Hydrobiologia 158: 167-172 (1988) J. M. Melack (ed.). Saline Lakes © Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands

167

Abundance and spatial distribution of Artemia salina in Lake Abert, Oregon Frank P. Conte' & Paul A. Conte 2 'Dept. of Zoology, Oregon State University, Corvallis, OR 97331, USA; 2Dircon Consultants, Dircon Inc., PO. Box 515, Corvallis, OR 97339, USA

Key words: Lake Abert, salt lake

Abstract In 1981- 82 Abert Lake had an area of 200 km 2, with a mean depth of 2.5 m and a total dissolved salt concentration of 82 g 1-1. The spatial distribution and abundance of the brine shrimp, Artemia salina, were monitored for 1981- 82. In 1981 during the midsummer months, with sampling primarily in the eastern side of Abert Lake, brine shrimp populations showed peak densities of 2-4 ind. 1- '. In 1982, peak abundance occurred in early July and ranged between 5 - 8 ind. 1- '. Lakewide estimates of brine shrimp derived from 14 collecting stations and assuming a uniform distribution over the lake resulted in estimates of lakewide abundance of 3 x 10"l adults with a total biomass of 7 x 106 kg. The major algal species in the lake was a benthic filamentous green alga (Ctenocladus circinnatus).

Introduction Lake Abert, Oregon, is a remnant of pleistocene Lake Chewaucan that left a shoreline more than 500 meters above the present day lake (Phillips & VanDenburgh, 1971; VanDenburgh, 1975). Lake Abert is cited as a classic example of a fault scarp with Abert rim, a 650 m fault block, rising abruptly from its eastern shore (Hutchinson, 1957). The dry conditions characteristic of the North American Great Basin have for the last several thousand years caused a decrease in freshwater inflows to the lake. Because of the reduction in the lake's volume the concentration of the lake's solutes has increased to a salinity in excess of 90 g 1- '. Since the early 1900s water has been diverted for agricultural irrigation from the Chewaucan River, the lake's only major freshwater tributary. In one extreme period of drought in 1937, the lake went to complete dryness. Lake Abert which lies in the western portion of the Pacific flyway (Bellrose, 1976) does serve as a staging and feeding area

for large numbers of migratory waterfowl and shorebirds. Quantitative estimates of the total abundance of Artemia in Abert Lake have not been done. The purpose of the present study was to determine the total abundance of brine shrimp based upon samples taken from several stations located in various regions of the lake. In addition, we attempted to ascertain the type of spatial distribution patterns for the population. From this data, an estimate of the lakewide abundance was calculated and the brine shrimp biomass was found to be approximately 7.0 x 106 kg. We evaluated the potential impact that the 1981- 82 commercial shrimp harvest together with estimate of avian consumption had on the adult population.

Study site description Abert Lake is a temperate lake (42°35 'N, 120°15 'W) and during the winter season water temperatures

168 range from 0 °C to 5 C, with periods of ice formation occuring at the southern end where the entrance of the Chewaucan River is located. In the spring the lake warms and an intermittent thermocline develops which breaks down rapidly due to waveaction. During midsummer, frequent thunderstorms cause rapid vertical and horizontal mixing and the shallow lake achieves an isothermal condition with nearly constant temperature (22 °C + 1 °C) prevailing for several weeks at a time. Gradual cooling of the lake does occur in late autumn. Abert Lake is predominantly a sodium chloride lake (Table 1). The total dissolved solids (TDS) in 1979 was 54 g 1- 1 and rose to values that exceeded 80 g 1-1 in 1980- 81. In 1982 the lake received large amounts of freshwater as a result of heavy spring inflows and the salinity fell to less than 40 g 1-1. The lake morphometry has been discussed by Phillips and VanDenburgh (1971) and they have published a detailed bathymetric map. We have modified this map (Fig. 1) based upon areal measurements of surface area and shoreline details that were derived from data obtained from EROS Data Center. Bathymetric data were obtained from an electronic echo sounder depth recorder and lead handline devices, aboard the collecting vessel. The total surface area of the lake measured approximately 200 km2 and the total volume of water contained in the lake was 675 x 106 m3. Geographic locations of highway markers, milepost signs, together with each of the 14 sampling stations, were marked by anchored flotation buoys (Herbst, 1980). At the time of the study, the maximum depth was 4.5 m and the mean depth was 2.5 m.

Methods The brine shrimp samples taken to establish the population densities utilized three types of zooplankton samplers. Description of each type of sampler and methods of use have been reported (Conte et al., 1987). Vertical and horizontal sampling was performed at all stations in order to evaluate spatial distribution patterns. Horizontal patchiness was assessed by calculating the coefficient of variation between stations and used the average density of vertical profiles (Table 3). Non-randomness distribution has been associated with coefficient of variation having values greater than 1.0 (P <0.05). Chemical analysis for Na, K, Ca and Mg were performed with a Instrumentation Laboratory Model 551 atomic absorption spectrophotometer and was calibrated against commercial standard solutions. 25 zm samples taken of filtered (0.45 micron Millipore) lake water were placed directly into cylindrical furnace cuvette's and analyzed under conditions prescribed by I.L. methods handbook.

