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Grand Teton National Park Report

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The submersed plant community in Jackson Lake is an important resource in Grand Teton National Park for both terrestrial and aquatic organisms. Since the early 1900's, Jackson Lake has been influenced by a series of drawdowns of varying magnitude that influenced the composition and extent of the macrophyte community in the littoral zone. While many physical conditions have been linked to macrophyte growth and distribution in lakes (e.g., light, sediments, nutrients, slope) recent work suggests that the magnitude and timing of water level fluctuations may be the most important factor regulating macrophyte community processes at shallower depths in regulated reservoirs (Brewer and Parker 1990; Gasith and Gafny 1990; Rerslett 1984, 1987). Because of the importance of the aquatic plant community in Jackson Lake as a food source and shelter for wildlife (waterfowl, fish,invertebrates, mammals), the factors affecting growth and recovery after disturbance have received periodic attention since the late 1960's (Hayden 1969; Brewer 1986; Brewer and Rerslett 1987; Brewer and Parker 1990; Brewer and Thompson 1994). Initial work on plant community dynamics in Jackson Lake has shown that the aquatic plant community in Jackson Lake has been exposed to a varied disturbance regime that has impacted recolonization, species diversity and peak biomass development. Because the extent of littoral habitat suitable for macrophyte growth in regulated lakes depends on the timing and magnitude of seasonal and long-term drawdowns, drawdown schedules and resulting fluctuating water levels may be selecting for a plant community characterized by low diversity and patchy distribution. Peak production under such conditions in Jackson Lake tends to be shifted into deeper waters (Brewer and Parker 1990; Brewer and Thompson 1994). Furthermore, clonal, weedy species with reduced value for wildlife (e.g., Elodea canadensis, Myriophyllum sibericum) have enhanced opportunities for dispersal under such a disturbance regime and are favored when maximum drawdown occurs during the period of peak standing crop. While historical records describing changing patterns of macrophyte distribution are available for Jackson Lake, few data are available to evaluate the influence of fluctuating water levels on the seed bed, and the potential for recruitment after disturbance from this source. The objective for our work was to establish protocols and collect preliminary data on the potential for recruiting macrophytes from the littoral seed bank by measuring germination of seeds from lake sediments.