Phytoplankton phosphorus deficiency and alkaline phosphatase activity in the McMurdo Dry Valley lakes, Antarctica

We assessed the nutrient (N, P) deficiency of phytoplankton from the perennially ice-covered lakes in Taylor Valley, Antarctica, with 14C-based bioassays. Phytoplankton in the near-surface waters from three of the four lakes displayed elevated photosynthetic carbon assimilation in the presence of added P relative to controls. Carbon assimilation in samples from deep chlorophyll layers was also enhanced by P addition in two of the lakes. No effect of N addition without simultaneous P addition was noted in any lake. Lake Bonney near-surface phytoplankton consistently showed a strong response to P and N 1 P additions, suggesting P deficiency. The near-surface phytoplankton of Lake Hoare responded to N 1 P but not N or P addition alone, suggesting a more general nutrient deficiency. Further investigation of phytoplankton phosphorus deficiency was carried out in the east lobe of Lake Bonney and in Lake Hoare. Water column dissolved and sestonic elemental ratios and planktonic alkaline phosphatase activity measurements corroborated the results of the bioassays. Despite the proximity of the lakes and their similarity in ice cover and watershed, a high degree of phytoplankton phosphorus deficiency was evident in Lake Bonney but not in Lake Hoare. We hypothesize that the difference in the degree of phytoplankton P deficiency between these lakes may not be due to differences in external P fluxes, but rather to differences in the internal cycling of phosphorus stemming from large differences in water column chemistry. The McMurdo Dry Valleys region of Antarctica is a unique environment in which the biological communities have had to adapt to some of the most extreme conditions available on Earth. The largest ice-free region on the continent, it is among the coldest and driest deserts in the world, having a mean annual temperature of #2208C and #10 cm yr21 precipitation (Heywood 1984). Within this desert are a number of lakes, many of which maintain a body of liquid water beneath a permanent ice cover. This ice cover protects the underlying water column from experiencing the extreme low temperatures and large temperature variations that are common in the environment above the lake ice (Spigel and Priscu 1998). These lakes offer habitats for specialized aquatic communities that have adapted to low light levels, lack of water column mixing, and intensely stratified chemical environments ranging from extreme oxygen supersaturation to anoxia (Simmons et al. 1993; Priscu et al. 1999). Life in the dry valley lakes is primarily microbial, with a conspicuous absence of metazoans (Takacs and Priscu 1998). The plankton of the dry valley lakes is composed primarily