post 1997). The Valparaiso Moraine Section topographically divides the Lake Michigan basin from the Kankakee River basin in northwest Indiana. Water from Silver Lake drains from an outlet located on its southeast corner (Fig. 1) to a ditch and water control structure, and then travels east through a ditch perpendicular and beneath Valparaiso Street, proceeding northward to Flint Lake, the largest lake in the Valparaiso Lakes Area watershed.
Silver Lake is found in Soil Region 7 characterized by the Blount-Morely-Pewamo association (Franzmeier 1997). These soil types are plentiful in the Valaparaiso Lakes Area watershed and were primarily formed from clayey glacial till after the retreat of the Wisconsinan glacier. With the exception of the western shore, which consists of Morely silt loam, much of the soil surrounding the immediate portions of Silver Lake, including the project site for the road extension, is Pewamo silty clay loam (Furr 1981). This very poorly-drained soil is commonly found in depressional areas and till plains in the Valparaiso Moraine Section. According to Furr (1981), this soil has severe limitations for building sites and roads because of its moderately slow permeability, which contributes to the accumulation of surface waters and its susceptibility to frost action potential; however, it is well-suited for water-tolerant vegetation.
The sediments of Silver Lake are composed of muck along the near shore areas, though remnants of peat occur in a few isolated areas especially toward the center of the lake. According to Joe Clifford (pers. comm.), a resident and property owner of land bordering the lake since the 1950s, Silver Lake was once an oak-hickory forest, having trees with diameter at breast heights (DBHs) of up to approximately 60 cm and an understory dominated by sassafras. Aerial photography from 1938 clearly shows that the current lake basin was a forested area (Fig. 2). In early November of 1953, the peat and sphagnum subsurface was ignited from a fire used to burn brush and debris from the clearing of land on the west side of the lake (Anonymous 1953a, 1953b, 1953c, 1953d). The peat burned down to the clay bottom of the original lake basin (Fig. 3) to a maximum depth of approximately 1 m, exposing the roots of the trees, which subsequently toppled over. Although some of the tree stumps and logs remain in the lake to this day, many were hauled out and chopped into firewood. The ultimate effect of this fire was to expose what likely represents the original post-glacial clay-lined lake basin. This basin, like many of those found in the northern United States, was probably originally formed as an ice block depression, which subsequently became a kettle hole lake following the last Wisconsinan glacial retreat. Although we believe it is not possible to accurately reconstruct the vegetational history of Silver Lake because of the loss of peat layers due to the fire, it is likely that the lake went through typical bog successional stages. It is somewhat ironic that the lake appears to have reverted back to earlier successional stages as a result of anthropogenic activity. Over the last 50 years, Silver Lake has slowly accumulated silt from runoff and organic matter from the growth of aquatic macrophytes, and is now largely covered by spatterdock.
[FIGURES 2-3 OMITTED]
Floristic survey and assessment.--In 2004, field surveys were conducted during the months of May-August to inventory the aquatic flora of Silver Lake. One survey was made each month to minimize temporal effects and to ensure that diagnostic characters, such as flowers and fruits, were present for positive identifications of specimens. Areas were surveyed using a point intercept method modified from Madsen (1999), which consisted of recording the presence of species in a radial area encircling a pre-selected waypoint. This method utilized a randomly selected point of origin for the construction of a grid. The grid consisted of 41 geo-referenced waypoints having an interval of 50 m that was generated in the Universal Trans Mercator (UTM) coordinate system North American Datum 1983 (NAD 83) using Trimble TerrsSync[TM] software (Fig. 4). The use of 50 m grid intervals provided adequate coverage for surveying Silver Lake because of its small size, almost... |
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