Response of an Aquatic Macrophyte Community to Fluctuating Water Levels in an Oligotrophic Lake

Abstract

ABSTRACT This study compares the species composition, biomass, and the influence of substrate composition on an aquatic macrophyte community in the meso-oligotrophic Lake Pend Oreille, Idaho under two winter drawdown regimes. Mean dry aquatic macrophyte biomass (AMB) in the drawdown zone (1.4 – 3.5 m) significantly increased from 39.9 gm−2 in 1990 to 99.2 gm−2 and 103.7 gm−2 with 2.1 m drawdowns in 1998 and 1999. Mean AMB deeper than 3.5 m did not significantly increase, suggesting the increased biomass in the drawdown zone can at least partially be attributed to increased winter survival. Myriophyllum sibiricum, Chara spp., and Potamogeton richardsonii were the most abundant aquatic macrophytes under the 3.5 m winter drawdown regime, while Chara spp., P berchtoldii, and P. crispus were most abundant under higher winter water levels. The exotic M. spicatum was present at one sample station and most prevalent in depths between 3.9 – 5.1 m. M. spicatum attained mean maximum densities in excess of 900 gm−2 by August, 1999 (one year after it was first observed in the lake). Logistic regression indicated a higher probability of finding clay and cobble substrates in the drawdown zone. On the clay substrates, there were significantly lower AMB (17.9 gm−2) than on sand (86.6 gm−2) or silt (129.0 gm−2) substrata with no plants observed on gravel or cobble substrata.