318 



ECOLOGICAL FACTORS: 



Spring warming of surface waters to 4°C again 

 causes temperature homogeneity and the spring 

 overturn. 



LAKE CLASSIFICATION 



Large bodies of standing water are classified in 

 many ways. One of the frequently seen, simplified 

 classifications uses artificial vs. natural as the primary 

 criterion of segregation. Artificial lakes have many 

 features that are unique to the individual waters; 

 however, most are poor habitats for organisms, owing 

 to their water level fluctuation and high turbidity. 



The natural lakes are further subdivided into clear 

 water lakes and impure water lakes. The two clear 

 water types are oligotrophic and eutrophic lakes. 

 Oligotrophic lakes (little-producing) usually are deep 

 and contain hypolimnion oxygen at all times. These 

 are geologically young lakes whose materials produce 

 few organisms. However, oligotrophic lakes through 

 succession mature into eutrophic (good-producing) 

 lakes. Eutrophic lakes have conditions that support 

 many organisms. These lakes are usually shallow. 

 However, because their hypolimnions are relatively 

 small (they represent a later stage than an oligo- 

 trophic lake, a stage in both a geomorphic and biotic 

 succession cycle reached after considerable sedi- 

 mentation and resultant filling), summer oxygen de- 

 pletion occurs at the bottom and many organisms die 

 to contribute to the organic richness of these waters 

 (Figure 17.12). 



Impure water lakes are of many types. Their 

 classification is not as logical as that of clear waters, 

 because the impure types normally do not represent a 

 geomorphic or biotic succession sequence. 



The main impure types are dystrophic (bad-produc- 

 ing), volcanic, alkali, and salt. Dystrophic are acid 

 lakes containing a particular chemical compound, 

 humic acid. The primary example is the bog lake. 

 When such lakes become filled with peat moss they 

 are called a peat bog or, sometimes, a moor. Vol- 

 canic lakes are either acidic or alkaline, depending 

 on the nature of the surrounding igneous rocks. 

 Alkali lakes and salt lakes are found mostly in desert 

 areas. The former result from waters draining 

 igneous areas; the latter, from waters draining sedi- 

 mentary areas (Figure 17.12). 



Certain additional information about the kinds of 

 natural lakes is required if each kind is to be ex- 

 amined in detail. 



Clear Wafer Habitats. These habitats fall into two 

 main groupings, life cycle and successional. The life 

 cycle group is related to the geomorphic cycle of a 

 lake. Youth is indicated by the oligotrophic take 

 whose subhabitats are largely limited to the life forms 

 previously mentioned. Early maturity, the eutrophic 

 lake, has greater numbers and diversity of organisms. 

 In addition, benthic aquatic vegetation becomes 

 dense and the substrate mucky or muddy. If this 

 vegetation is strictly herbaceous, the habitat is a 

 marsh; if it contains trees, a swamp. Later geomorphic 

 maturity is first indicated by the pond stage and then 

 the temporary pond stage. Finally, old age occurs 

 when the area dries completely and becomes part of 

 the land. 



Within these geomorphic stages there usually is a 

 shoreward zonation of benthic life into progressively 

 later successional stages. These stages may be con- 

 sidered individual habitats. In actual practice, one 

 should first classify clear water habitats on the basis 



Oligotrophic 



Eutrophic 



Dystrophic 



Vol conic 



communities 

 poorly defined 



communities 

 v\^ell-def ined 



mostly peat moss 

 or sedge 



varioble 

 communities 



Alkcl i or So I ine 



alkali marsh 

 and sink 



Land 



Figure 17.12 Characteristics of natural tresh-water lakes and some indication of their path of 

 development to a land habitat. 



