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

Environmental Engineering Reference

In-Depth Information

(or not inluenced by) the settling of other particles, in a quiescent luid. So, settling is also inversely

proportional to the deviation of an object from a sphere (form resistance). Think of skydivers. If they

are falling head down with their arms by their sides, frictional (form) resistance is minimized and

their fall velocity is great (about 160-180 mph in the lower atmosphere). If the skydivers fall with

their body perpendicular to the ground and with their arms stretched out, the terminal fall velocity

is substantially decreased (to about 120 mph).

Similarly, phytoplankton may minimize their fall velocity by taking advantage of colder periods

(e.g., spring or fall in contrast to summer), minimizing the density difference between their bodies

and the water, and reducing their size. Since settling decreases nonlinearly with the square of the

radius, the most effective method for reducing settling velocities is to reduce size. A common clas-

siication system by size is illustrated in Table 15.3. So, Cyanobacteria, which include some of the

smallest phytoplankton, may have some competitive advantage over large phytoplankton in some

cases.

The ratio of surface area to volume also increases as size decreases, which may facilitate the

surface exchange of gases and nutrients as well as decrease settling rates. For picoplankton and

nanoplankton, the transfer of material to and from cells is almost entirely by molecular diffusion

(Wetzel 2001). Size may also impact the ease with which the phytoplankton may be eaten, such as

by ish and zooplankton. The microplankton and macroplankton are not as vulnerable to predation

(Wetzel 2001) but they are less eficient at nutrient uptake and more vulnerable to settling.

The size of phytoplankton may also impact the manner and accuracy of their enumeration. Sigee

(2005), for example, suggested that in the past the ecological role of picoplankton and nanoplankton

has been underestimated due to the dificulty of their detection and enumeration using conventional

microscopy.

In addition to their size, phytoplankton can also decrease their rate of settling by varying their

shape. Some of the potential strategies are to form groups or colonies, or to become asymmetrical

where the form resistance decreases with increasing asymmetry.

In addition, many phytoplankton may actively inluence settling velocities. Wetzel (2001), for

example, indicated that microphytoplankton (Table 15.3) may be separated into two broad types of

ecological strategies, those that are motile and those that are not motile. The motile forms include

those with gas vacuoles, such as some Cyanobacteria. By regulating their gas vacuoles, these algae

may become negatively buoyant and rise through the water column. Flagella may also be used

for motility. The nonmotile forms may depend on the turbulence mixing of the water column to

maintain their position within the photic zone. Other strategies include the accumulation of fats, the

reduction of density by altering the cellular ion content, and the production of gelatinous sheaths

TABLE 15.3

Size of Phytoplankton

Linear Size (Cell or

Colony Diameter, λ m)