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Apparently the algae in Silver View Pond are still obtaining enough nitrogen, because there is a statistically significant relationship between total nitrogen and and the quantity of algae, as measured by chlorophyll (r =0.88; P< 0.01)( Figure 8). Presumably, at higher algal densities algae are running short of nitrogen at which time blue - greens begin to obtain nitrogen directly from the air through the process of nitrogen fixation. On two sampling dates, dissolved nitrogen compounds (nitrate plus nitrite) were measured, and five of the six samples were below the detection limit of 50 ug/L ( see Table 2), indicating that the algae were taking up most of the available nitrogen. Algae Given that the system seems to be slightly nitrogen -limited, one would expect blue-green algae to dominate the phytoplankton of Silver View Pond. However, on every sampling date, the algae population of the pond was dominated by a non -blue-green species, Ce_ratium (Table 3). The balance of the arae on each date were either blue - greens or green algae. However, except for :Anabaena, the blue -greens observed in were of a type that can not obtain nitrogen from the air. These results apply to water samples obtained from the middle of each arm of Silver View Pond. Table 3. Estimated composition (by biomass) of phytoplankton in Silver View Pond, 1989. Date % N -Fixing BGa % other BGb % Ceratium % Green 6/24 5 10 80 5 7/22 0 30 60 10 8/13 0 0 75 25 9/2 0 50 50 0 a The only nitrogen -fixing blue-green algae (N -fixing BG) observed was Anabaena. b Non -nitrogen -fixing' blue-green algae (non -ti -fixing BG) observed were Oscillatoria or Lynob a and `licrocystis. In contrast, colonies of blue-green algae were observed floating around the margins of the pond on both June 24 and September 2. On June 24 large colonies of Oscillatoria or L.yri bLa (both non nitrogen -fixing blue -greens) and filamentous green algae were observed floating at the surface. On September 2 numerous (at least 50) colonies of the blue-green alga Nostoc had been blown to the shore of the western arm. Nost_oc colonies are hard, hollow green spheres slightly smaller than a ping-pong ball. When examined under a microscope, the colonies were rich with heterocysts, which are the special cells that fix nitrogen. 86