Biota of Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 429

mixing, so settling is minimized. Similarly, in the fall with fall turnover, waters are cooler, mixing
is again high, and nutrient concentrations are high. Diatoms are capable of responding quickly and
taking competitive advantage of such conditions, during which their populations “bloom.” Following
the spring bloom, nutrients become depleted, particularly the silica for diatoms, there is an increase
in herbivores, and then the diatom populations are “bust.” During these conditions, green algae
often become dominant. Green algae are good competitors for nutrients and many have lagella or
gas vacuoles to offset higher sinking rates. During late summer and early fall, the Cyanobacteria
often dominate. In the early fall, herbivore populations are large, but Cyanobacteria are not very
palatable. While nutrients are low, they can ix nitrogen. So, they bloom. Then, during winter with
lower temperatures and increased vertical mixing, the population densities for all algae are typically
low. However, winter productivity, such as under ice, may be signiicant in many systems.
Another more speciic example of the complications in seasonal variations in spatial distribu-
tions is with fall blooms of the Cyanobacteria Planktothrix rubescens. This Cyanobacteria, like red
algae, contains the pigment phycoerythrin, which relects red light and absorbs blue light. So, blooms
of this alga give water the color of deep red, rich wine (Sappington 2009; Figure 15.16). By absorbing
blue light, which penetrates deeper into the water column than the light of longer wavelengths,
these Cyanobacteria may commonly occur at greater depths than other phytoplankton. For example,
Sappington (2009) discussed the distribution of these Cyanobacteria in Missouri lakes, where they
may commonly occur in the metalimnetic zone during the summer months (Figure 15.17). As illus-
trated in Figure 15.17, during much of the year they may not occur at the depth of the irst arrow
(12 ft.) where there is still approximately 20% of surface light available and temperatures are near
that of the surface. However, in the metalimnetic zone (36 ft.), where there is little or no light and
the temperature is cooler, P. rubescens are at their greatest density. So, blooms, if they occur, do so

Te mp (F)
Light (%I)

40
1

50 60 70 80

10 100

Light

12

P. Rubescens
density

36

60

84

Te mperature

Low High

P. Rubescens density

FIGURE 15.17 Illustration of the summer distribution of light, temperature, and the Cyanobacteria in a
Missouri lake. (From Sappington, A., Red algae, The Water Line , 13, 1-2, 2009.)