Results and discussion In 1981, due to the restriction of boat launching sites the lake was arbitrarily divided into three sampling regions: a lower region (stations 1 through 7), a middle region (stations 8 through 10) and upper region (stations 11 through 14) as shown in Fig. 1. The peak abundance of adult brine shrimp occurred during midsummer (Table 2). In 1981, the peak abundance was found to be bimodal: peaks occurring on 21 August and 3 September. In 1982, peak

Table 1. Chemical composition of Lake Abert. Concentrations in g 1- . TDS is total dissolved solids. Study (year)

Phillips (1961) Herbst (1980) Present

Field pH

9.7 10.0 9.9

T.D.S

41 54 82

Cations

Anions

Na

K

16.3 34.9

Ca

Mg

0.5

-

-

1.1

<1

<1

Cl

SO4

HCO3

CO 3

14.3 15.1 32.8

0.7 <1.0

2.4 2.7

6.5 6.4

169

LAKE ABERT, OREGON

U.S.A.

I meter contour orth

mp 75

mp 78

81 1298.4 m 2.5 m

=4.5 m mp 84 m

mp 86

mp=milepost markers along HWY 395 Fig. 1. Bathymetric map depicting the geographical station.

abundance was found on 14 July. The total lakewide population of brine shrimp derived from the 14 collecting stations was estimated to be 3.4 x 101 adults with an estimated biomass of 6.6 x 106 kg. Vertical stratification of brine shrimp between the surface layer and the bottom was not present. The horizontal spatial distribution of the brine shrimp population appears quite patchy during periods of peak abundance (Table 3). Angiosperms are absent at Lake Abert and there is very little evidence of large populations of

phytoplankton in the lake. The dominant alga is the filamentous green alga Ctenocladus circinnatus which grows on muddy bottom or on submerged rocks near the shoreline. Ctenocladus reproduces asexually by division and elongation of vegative cell filaments or through motile zoospores. These zoospores together with the diatoms Naviculaand Nitzschia and the blue-green filamentous algae Anabena and Oscillatoriaoften form large mats on the slightly submerged mud flats. At times, tiny fragments of these algal-microbial mats would break loose from

170 2 Table 2. Horizontal distribution of adult brine shrimp abundance (ind x 103 m- ) of Lake Abert brine shrimp for during 1981 and 1982. Data are means of three replicate pumpings.

Month/date 1981

July 31 Aug 21 Aug 28 Sept 3 Oct 16 Nov 14 1982 May23 June 11 June 28 July 14 Aug 12 Sept 10

Stations #1

#2

#3

#4

#5

#6

#7

#8

#9

#10

#11

#12

#13

1 2 1 1 <1 0

1 3 2 1 <1 0

2 2 2 1 <1 0

3 3 1 1 <1 1

3 3 2 1 <1 0

4 2 1 3 I 0

3 3 2 4 I 0

1 1 1 2 <1 0

1 1 1 2 <1 0

2 2 2 2 2 <1

3 3 2 5 <1 <1

1 3 2 3 <1 0

2 3 1 5 <1 0

2 1 0 -

#1

#2

#3

#4

#5

#6

#7

#8

#9

#10

#11

#12

#13

#14

<1 1 1 1 <1 0

<1 1 1 1 <1 0

<1 1 4 3 <1

<1 2 3 4 <1

<1 1 3 4 <1 0

3 6 <1 -

2 1 -

2 2 3 <1 0

<2 2 1

1 1 1

1 1 1 1 -

1 0 0

-

< 1

5 7 -

the bottom and form a raft of floating algal spheres. The algal balls are actively sought after by feeding brine shrimp as shown in Fig. 2D. Commercial harvesting of brine shrimp from lake Abert began in 1979-80 by Keith Kreuz, operator and owner of Oregon Desert Brine Shrimp Co., Lakeview, Oregon. He has provided data on the daily harvest of adult brine shrimp taken from an area approximately 3 000 ha in size and bounded by stations

Table 3. Horizontal patchiness of brine shrimps lakewide for 1981 - 82. Mean density/ Total station 3 (Ind. x 103 x m- )

Coefficient of variation (v/m)

