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.