THE IMPORTANCE OF NITROGEN IN PYRAMID LAKE (NEVADA, USA), A SALINE, DESERT LAKE

The increase in human development in the downstream portion of the Pyr amid Lake drainage basin has resulted in increased nutrient loading to the lake. Since this is a deep, terminal lake, concern over nutrient build up and change in trophic status exists. On the basis of lake che mistry which shows consistently high concentrations of total reactive- P (mean = 55 mug P l-1) relative to dissolved inorganic-N (DIN) (mean = 15 mug N l-1), it has been hypothesized that Pyramid is N-limited. H owever, no systematic study of nutrient limitation had been undertaken . Nutrient enrichment bioassays conducted throughout an entire year cl early showed that additions of DIN resulted in a 350-600% stimulation of chlorophyll production. Phosphate, when added singly or in combinat ion with DIN, had no effect. This positive response to N-addition was significant at all times of the year except, (1) immediately after com plete lake mixing in February when a large pool of hypolimnetic nitrat e was injected into the euphotic zone, and (2) during a fall bloom of the nitrogen fixing species Nodularia spumigena. The positive response to N-addition in the bioassay experiments was strong between March an d November. However, the seston exhibited only a gradual depletion of nitrogen relative to carbon over this same period. PN:PC ratios sugges ted no N-deficiency in phytoplankton biomass in February, March and Ap ril, moderate N-deficiency in May, June and July and, severe N-deficie ncy from August until winter turnover. The appearance of nitrogen fixi ng blue-green algae in September supports the hypothesis of N-limitati on in the summer-autumn. In evaluating the nutrient status of a lake, the concepts of nutrient stimulation versus nutrient deficiency versus nutrient limitation must clearly be defined.