THE ENVIRONMENT 



319 



of geomorphic stage, then on the basis of life fornns 

 within the lake, and finally, on the basis of benthos 

 subdivision into developmental stages from open 

 water to shore. 



The variety of successional habitats, according to 

 their progress in succession, are submerged stage, 

 floating-leaved stage, emergent stage, saturated soil stage, 

 moist soil stage, and climax. This classification of hy- 

 drophytes is slightly different from that to be given 

 later for water habitat, or hydrarch, succession 

 where the general similarities between kinds of suc- 

 cession are stressed. In further discussion the satu- 

 rated soil and moist soil stages are not emphasized. 



Impure Water Habifafs. Impure water habitats 

 show a great deal of local variation and little re- 

 lationship to one another. Dystrophic lakes have many 

 successional habitats that might be termed open water, 

 moss stage, peat bog, bog swamp, and bog forest stage. 

 Volcanic lakes, owing to the diversity of their waters, 

 can have many habitats. These habitats can be 

 named according to the appearance of the plants or 

 any other convenient aspect of the waters. Alkali 

 lakes may have little or no life in the waters. How- 

 ever, the margin regularly has an alkali marsh and 

 adjacent land usually has scrub vegetation in the 

 form of an alkali sink community. Saline lakes may 

 contain little more life than brine shrimp (Anostraca). 

 However, the margins generally have the same sort of 

 communities as those found around alkali lakes. 



MOVING WATERS 



Most of the factors operating in standing water are 

 also important in running water. However, tempera- 

 ture layering is insignificant in most streams. Of 

 greater importance is the rate of water flow; many 

 animal adaptations are directly related to this. 



Stream classification is based upon geological age. 

 However, within young, mature, and old streams is a 

 diversity of habitats. Unfortunately these habitats 

 cannot be segrated in any simple, meaningful man- 

 ner. This is the case because a multitude of ecologi- 

 cal factors produce complex environments that are 

 not easily categorized. For example, falls, rapids, 

 and plunging pools of one young stream might pos- 

 sess entirely different organisms, however in another 

 young stream these situations might display no such 

 distinction. 



This does not mean that stream habitats defy 

 segregation. One can study physical factors of areas 



having unique organisms and seek environmental 

 correlations. Perhaps the habitats can be named 

 according to these unique situations. In addition, 

 quieter waters of streams normally display the water- 

 to-land sequence of stages that are found in stand- 

 ing waters. As in standing waters, these stages of 

 succession are unstable and developmental (serai), 

 each being replaced by another stage until the stable 

 climax is reached. 



FRESH-WATER COMMUNITIES 



Oligofrophic Lakes. These young, "little-produc- 

 ing," clear water lakes have a poor definition of suc- 

 cessional stages. For practical purposes, any clear 

 water lake having neither clearly defined open water; 

 submerged, floating-leaved, and emergent stages; nor 

 life in the form of neuston, plankton, nekton, 

 periphyton, and benthos is an oligotrophic lake. 

 However, such lakes may have well-developed 

 saturated soil and moist soil stages as described 

 under eutrophic lakes. 



Eutrophic Lakes. These "good-producing" lakes 

 are geologically mature and have a great diversity of 

 life. Also, zonation of life is clearly marked, the 

 stages being indicative of the hydrarch succession 

 that will eventually fill these bodies of water. From 

 first (at or near deepest water) to last (a land habi- 

 tat), the stages are (Figure 17.13): 



Open Water Stage. In open water there are many 

 plants and animals, occurring as neuston, plankton, 

 nekton, and benthos. A fifth category, periphyton, 

 can also be recognized. 



■Submerged Stage. This community may be in- 

 fluenced by the presence of neuston, plankton, nek- 

 ton, and benthos similar to those in the open water 

 stage. However, most typical of the submerged-stage 

 organisms are submerged anchored hydrophytes, 

 plants with thin, dissected, and/or linear leaves. 

 Typical plants of this habitat are pondweeds (Pota- 

 mogeton) and stonewort (Chara). The maximum depth 

 of this stage is limited by sunlight penetration and 

 the minimum depth is limited by invasion of the next 

 stage. 



Floating-leaved Stage. Although organisms in- 

 habiting the four microhabitats are present, most 

 characteristic here are the floating-leaved anchored 

 hydrophytes. These plants often have leaves of two 

 types. Their underwater leaves are similar to those 

 of submerged anchored hydrophytes but the floating 



