Crater Lake National Park Whitehorse Pond Limnological and Vascular Plant Study, 1993



Crater Lake National Park contains many unique environs. The collection of ponds on the top of Whitehorse Bluff is one such special area. In 1993 the Crater Lake Natural History Association sponsored this environmental research project designed to continue work begun by Roger Brandt in 1992. This study focused on the physical, chemical, and biological characteristics of the ponds themselves and includes a survey of the flora found on the Bluff.

The ponds were visited between July 14 and September 10, 1993. In the first of several field trips the ponds were found to be close to full of water and teeming with life. The ponds were found to support healthy populations of dragon flies, water striders, invertebrates, many types of aquatic insects, frogs, toads, salamander and their tadpoles, moss and other aquatic plants, and many types of plankton. Later in the summer all but two ponds were completely dry. One challenge in surveying the ponds was simply to identify the individual ponds.

The Whitehorse Ponds are located within a mosaic of forest communities of which red fir and lodgepole pine forest were the most important. The dominant overstory tree was Shasta red fir (Abies magnifica var. shastensis) which, in combination with mountain hemlock (Tsuga mertensiana), provided a nearly closed canopy over large areas of the bluff top. The single day’s floral survey documented twenty-nine taxa in and around the ponds.

Study of plants in the Whitehorse Bluff area over a summer would surely add to the listing begun by David Hartesvelt. He suggested that the bryophytes alone are deserving of a more complete survey. The bryophytes were observed but not documented here.

The ponds themselves supported a limited flora of vascular plants. Two aquatic plants, western quillwort (Isoetes occidentalis) and small bur-weed (Sparganium natans), were observed in the largest and deepest of all the ponds. These species were not observed in any other ponds. Two additional species, water sedge (Carex aquatilis) and narrow-spiked reedgrass (Calamagrostis inexpansa), were observed growing as emergent vegetation along the shallow margins of most ponds occurring on the White Horse Bluffs.

In this brief period the water temperatures varied from 13 to 240 C, the acid concentration or pH varied from 5.55 to 6.20, dissolved oxygen levels were low and varied from 4.5 to 6.7 mg/L and the conductivity of the pond water varied from 7.6 to 16.6 puMHO/cm.

Chemical concentrations paralleled the concentrations of a bulk deposition (precipitation) study completed in September 1988 (Larson, 1993). All chemical species determined were of similar concentration except nitrate and sulfate ions. Nitrate ion was found to be 18 times less concentrated in the ponds than in precipitation. Nitrate ion, an important nutrient, was probably being taken up by plants in and around the ponds. Sulfate ion was also found in very small concentrations in the ponds about 100 times less than in Park precipitation. Total phosphate, sodium, potassium, calcium, magnesium, and chloride were all similar in concentration when compared to precipitation.

When Crater Lake water chemical specie concentrations were compared to pond water concentrations, they ranged from similar, as in total phosphate, to 50 times greater for alkalinity. All the other chemical species were in the 8 to 20 times range greater in the Lake. The ponds are probably fed by precipitation alone. Changes in the quality of the precipitation would certainly affect the ponds.

Phytoplankton were sampled three times. The two samples taken on August 9 from two separate ponds were very similar in population with nine taxa identified in each and biovolumes of 225,000 and 350,000 Rm3/L. The single sample from September 10 contained only four taxa but had a biovolume of 14,300,00 Rm3/L. There was a great diversity and biovolume of phytoplankton for such small water bodies. Further study will probably reveal that this study underestimates the true diversity in the phytoplankton community.

A review of all the plankton data suggests that the Whitehorse ponds were eutrophic in quality with a high amount of organic material present. The pond color supports this as well as the presence of the euglenoids that require certain organic materials to live. Phytoplankton cell densities increased 30 times in September due to a reduction in nutrients, higher temperatures, and greater light intensities as Bob Truitt has suggested. Chemical analyses do not support the nutrient suggestion. However, it has been documented that later in the summer the number of phytoplankton species decrease and the cell densities increase. More study into this trend would reveal interesting relationships.

Zooplankton were more diverse than the phytoplankton. Similarity indices indicated that different ponds also had unique zooplankton communities. Zooplankton feed on the smaller phytoplankton. There was a documented difference in the zooplankton assemblages on a pond’s surface and on the pond’s bottom. This was seen to be true even in very shallow ponds about 1 m deep. The large diversity in zooplankton depended little on the date of collection. A greater number of samples through time and for each pond would also document very interesting trends in zooplankton community structure.

Future study of the Whitehorse Ponds would document changes in the ponds due to changes in this small watershed.