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In an approach virtually identical to that taken 18 years earlier (Cathey et al. 1981, 1982), artificial substrates were collected from six main sampling sites between 8 and 22 January 1996 to characterize the protozoan communities of two McMurdo Dry Valley lakes. Samples were collected at paired shallow- and deep-water sampling holes in central Lake Hoare, western Lake Hoare, and western Lake Fryxell. We evaluated the degree of similarity in protozoan community composition at several spatial scales and compared our taxonomic findings with those previously obtained by Cathey et al. (1981, 1982).
Polyurethane foam units (PFUs) were used to collect protozoa at each site (Pratt and Kepner 1992). Triplicate PFUs were placed at three or four depths at each site, and deepest PFUs were actually in contact with lake bottoms. Substrates were colonized in situ for 3 weeks. For PFUs placed in Lakes Hoare and Fryxell, Cathey et al. (1982) reported that the time required to obtain 90 percent of the equilibrium species number of protozoa was 16 days. Thus, a 3-week colonization period should permit an adequate assessment of nearly all potential colonizers. PFUs allow replication and often provide richer collections than natural substrates. Three or four PFUs provide as good an estimate of species richness at a site as 10-12 samples from natural substrata (Pratt, Horowitz, and Cairns 1987).
Subsamples were systematically evaluated for taxonomic composition by direct microscopic observation of live material. Protists were identified to species where possible and all were identified at least to genus. Standard protozoological keys were used (e.g., Kudo 1966; Lee, Hutner, and Bovee 1985). Identified protozoa, classified according to the system of Levine et al. (1980), were also classified into functional feeding groups (Pratt and Cairns 1985).
In total, 41 living protozoan taxa were observed in Lakes Hoare and Fryxell during this study. Greatest richness (24 taxa) was observed at the shallow Fryxell site. On a site-by-site basis, taxonomic richness was greatest in west Fryxell (32), followed by central Hoare (18), and then west Hoare (15). Removing "rare" taxa (those occurring in less than 10 percent of samples) left a total of 21 taxa from all sites for consideration. Removal of rare taxa acts as a noise filter by eliminating records based on single observations. Presence/absence data were used to calculate and compare community similarity indices using a permutation procedure (Pratt and Smith 1991, pp. 91-103). Jaccard's coefficient of community similarity was used to compute a test statistic comparing the mean similarity of replicate PFUs at one depth, sampling hole, site or lake, to the between depth, sampling hole, site, or lake similarity. Thus, detailed comparisons of community composition have been made at a variety of spatial scales.
Based on comparisons of calculated Jaccard's coefficients, several significant differences in community composition were observed (p<0.05, table). All differences were significant whether "rare" taxa were, or were not, included in the analyzed data set. Differences between sampling holes within Lake Fryxell were as great as those between sampling holes in different lakes. As expected, the greatest differences frequently involved samples collected from deeper, anaerobic waters, which harbor few species of protists. Assemblages from the shallow Fryxell hole were significantly different from samples collected below the oxycline at the deep water hole, whereas aerobic zone samples from the deep hole were not different from anaerobic samples collected from the same hole. Using the presence/absence metric employed here, we detect significant differences in protozoan community structure at spatial scales of as little as 3 meters (m) in the vertical (table).
Euclidean distance measures were also used to evaluate community similarity between sampling holes. Mean-standardized abundance indices, based on frequency of observation, were used to calculate distance measures incorporating data from all sites and depths. Distance measures indicate that communities at proximate sampling holes were most similar to one another and that sampling holes in Lake Hoare clustered together in terms of community composition ( figure). A distance of zero would mean that the exact same taxa were found in the exact same relative frequencies at two different sites (i.e., that communities at the two sites were identical).
No relationship was found between taxonomic richness and water column chlorophyll-a concentration (data collected roughly 1 week after collection of PFUs). Richness in the aerobic zone of the more-productive Lake Fryxell was consistently higher than at other sites, however. This richness was also the case during the 1978-1979 season (Cathey et al. 1981).
As is typical, these data show a skewed distribution of species abundances with numbers of species, that is, many more infrequently occurring than frequently occurring taxa were noted. On a sample-by-sample basis, the most frequently encountered taxa overall were the suctorian, Sphaerophrya sp., the kinetoplastid flagellate, Bodo globosa, the chrysomonad, Ochromonas sp. (probably O. minuta or O. miniscula), the volvocid, Chlamydomonas globosa, the cryptomonad, Chroomonas lacustris, and the peritrich ciliate, Vorticella mayeri; all of which were observed in roughly half the PFUs collected. Other common bodonid, chlamydomonad, and chrysomonad species were also frequently encountered and phytoflagellate groups were often numerically dominant in our collections. The ubiquitous nature of most of these taxa has been previously noted and the majority of species encountered also occur in temperate freshwater habitats.
Protozoa were classified into functional feeding groups, or guilds, used to examine further spatial patterns in community structure. Primary-producing flagellates (frequently cryptomonads and chlamydomonads) and bactivores dominate these lakes. Cathey et al. (1982) also found that phytoflagellates were the most abundant initial colonizers of PFUs. Functional feeding group distributions (as proportion of identified taxa) formed no consistent pattern with depth and, in general, similar proportions of the various feeding groups were found at each sampling hole. Organisms in functional feeding groups other than bactivores and primary producers constituted only 16 and 19 percent of taxonomic identifications made in lakes Fryxell and Hoare, respectively.
Further comparisons with historic taxonomic records are currently underway as are similar colonization studies in Lakes Bonney (east and west lobes), Joyce, Hoare, and Fryxell.
This research was supported by National Science Foundation grant OPP 92-11773.
References
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