| Abstract: | Algal nutrient enrichment bioassays were conducted between May 1975 and August 1978 using water samples collected from Chautauqua Lake, New York. Photosynthetic fixation rates of natural phytoplankton assemblages were enhanced by additions of phosphorus and nitrogen, although enrichment with other nutrients had no significant stimulatory effect on algal photosynthesis. Whereas phosphorus stimulated in spring and early summer, both nitrogen and phosphorus enhanced photosynthesis in midsummer and fall. Relative to the effect of phosphorus enrichment, enhancement of photosynthesis by nitrogen during the summer and fall was highest in the northern part of the lake. During the period of ice cover, photosynthesis did not appear to be limited by nutrients in that nutrient additions (P, N, Si, C, Fe, trace metals) did not enhance fixation rates. Observed temporal fluctuations in the response of the algae to P and N correlated with changes in the lake water N:P ratio as well as with temporal changes in dissolved orthophosphate and nitrate-nitrite nitrogen. The N:P ratio decreased drastically in the summer and remained at ca. 10 or less through mid-fall, suggesting that N concentrations were inadequate for the non-N-fixing phytoplankton. Studies over 3 yr indicate that states of P and N limitation undergo time-space fluctuations that occur in a cyclic pattern in the surface waters of Chautauqua Lake. |