1981 July 31 August 2 August 28 September 3

2.07 2.29 1.43 2.38

0.48 0.30 0.29 0.95

1982 June 11 June 28 July 14

1.27 2.54 3.00

0.22 0.83 1.67

Month/Date year

1 5 6 0

<1 -

0

Average

+I SE ±

2.1 2.3 1.4 2.4 <1.0 0

+0.5 ±1.0 ±0.6 ± 0.8 -

Average

± 1 SE

<1.0 1.3 2.5 3.0 <1.0 0.0

±1.0 ±0.8 ±0.9 -

#14

11 through 14. Annual yields computed from daily harvests of brine shrimp taken by fyketype nets towed by outboard motor powered rafts are estimated as being: 4.9 x 103 kg in 1981, 4.1 x 103 kg in 1982 and 3.4 x 103 kg in 1983. Since the surface area harvested is approximately 15°70 of the total lakes surface, the harvest is estimated to be 0.05 percent of the estimated shrimp biomass. Therefore, this commercial harvest represented a very small removal of the adult brine shrimp population. In 1981 Kathryn Boula and Robert Jarvis (Dept. of Fisheries and Wildlife, Ore. St. Univ.), in cooperation with the present investigators, conducted a foraging ecology program at Lake Abert in order to assess the importance of the lake as a feeding and staging area for migratory birds. They have published a preliminary report on the results of this study (Boula and Jarvis, 1984). The two principal bird species found to be the major consumers of brine shrimp adults and cysts in Lake Abert are the Northern Shoveler (Anas clypeata) and the Eared Grebe (Podicepsnigricollis).Analyses performed on the stomach contents of Shoveler and Eared Grebes showed that brine shrimp composed some 30% and 17°7o of the respective diets. The weight of brine shrimp adults and cysts contained in stomachs of the

171

Fig. 2. Photographs of Abert Rim (A) depicted as a fault scarp (B) with migratory shorebirds (C) and feeding shrimp located among balls of filamentous algae (D).

two species of birds was approximately 20 mg + 5 mg dry weight per bird. If one assumes that this reflects a single feeding, then a feeding rate of 5 times per hour would give an hourly brine shrimp consumption of an average 0.3 g wet weight. If the bird spends 5 to 8 hours per day actively feeding, then the daily consumption per bird is 1.5 to 2.5 g of shrimp plus cysts. The bird census taken at the lake in 1982-83 during the peak period of migration for the Shoveler and Eared grebe indicated that an average of 2000 to 5000 individuals per day were utilizing this habitat and that foraging on the lake invertebrates lasted for a 60-90 day period. Therefore, the maximum utilization of brine shrimp by avian predation is estimated at 1125 kg per migratory period. This consumption is only approximately 25 percent of the commercial harvest for 1981- 83 and represents a very small (< 0.01 percent)

proportion of the biomass of adult Artemia in Lake Abert. These foraging estimates for the Eared Grebes can be compared to the study conducted by Cooper and co-workers at Mono Lake, CA (Cooper et al., 1984) but it is difficult to evaluate how the two populations impact their respective food resource since censusing Eared Grebe populations is a most difficult task.

Acknowledgements A special thanks are extended to Bill Otani, Keith Kreuz, Steve Denny, Bob Jarvis, Kathy Boula and David Herbst, all of whom gave generous amounts of their time and materially assisted us during the course of the investigation. This study was aided financially by a grant from NOAA to O.S.U. Sea

172 Grant Program under Aquaculture Project No. R/Aq 39. Field instruments and laboratory facilities were provided by the Oregon Department of Fish and Wildlife (ODFW) and Dircon Consultants, Corvallis, Oregon to conduct water chemistry and zooplankton monitoring. Harry H. Wagner, Larry Bisbee, Larry Korn, Frank Newton and William Haight, all members of the ODFW administration provided cooperation and enthusiasm throughout the study. Lastly, appreciation is given to William Q. Wick, director of Sea Grant, for his belief that Lake Abert is truly Oregon's inland sea. We thank Julie Valenter for typing the manuscripts.

References Bellrose, F. C., 1976. Ducks, geese and swans of North America (2nd). Stackpole Books, Harrisburg, PA. 544 pp. Boula, K. M. & R. L. Jarvis, 1984. Foraging ecology of fallmigrating waterbirds, Lake Abert. Dept. of Fish. and Wildlife,

Ore. St. Univ. Spec. Rpt., 29 pp. Conte, E P., R. Jellison &G. L. Schnoor, 1987. Abundance and spatial distribution of brine shrimp Artemia monica in Mono Lake, California. Hydrobiologia. This volume. Cooper, S. D., D. W. Winkler &P. H. Lenz, 1984. The effect of Grebe predation on a brine shrimp Artemia monica population. J. Animal Ecol. 53: 52-64. Herbst, D., 1980. Ecological physiology of the larval fly, Ephydra hians, an alkaline salt lake inhabiting ephyrdrid (Diptera). M.S. Thesis, Oregon State University. 65 pp. Hutchinson, G. E., 1957. A treatise on limnology. V61. 1. J. Wiley and Sons, N.Y. 320 pp. Malone, B. G. &D. J. McQueen, 1983. Horizontal patchiness in zooplankton populations in two Ontario kettle lakes. Hydrobiologia 99: 101-124. Phillips, K. N. &A. S. VanDenburgh, 1971. Hydrology and geochemistry of Abert, Summer, and Goose Lakes and other closed-basin lakes in southcentral Oregon. U.S. Geol. Survey Prof. Paper 502-B. 86 pp. Pielou, E. C., 1977. Mathematical Ecology. J. Wiley and Son, N.Y., 385 pp. VanDenburgh, A. S., 1975. Solute balance at Abert and Summer lakes, southcentral Oregon. U.S. Geol. Survey Prof. Paper 502-C. 29 pp.