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Given the extreme seasonality experienced by the high-latitude, perenially ice-covered lakes of Taylor Valley, Antarctica, one might expect the availability of light alone to dictate the growth of their phytoplankton communities. It has been demonstrated that photosynthesis of Lake Bonney phytoplankton is not saturated by in situ irradiance (Lizotte and Priscu 1992); however, the maximum quantum yield of photosynthesis exhibited by these phytoplankton decreases with distance from the nutricline (Lizotte and Priscu 1994). Nutrient deficiency, therefore, has been suggested as a control on photosynthetic production in Lake Bonney (Lizotte, Sharp, and Priscu 1996). Similarly, nutrient bioassays have revealed enhanced photosynthetic carbon assimilation in incubations amended with ammonium and/or phosphate in all four of the Taylor Valley lakes examined (east and west lobes of Lake Bonney, Lake Hoare, and Lake Fryxell; Priscu 1995). Phosphate additions stimulated a dramatic and highly significant increase in photosynthesis in samples from Lake Bonney, whereas significant stimulation in samples from Lake Hoare and Lake Fryxell occurred only with simultaneous additions of ammonium and phosphate (Priscu 1995). These bioassay data, along with estimates of nitrogen and phosphorus flux ratios to the trophogenic zones of these lakes, have indicated a strong phosphorus deficiency in Lake Bonney phytoplankton and only mild phosphorus deficiency and/or nitrogen deficiency in Lake Hoare and Lake Fryxell phytoplankton (Priscu 1995).
During the 1995 Winfly deployment of the McMurdo Dry Valleys Long-Term Ecological Research program (McMurdo LTER), we undertook a preliminary study of the role of phosphorus in the ecology of Taylor Valley lake ecosystems. We report here some initial results of this phosphorus study from the highly density-stratified Lake Bonney (east lobe) and the fresh-water Lake Hoare. Fieldwork was carried out at Lake Hoare from 7 September to 19 September 1995 and at Lake Bonney from 24 September to 19 October 1995. Water samples were collected through a melthole in the ice according to standard LTER procedures. Soluble reactive phosphorus (SRP) was analyzed by a standard manual colorimetric method (Strickland and Parsons 1972) and particulate phosphorus (PP) was measured as SRP after high-temperature combustion and acid hydrolysis of the filtered samples (Karl et al. 1991, pp. 71-77). The enzymatic activity of alkaline phosphatases was estimated using the organic monophosphate ester 4-methylumbelliferyl phosphate (MUP) as a substrate analog (Pettersson and Jansson 1978).
Water column concentration profiles of SRP and PP display several important differences between Lake Bonney (east lobe) and Lake Hoare ( figure 1). SRP concentrations in the trophogenic zones (from the base of the ice cover to about 20 meters depth) are vanishingly small in both lakes, but PP is substantially higher in Lake Hoare, especially immediately below the ice. The decreasing trend with depth suggests that this sestonic PP is associated with phytoplankton. Although Lake Bonney was sampled later in the growing season than was Lake Hoare, no accumulation of PP is noted above 18 meters, suggesting that phytoplankton in Lake Bonney are growing with a low cellular phosphorus quota. Beneath the trophogenic layer of Lake Bonney, both SRP and PP concentrations are elevated, indicating SRP remineralization from organic detrital particles and/or SRP desorption from mineral phases in the dense brine of the deep layer. Lake Hoare exhibits no such detrital PP signature but does show elevated SRP in the deepest sample, where anaerobic decomposition of sedimentary organic material is likely contributing to the SRP pool. The deep SRP pools of these lakes may be a source of phosphorus to the trophogenic layers, but the extreme density stratification in Lake Bonney severely restricts the potential upward diffusion of SRP; conversely, the lack of density stratification in Lake Hoare may well allow a significant flux of deep SRP to the trophogenic zone (Priscu 1995).
The enzymatic activity of alkaline phosphatase (APase) is also greatly different between the two lakes. Substrate-saturated APase activities measured at 5 meters in Lake Bonney were an order of magnitude higher than those in Lake Hoare ( figure 2). Considering that Lake Hoare was sampled earlier in the growing season, the APase activities on a per-cell basis are likely to display an even greater disparity between the two lakes. APase activity in Lake Bonney decreases with depth and is appreciably inhibited by the addition of phosphate ( figure 3). The decrease in APase activity with depth is partly due to a decrease in biomass with depth, but the degree of inhibition by phosphate addition increases with distance from the chemocline, suggesting a greater degree of phosphorus deficiency in shallow populations. Size fractionation experiments (data not shown) reveal that the measured APase activity belongs mostly to bacterial and algal size classes, with very little free dissolved activity.
The above results support earlier studies that have suggested a high degree of phosphorus deficiency in the phytoplankton of Lake Bonney but only mild phosphorus deficiency in Lake Hoare phytoplankton. Further examination of the emerging data set will focus on quantification of phosphorus fluxes to the trophogenic zones of these lakes and on the potential bioavailability of different inorganic and organic phosphorus pools.
This research was supported by National Science Foundation grants OPP 94-19413 and OPP 92-11773 awarded to J.C. Priscu. We thank E. Adams, R. Edwards, C. Fritsen, D. Gordon, A. Lundberg-Martell, C. Takacs, C. Wolf, and the staff of Antarctic Support Associates for field and laboratory assistance.
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