The Limnetic Zone
The open water is a world of minute suspended organisms that are the plankton. Dominant are the phytoplankton and among them the diatoms, the desmids and the filamentous green algae. Because these tiny plants carry on photosynthesis in open water, they are the base of the food chain upon which the rest of the limnetic life depends.
Suspended with the phytoplankton are the zooplankton and animals. These organisms graze upon the minute phytoplankton. They form an important link in the energy flow in the limnetic zone. Some characteristic animals are the rotifers, copepods, and cladocerans.
The rotifers, or so called wheelbearers, have a circlet of moving cillia that look like two rapidly rotating wheels at the front of the body. The cilia pull algae and protozoa into the pharynx where the food is crushed and ground. The crustaceans, cladocerans and copepods filter phytoplankton, bacteria and detritus out of the water by means of a comb like setae on the thoric appendages.
Phytoplankton and zooplankton movements and distributions in the water column are influenced largely by physical forces. Vertical distribution or stratification of plankton organisms is influenced by the physiochemical properties of water, especially temperature, oxygen, light and current.
Light of course sets the lower limit at-which phytoplankton can exist. Since they are unable to determine their own position in the water, these organisms must either float or sink. The ability to float depends upon the specific gravity of the organisms in relation to the specific gravity of the water. If the specific gravity of the organism is the same as the water, it will float. If the specific gravity is greater than the organism, it will sink.
The rate at which a small body sinks varies directly with (1) the weight in excess of equal volume of surrounding water, and (2) the body shape indirectly with viscosity.
Many plankton organisms have adaptations for staying afloat or remaining suspended. Fresh water diatoms and desmids have thin, silicaceous cells which reduce their weight. Some phytoplankton organisms may live in gelatinous envelopes that have about the same specific gravity as water.
Others may have gas vacuoles or oil droplets in their bodies. Many plankton organisms have surfaces set with spines, ridges, horns, or setae or possess an elongated body. All of these body shapes increase the total surface area in relation to weight. However, diffusion currents more than any floating device keep the plankton from sinking.
The currents maintain a mixing action that not only holds the organisms in suspension, but also prevents a persistent stratification of plankton in the epilimnion. Phytoplankton can limit light penetration by its own growth and thus reduce the depth at which it can live.
As the light zone becomes increasingly shallow, the phytoplankton can absorb more light and organic production is increased. But within these limits, the depths at which various species can live are influenced by the optimum conditions for their development.
For instance, some phytoplankton live just beneath the water surface while others are more abundant a few feet beneath and again those requiring colder temperatures live still deeper. Cold water plankton, in fact, are restricted to those lakes in which phytoplankton growth is scarce in the upper region and in which the oxygen content of the deep water is not depleted by decomposition of organic matter. Many of these cold water species never move up through the metalimnion.
Since animal plankton feed on the minute plants, most of the zooplankton are concentrated in the trophogenic zone. Because many of them are capable of independent movement, zooplankton exhibit stratification that often changes seasonally.
In the winter, some zooplankton forms are spread evenly over considerable depths. During the summer, zooplankton concentrate in layers most favorable to them and to their stages of development. During the summer, some animal plankton undertake a vertical migration during some part of the 24 hour period. Depending upon the species, and their stage of development, zooplankton spends the night or day in the deep water or on the bottom and move up to the surface during the alternate period to feed on phytoplankton.
During the spring and fall overturns, plankton can be evenly distributed. In spring, when surface water warms and stratification again develops. Then phytoplankton have access to both nutrients and light.
The fish and some invertebrates are free to a large extent from the action of weak water currents and are capable of moving about at will. In the limnetic zone, fish make up the bulk of the nekton. Their distribution is influenced mostly by food supply, oxygen and temperature. During the summer, large mouth bass, pike and muskyl lunge inhabit the warmer epilimnion waters where food is abundant. In winter they retreat to deeper water.
Lake trout on the other hand move to greater depths as summer advances. During the spring and fall overturn, when oxygen and temperature are fairly uniform throughout both warm water and cold water fish can occupy all levels.