Phytoplankton cell densities were somewhat higher, about 475 cells/ml, at the surface than in the May collection. A wide variety of algal species were present in Bourgeau Lake in September. The phytoplankton was dominated by the blue-green algae Aphanocapsa sp. (36^ at 0 m, 57% at 1 m, 20% at 4 m), Chroococcus sp. (24% to 0 m, 42% at 1 m, 52% at 4 m, 35% at 8 m), and Anabaena flos-aquae (18% at 0 m, 14% at 1 m). The blue-greens Coelosphaerium sp. (14% at 0 m, 12% at 1 m, 23% at 4 m), and Oscillatoria sp. (3% at 0 m, 11% at 1 m) were also well represented. At 8 m, unidentified centric diatoms and Oocystis sp. each composed 24% of the algal population. The greens Lagerheimia subsalsa and Cosmarium cf. pygmaeum each made up 12% fo the phytoplankton in the 8 m depth.
The algae represented by high cell counts in the May collections of Bourgeau Lake were Sphaerocystis schroeteri and Quadrigula sp. Smaller numbers of Anabaena flos-aquae and Fragilaria were present. The most abundant phytoplankton in the September collection were the blue-greens Aphanocapsa elachista, Chroococcus sp., Anabaena flos-aquae, Coelosphaerium and Oscillatoria.
Sphaerocystis schroeteri is frequently associated with eutrophic conditions (Taylor et al. 1981, Reynolds 1984A) and is common in ponds and shallow fertile lakes. It is also sometimes important in unproductive oligotrophic lakes (Hutchinson 1967, Happey-Wood 1988). Studies by Reynolds (1983) indicated that Sphaerocystis depends upon water turbulence for suspension and population maintenance in the euphotic zone. The reduction in depth of the euphotic zone (shading) by a large population fo blue-green algae could prevent the increase in Sphaerocystis populations when nutrient availability would otherwise be favorable. Large numbers of Sphaerocystis colonies may develop as a result of grazing by zooplankton because Sphaerocystis is too large to be eaten by many zooplankters (Harris 1986). The develpment of Sphaerocystis is also favored by increased thermal stability (Happey-Wood 1988). The temperature profile of Bourgeau Lake shows that the lake had begun to stratify. Reynolds (1984b) considers the assemblage of Sphaerocystis and Oocystis to be typical of mesotrophic waters.
Fragilaria is frequently associated with eutrophication in temperate lakes (Taylor et al. 1981). Although Stoermer and Yang (1970) called the diatom eurytopic, the authors observed that it did not flourish in highly polluted waters that contained high nitrogen and phosphorus ratios. Stevenson (1985) reported that Fragilaria crotonensis is less competitive in waters that have abundant phosphorus but are poor in nitrogen or silicon.
The portion of the phytoplankton represented by Anabaena was similar in the May and the September collections. High populations of the blue-green Anabaena flos-aquae are common in productive temperate waters in summer (Hutchinson 1967), particularly in waters enriched with phosphorus and organic matter (Paerl 1988). Maximum bloom development occurs in thermally and sometimes chemically stable, stratified water columns (Reynolds 1980). Because Anabaena flos-aquae is capable of fixing atmospheric nitrogen, it is able to thrive under conditions of low nitrogen availability.
Some heterocysts were observed in filaments of Anabaena in both the May and September collections. During periods of nitrogen deficiency (and, usually, phosphorus availability), vegetative cells some blue-green algae become heterocysts after undergoing morphological and physiological changes. The heterocysts contain the nitrogenase enzyme complex involved in nitrogen-fixation (Paerl 1988). Consequently, the presence of heterocysts in blue-green algal populations in May and September is an indication of nitrogen depletion in the waters of Bourgeau Lake.
High populations of Aphanocapsa, also a blue-green, often develop in hypertrophic conditions and the alga is considered indicative of high nutrient nloading Reynolds 1984a). The blue-green Chroococcus is a common subdominant alga in mesotrophic to hypertrophic waters. It is often associated with nuisance bloom genera, but it seldom forms blooms by itself (Paerl 1988). Chroococcus is not a nitrogen-fixing genus. Oscillatoria is common in mesotrophic to hypertrophic lakes, ponds and stagnant waters. Although the genus does not produce heterocysts, some species of Oscillatoria are able to fix atmospheric nitrogen (Paerl 1988).
In summary, the algal assemblages found in two phytoplankton samples from Bourgeau Lake, together with the moderate algal cell densities, indicate that the lake is probably mesotrophic.
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Spring , 10 May 1987 |
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| Sample Depth | Phytoplankton | Algal Cells per ml | Percentage |
|---|---|---|---|
| Surface | Sphaerocystis schroeteri | 74 | 35% |
| Quadrigula sp. | 49 | 23% | |
| Quadrigula cf. closterioides | 35 | 16% | |
| Anabaena flos-aquae | 23 | 11% | |
| Centric diatoms | 11 | 5% | |
| Chroococcus sp. | 11 | 5% | |
| Oocystis sp. | 10 | 5% | |
| 4 Meters | Sphaerocystis schroeteri | 79 | 39% |
| Quadrigula sp. | 59 | 29% | |
| Centric diatoms | 39 | 19% | |
| Anabaena flos-aquae | 39 | 7% | |
| Chroococcus sp. | 8 | 4% | |
| Oocystis sp. | 5 | 2% | |
| 8 Meters | Chroococcus sp. | 31 | 63% |
| Fragilaris sp. | 9 | 17% | |
| Unidentified green flagellate | 5 | 9% | |
| Quadrigula sp. | 4 | 8% | |
| Centric diatoms | 1 | 2% | |
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| Sample Depth | Phytoplankton | Algal Cells per ml | Percentage |
| Surface | Aphanocapsa elachista | 170 | 36% |
| Chroococcus sp. | 114 | 24% | |
| Anabaena flos-aquae | 84 | 18% | |
| Coelosphaerium kutzingianum | 67 | 14% | |
| Oscillatoria sp. | 15 | 3% | |
| Gloeocapsa rupestris | 9 | 2% | |
| Dinobryon cylindricum | 6 | 1% | |
| 1 Meter | Aphanocapsa sp. | 209 | 57% |
| Chroococcus sp. | 140 | 42% | |
| Anabaena flos-aquae | 50 | 14% | |
| Coelosphaerium kutzingianum | 43 | 12% | |
| Oscillatoria sp | 39 | 11% | |
| Dinobryon cylindricum | 12 | 3% | |
| Chroococcus sp. | 12 | % | |
| Denticula sp. | 5 | 1% | |
| 4 Meters | Chroococcus sp. | 89 | 52% |
| Coelospherium sp. | 40 | 23% | |
| Aphanocapsa elachista | 34 | 20% | |
| Unidentified green alga | 6 | 3% | |
| Dinobryon cylindricum | 4 | 2% | |
| 8 Meters | Chroococcus sp. | 3 | 35% |
| Unidentified centric diatom | 2 | 24% | |
| Oocystis sp. | 2 | 24% | |
| Lagerheimia subsalsa | 1 | 12% | |
| Cosmarium cf. pygmaeum. | 1 | 12% | |