Trophic State Evaluation of Jackson Lake in Grand Teton National Park

Abstract

This study is the continuation of an evaluation of the trophic state of lakes located in Grand Teton National Park, Wyoming. The original 1995 study was motivated by concern that the water quality of the lakes within the Park may be declining due to increased human usage over the past several years. A trophic state evaluation, featuring nutrient and chlorophyll-a analyses, was chosen because it is believed to be a sound indicator of the lakes' overall water quality. In this 1996 study, a thorough evaluation was made of Jackson Lake. This summary is taken from the complete 100 page report which is available from Woodruff Miller at Brigham Young University or Hank Harlow at the University of Wyoming. In most cases water samples were taken four times during the summer of 1996, in June, July, August, and October. Jackson Lake was sampled at eight different locations on thesurface and at depths near the bottom. The lake inlet and outlet were also sampled four times. Jackson Lake was sampled from a motor boat which also provided a means to measure the lake transparency and depth. The chlorophyll-a and nutrient concentrations were analyzed by the Utah State Health Department, Division of Laboratory Services. Jackson Lake was evaluated using the models of Carlson, Vollenweider, and Larsen­Mercier. The nature of the Larsen-Mercier and Vollenweider models, based on system inflow and outflow data, is such that they yield one trophic state assessment of the lake per inflow and outflow sample set. The Carlson Trophic State Indices (TSI), on the other hand, are based on in situ properties of the water at any point in the lake. Consequently, while there are four Vollenweider and four Larsen-Mercier evaluations for Jackson Lake, individual Carlson evaluations were made for the eight sample sites around the lake at the surface and at depth, and an evaluation for the lake as a whole was constructed using averages taken from the site evaluations. This allowed us to examine the relative water quality of different portions of the lake at different time periods.