Hydrogen-ion concentration 



LAKE BIOCIES 



The acidity or alkalinity of water depends on 

 the ratio between the H+ (or hydronium, H3O + ) 

 and OH- ions. The amount of acidity or alkalinity 

 is commonly expressed in terms of potential hydro- 

 gen ions in a pH scale. The values on this scale rep- 

 resent the logarithm of the reciprocal of the normal- 

 ity of free hydrogn ions. When the number of H + 

 ions is equal to the number of OH" ions, the pH 

 value is 7, the value which represents absolute neu- 

 trality. All pH values less than 7 indicate a greater 

 number of H+ ions than OH^ ions, which is to say 

 the closer the pH value approaches 0, the more acid 

 the water. Above pH 7, there is a preponderance of 

 OH~ ions; the higher the pH value, up to 14, the 

 more alkaline is the water. 



The hydrogen-ion concentration of most unpol- 

 luted lakes and streams is normally between pH 6.0 

 and 9.0, but extreme values of pH 1.7 and pH 12.0 

 occasionally occur (Hutchinson 1957). In some 

 bodies of water, the pH value fluctuates consider- 

 ably. Hydrogen-ion concentration increases (low pH 

 values) with active decomposition of organic matter. 



In general, aquatic animals can tolerate great 

 changes in pH, although the range of toleration varies 

 between species. Mollusks are not ordinarily found 

 in acid lakes, but some snails can survive pH as low 

 as 6, and the fingernail clam Pisiduim down to pH 

 5.7. At the lower pH values, the shells of mollusks 

 become thin, fragile, and transparent, but it is be- 

 lieved that the cuticular covering is partially protec- 

 tive and prevents complete dissolution of the calcium 

 carbonate by the acid (Jewell and Brown 1929). In 

 Campeloma snails, the apex of the shell may com- 

 pletely dissolve, exposing the apex of the visceral 

 mass. Most fish can tolerate pH 4.5 to 9.5 provided 

 there is plenty of oxygen (Brown and Jewell 1926, 

 Wiebe 1931), and many invertebrates will tolerate 

 even greater extremes. Fish as individuals become 

 acclimated to certain pH values, and will select those 

 values when given choice in a gradient. Such ac- 

 climation of individuals may have an effect on their 

 choice of natural habitats, although when forced into 

 a habitat with a different hydrogen-ion concentration, 

 they change their acclimation. Although the direct 

 ecological importance of differences in hydrogen-ion 

 concentrations is doubtful, the measurement of pH 

 may serve as an index of other environmental con- 

 ditions, such as the amount of available carbon di- 

 oxide (with which it varies inversely), dissolved 

 oxygen (with which it varies directly), dissolved 

 salts, etc. Sometimes the difference in species of 

 plankton found in bodies of water with permanently 

 different pH values, for example in granite and lime- 

 stone, is very striking (Reed and Klugh 1924). 



If we reserve ponds and peat bogs to sep- 

 arate consideration, there remain two major lake 

 communities. They differ in species composition, 

 abundance of organisms, distribution of niches, pro- 

 ductivity, and physical characteristics. Inasmuch as 

 these two communities correspond fairly well to the 

 oligotrophic and eutrophic types of lakes, we may 

 name them simply the oligotrophic and eutrophic lake 

 biocies. Various facies of each community, or inter- 

 mediate types (Deevey 1941) are affected by varia- 

 tions in the abundance of component species and 

 correspond to differences in temperature, depth, fer- 

 tility, and other features of the habitat. The com- 

 munities that occur in dystrophic lakes ; for instance, 

 are an impoverished facies of the eutrophic lake 

 biocies. In spite of taxonomic differences in constitu- 

 ent species, each lake biocies contains organisms be- 

 longing to the same life-forms and with similar mores 

 so they may be discussed together. 



Depending largely on their morphological adap- 

 tations and behavior, aquatic organisms are, for con- 

 venience, divided into plankton, neuston, nekton, and 

 benthos, although the differences between the groups 

 are not precise. Seston is a collective term that in- 

 cludes all small particulate matter, both living and 

 non-living, that floats or swims in the water. Plank- 

 ton are free-floating or barely motile organisms, 

 either plant (phytoplankton) or animal (sooplank- 

 ton), that are readily transported by water currents. 

 Most plankton are microscopically small, although 

 some forms are visible to the unaided eye. Species 

 that can be caught with a net are called net plankton 

 to distinguish them from the minute varieties that 

 pass through No. 20 silk bolting cloth meshes. The 

 latter include most protozoan, bacterial, and fungal 

 forms, collectively called nannoplankton. Organisms 

 that depend on the surface film for a substratum are 

 called neuston and are more important in the quiet 

 waters of ponds than in lakes. Nekton are larger 

 animals that are capable of locomotion independent 

 of water currents. Aquatic birds that swim and dive 

 are included in this group. Benthos organisms are 

 attached to or dependent on the bottom for support ; 

 there are sessile, creeping, and burrowing forms. 



PLANKTON 



Fresh-water plankton (Welch 1952, Pennak 

 1946, Davis 1955) includes representatives from 

 the photosynthetic algae, Bacillariaceae (diatoms), I 



Myxophyceae (blue-green), and Chlorophyceae 

 (green), and occasional other form such as Wolffia 

 among the higher plants ; the non-photosynthetic bac- 



66 Habitats, communities, succession 



