PHYTOPLANKTON NUTRIENT DEFICIENCY IN LAKES OF THE MCMURDO DRY VALLEYS, ANTARCTICA

1. The influence of inorganic nitrogen and phosphorus enrichment on ph ytoplankton photosynthesis was investigated in Lakes Bonney (east and west lobes), Hoare, Fryxell. and Vanda, which lie in the ablation vall eys adjacent to McMurdo Sound, Antarctica. Bioassay experiments were c onducted during the austral summer on phytoplankton populations just b eneath the permanent ice cover in all lakes and on populations forming deep-chlorophyll maxima in the east and west lobes of Lake Bonney. 2. Phytoplankton photosynthesis in surface and mid-depth (13 m) samples from both lobes of Lake Bonney were stimulated significantly (P < 0.01 ) by phosphorus enrichment (2 mu M) With further stimulation by simult aneous phosphorus plus NH4+ (20 mu M) enrichment. Similar trends were observed in deeper waters (18 m) from the east lobe of Lake Bonney, al though they were not statistically significant at P < 0.05. Photosynth esis in this lake was never enhanced by the addition of 20 mu M NH4+ a lone. Simultaneous addition of phosphorus plus nitrogen stimulated pho tosynthesis significantly (P < 0.01) in both Lake Hoare and Lake Fryxe ll. No nutrient response occurred in Lake Vanda, where activity in nut rient-enriched samples was below unamended controls; results from Lake Vanda are suspect owing to excessively long sample storage in the fie ld resulting from logistic constraints. 3. Ambient dissolved inorganic nitrogen (DIN) (NH4+ + NO2- NO3-): soluble reactive phosphorus (SRP) ratios partially support results from bioassay experiments indicating strong phosphorus deficiency in Lake Bonney and nitrogen deficiency in Lakes Hoare and Fryxell. DIN:SRP ratios also imply phosphorus deficie ncy in Lake Vanda, although not as strong as in Lake Bonney. Particula te carbon (PC):particulate nitrogen (PN) ratios all exceed published r atios for balanced phytoplankton growth, indicative of nitrogen defici ency. 4. Vertical nutrient profiles in concert with low advective flux , indicate that new (sensu Dugdale & Goering, 1967) phytoplankton prod uction in these lakes is supported by upward diffusion of nutrients fr om deep nutrient pools. This contention was tested by computing upward DIN:SRP flux ratios across horizontal planes located immediately bene ath each chlorophyll maximum and about 2 m beneath the ice (to examine flux to the phytoplankton immediately below the ice cover). These flu x ratios further corroborated nutrient bioassay results and bulk DIN: SRP ratios indicating phosphorus deficiency in Lakes Bonney and Vanda and potential nitrogen deficiency in Lakes Hoare and Fryxell. 5. Neith er biochemical reactions nor physical processes appear to be responsib le for differences in nutrient deficiency among the study lakes. The d ifferences may instead be related to conditions which existed before o r during the evolution of the lakes.