A COMPARISON OF METHODS FOR ASSESSMENT OF NUTRIENT DEFICIENCY OF PHYTOPLANKTON IN A LARGE OLIGOTROPHIC LAKE

Short-term (h) and long-term (d) changes in phytoplankton community physiology and biomass in response to nutrient enrichment were used concomitantly as bioassays of phytoplankton nutrient deficiency in oligotrophic Flathead Lake, Montana, 6 times over the course of a year. Long-term bioassays consisted of nutrient amendments to epilimnetic water in 20L containers which were subsequently monitored for algal growth. Short-term bioassays included measurements of NH4 + stimulation of dark carbon fixation, measurement of PO4 3- and NH4 + uptake over time to assess depletion of internal pools and stimulation effects of PO4 3- on NH4 + uptake and NH4 + on PO4 3- uptake. During thermal stratification, simultaneous additions of NH4 + and PO4 3- in long-term bioassays caused significant increases in chlorophyll a concentration, photosynthetic 14CO2 uptake, and particulate N concentration within 4.5d; single additions of NH4 + or PO4 3- had little or no effect. During winter mixing there was little evidence for N or P deficiency in either short- or long-term bioassays. In general, short-term bioassays did not consistently agree with each other or with long-term bioassays. It may be necessary to elicit growth of phytoplankton with nutrient additon to make definitive statements regarding nutrient deficiency. -Authors