productivity increases as early as February resulting in the high spring saturation levels of oxygen in the lake. Blooms of algae and zooplankton often follow winter turn-over. Increasing nutrients and warmer temperatures continue to favor productivity until predation and excessive heat drive productivity lower in the water column by late summer and fall.
For August, secchi depths (table I, map 1) reflected heightened productivity from phytoplankton (see chlorophyll levels, table I, fig 5), suspension of sediments from the spring flood (a minor contributor this year with lower inflows), as well as some whiting
of the lake in the lacustrine zones from super-saturation of calcium carbonate, a fairly common occurrence in late summer. Chlorophyll values ranged from 0.5 to 3 mg/m3 in the down-lake portions of the reservoir, 2 to 10 mg/m3 in the transitional portions, and 5-10 mg/m3 in the inflows, reflecting seasonal peaks.
The ranges of nutrients (figure 7) and primary productivity are indicative of a large reservoir system with distinct zones (riverine to lacustrine) and this generally demonstrates a system which transitions from moderate or mesotrophic productivity in the transition zone to primarily oligotrophic (low nutrient, low productivity) in the lower reaches of the lake.
Findings on the reservoir included some unusual events on-shore. In May, the appearance of mosquitoes in two bays at the lower end of the lake was experienced for the first time in many years. This may be an effect of the fairly stable lake elevation during this spring. On the first night of the August trip, Mark Anderson of Glen Canyon National Recreation Area had a misadventure with a rattlesnake. I thought I kicked a cactus
. A sound bite on the toe lead to a midnight helicopter out, but he returned to the trip in a day and a half, gaining the Trooper Award. Other unusual wildlife included an impressive but fairly benign giant hairy scorpion Hadrurus spadix and the rarest sighting, a freshwater jellyfish, Craspedacusta sowerbyi. The jellyfish was found by a park visitor in Oak Canyon. This is the only known species of freshwater jellyfish in the U.S., but though seen on rare occasions, it appears not to be documented in Lake Powell.
Acknowledgments:
The Integrated Water Quality Program wishes to thank Glen Canyon National Recreation Area for assistance of personnel and equipment. Appreciation is extended to GCMRC staff and the many supporters of the IWQP and volunteers who assisted in collections.
For further information, please contact:
Grand Canyon Monitoring & Research Center
Susan Hueftle
shueftle@usgs.gov
2255 N. Gemini Dr.
Flagstaff, AZ 86001-1600
Phone: 520.556.7460
Fax: 520.556.7368
S
Map 1: August '00 secchi depths across Lake Powell with map of stations sampled in WY2000. Lake length along the main channel is 272 km (154 miles) from dam to Dark Canyon.
GCMRC - Annual IWQP Report September 2000 8