Environmental Control of Primary Productivity in Alaskan Tundra Ponds

The photosynthetic response of tundra pond algae to various combinations of temperature, light intensity, and phosphate concentration was measured at weekly intervals during the 1973 summer. In these small (50 m diam by 20 cm deep) ponds near Barrow, Alaska the epipelic algae had a higher temperature optimum (> 20 degrees C) for photosynthesis than did the phytoplankton (14 degrees C) but the epipelic Q_(10) (2.5—12.5 degrees C) for photosynthesis was only 2.2, compared to a value of 3.0 for the planktonic algae. Thus the epipelic algae seemed to be adapted to the sediment environment where temperatures were usually higher than temperatures in the water. The plankton algae, in contrast, appeared to consist of species which photosynthesized more efficiently at the lower temperatures of the pond water. The photosynthetic half—saturation light intensity, I_(0.5), was temperature dependent, increasing as much as threefold in the epipelic experiments (from 0.04 to 0.12 ly° min — 1 ) over a 10 degree temperature range. No diurnal change in I_(0.5) was observed but it declined steadily for both algal groups throughout the summer in response to declining illumination. This decrease in I_(0.5) probably resulted from an increase in the chlorophyll—to—carbon ratio of the algae. Light inhibition was common at low temperatures in the planktonic experiments but never occurred in the epipelic experiments. Short—term phosphate enrichment experiments resulted in no detectable response in either plankton or epipelic algal photosynthesis. However, after whole pond fertilizations large increases in algal biomass and productivity occurred within several days. Thus, the ponds were phosphate—limited but there was a time lag in the response to the nutrient addition.