AQUATIC PLANT CHARACTERIZATIONBrazilian Elodea

Lake Leland Integrated Aquatic Plant Management Plan

 


During August and September of 1997, Jefferson County Conservation District personnel characterized the aquatic plants of Lake Leland by conducting distribution and abundance surveys. The purpose of these surveys was to characterize and quantify the aquatic plant community in sufficient detail so as to be able to distinguish temporal changes in the distribution and abundance of Brazilian elodea and other aquatic plants. The surveys provide baseline data which can be used to evaluate control treatments (or no treatments).

METHODS

Distribution Monitoring

Initially, 27 transect sites, 150 meters apart, were flagged around the 4050 meters of shoreline (Figure 15). Plants were later sampled at 0.5, 1, 2, and 3 meter depths along the transects, perpendicular to the shoreline. Plant samples were collected from a boat by throwing out a "plant sampler" perpendicular to the transect (parallel to the shoreline) and retrieving it. The "plant sampler" consisted of two iron bow rakes (each with fourteen 2.25 inch tines spaced 1.0 inch apart) attached to each other back to back; one handle was completely removed and the other was shortened to 8 inches. A rope was attached to a screw-eye in the end of the shortened handle. Plants were identified and assigned an index of abundance (1=sparse, 2=moderate, 3=dense). Photos were taken to document examples of the abundance indices (Figure 16). In addition to using the "plant sampler," surveyors used visual estimates in assigning abundance indices where plants could be readily observed and identified.

Abundance Monitoring

Plant biomass was monitored at five sites (Figure 15). At each site, three transects were established parallel to the shore; one at 1 meter depth, one at 2 meters, and one at 3 meters. Transects were marked by a 25-meter floating line with an anchor and float at each end. Three samples were taken along each transect at places determined by a random number generator. Weighted lines were suspended from the marked places to pinpoint the sample location on the lake bottom. A SCUBA diver positioned a 0.5 m x 0.5 m quadrat sampler on the bottom with the weighted line in the center. The quadrat sampler, constructed of PVC pipe, actually had three sides for ease of positioning the sampler around the line. Using scissors, the SCUBA diver cut the plants within the quadrat just above the roots and placed them in a "goody" bag. At the surface the contents of the "goody" bag were transferred to a labeled plastic bag. The plastic bags were taken to a processing station on shore where the plants were sorted and identified and then placed in tarred, labeled paper bags.

Laboratory work was conducted at the US Geological Survey’s Marrowstone Field Station in Nordland, Washington. Here the bags were placed in a drying oven at 105o C for 24 to 72 hours. After reaching constant weight (checked by periodic weighings), the bags were weighed to the nearest hundredth of a gram, tare weights subtracted, and the "dry plant weights" recorded. In the distribution monitoring, District personnel were unsure in differentiating between coontail (Ceratophyllium demersum) and stonewort (Nitella spp). All were identified as coontail in the distribution monitoring. Both plants were identified by Ecology personnel in the abundance monitoring.

RESULTS

Brazilian elodea occurred at one or more depths in 85% of the 27 distribution transects (Figure 17). The plant was densest in the southern end of the lake, where it was first observed in 1994 (Figure 18). All transects in the area south of the neck had abundance indices of 3 (dense) at 2 or more depths. In contrast, transects in the main body of the lake had indices of mostly 1’s (sparse) and a few number of 2’s (moderate), and 4 transects were void of Brazilian elodea.

Besides Brazilian elodea, 26 other plant species and sponges were identified in transect samples (Figure 17, Table 1). When all depths were combined, Brazilian elodea had the highest frequency of occurrence (85%), followed by common elodea (78%), yellow water-lily (Nuphar lutea) (70%), reed canary grass (67%), yellow flag iris (63%), fern leaf pondweed (59%), whitestem pondweed (48%), coontail and/or stonewort (33%), and hardhack (Spiraea douglasii) (30%). Other plant species and sponges were less frequent (<30%).

At the 0.5 meter depth, predominant plants consisted of reed canary grass (67%), Brazilian elodea (59%), yellow flag iris (59%), common elodea (44%), and yellow water-lily (33%) (Figure 19). At 1 meter, the most common plants were Brazilian elodea (59%), common elodea (52%), yellow water-lily (48%), fern leaf pondweed (37%), and yellow flag iris (30%). At 2 meters, Brazilian elodea (41%), common elodea (41%), and whitestem pondweed predominated. And at 3 meters, only Brazilian elodea (30%) was appreciably frequent.

Yellow flag iris occurred mainly in the southern and northern ends of the lake at 0.5 and 1 meter depths (Figure 20). Reed canary grass occurred on all transects in the southern end of the lake and intermittently along the main lake shoreline at 0.5 and 1 meter depths (Figure 21).

Of the five biomass sample sites, the highest dry weights occurred at site 1 in the southern end of the lake (Figure 22, Table 2). At this site, Brazilian elodea predominated at 3 meters (43 gm/m2) and at 2 meters (223 gm/m2). Whereas at 1 meter, fern leaf pondweed had the highest average dry weight (88 gm/m2), followed by yellow water-lily (37 gm/m2), Brazilian elodea (33 gm/m2), common elodea (13 gm/m2), coontail (8 gm/m2), stonewort (1.1 gm/m2), and thin leaved pondweed (0.2 gm/m2).

Brazilian elodea outweighed other plant species in samples from site 5 on the eastern side of the neck (Figure 22, Table 2). Plant abundance at this site was greatest at the 1 meter depth where Brazilian elodea averaged 14 gm/m2 and common elodea averaged 9 gm/m2. At site 3, yellow water-lily (42 gm/m2) predominated the 1 meter depth and whitestem pondweed (9 gm/m2) predominated at the 2 meter depth. No appreciable plant biomass occurred in samples from sites 2 and 4. Brazilian elodea was found in trace amounts at these sites. 

DISCUSSION

Based on these results, Brazilian elodea appears to rank highest both in terms of distribution and abundance. And this has occurred in only four years from the time it was first noticed. Brazilian elodea is unquestionably a very competitive plant, probably the most competitive submersed plant in Lake Leland. It grows well at all depths within the littoral zone (about 0-10 feet deep). At the present rate of increase, it probably will eventually occupy a large part of Lake Leland’s littoral zone.  


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Contact Kathy Hamel by e-mail at kham461@ecy.wa.gov