SEASONAL CHANGES IN THE ATTACHED ALGAE OF FRESHWATER AND SALINE LAKES IN THE LOWER GRAND COULEE, WASHINGTON1

Abstract

A two year investigation of two freshwater alkaline lakes and two saline lakes in the Lower Grand Coulee has provided a quantitative and qualitative record of many of the seasonal changes occurring in the non‐planktonic algae. A glass plate method satisfactorily recorded quantitative changes in the attached algae. The plates (28 by 28 cm) were submerged for 2 to 4 weeks at various depths. Dry weight and ash‐free dry weight of the predominantly algal attachment materials on plates were determined, and the ash‐free weight was expressed as a production rate. The ash from freshwater lakes was composed normally of intact diatom frustules. The ash from saline lakes was also diatomaceous, but the weakly silicified frustules were deformed during ashing. Proportional counts of the various species in freshwater lakes were made using the ash. The evaluation of relative dominance was based on the mathematical product of the cell count and a calculated volume factor for each species. The glass‐plate production technique was examined at some length and it was concluded that a 2‐week submergence period was most satisfactory, that glass was not unduly selective, that a horizontal position was satisfactory, and that the method was best suited to the freshwater lakes.In freshwater lakes (200–450 ppm T.D.S.) results clearly show a bimodal production curve with the peak generally higher in the spring than in the fall. Lowest production invariably occurred in late summer. High production values in the spring were commonly near or above 500 mg/m2/day. Comparison of the results of two years shows sizable differences in total production at comparable times of year, but the seasonal distribution of species followed the same general pattern both years. Cymbella affinis, C. cistula, C. mexicana, Diatoma elongatum, Fragilaria vaucheriae, Gomphonema eriense, Nitzchia spp., Synedra acus, and S. ulna were characteristic spring dominants in the freshwater lakes. These diatoms were replaced in summer mainly by Epithemia sorex, E. turgida, and Rhopalodia gibba. A few “spring” species were common in one lake in summer but only at greater depths. TheNine per cent of the 275 algal taxa collected in the Lower Grand Coulee occurred in both freshwater and saline lakes. Although the saline lakes were not rich in numbers of species, production rates were similar to and even higher than production rates in the freshwater lakes. In the two saline lakes (Lake Lenore‐10,000 ppm and Soap Lake—25,000 ppm) there was a pronounced diatom pulse in the fall. Production rate values as high as 550 mg/m2/day were achieved during this season. In Soap Lake a winter pulse reached a value of 1000 mg/m2/day which was followed by a distinct spring pulse, both absent in Lake Lenore. The dominants on glass plates in both lakes were species of Nitzschia and Amphora. Blue‐green algae were also common and were conspicuous for their lack of seasonality.