112 



PROBLEMS OF LAKE BIOLOGY 



lowing. The least growth of rooted aquatic 

 plants is upon nearly pure sand, cobbles, 

 and hard rock, Avhile the most prolific 

 growth is upon the soft slimy sedimentary 

 peat, or organic mud, or upon the soft, silty, 

 s^ or clayey inorganic muds. 



The genus Potamogeton is most abundant 

 in calcareous and alkaline waters and on 

 the soft or fluid organic "mud," and on 

 the soft marl. Some species, however, were 

 observed on all types of bottom except in- 

 durated rock. One species occurs in soft or 

 slightly acid water. 



Niimplwzanthns and Castalia appeared 

 to be most abundant in soft, fluid, organic 

 mud. The waters ranged from very soft to 

 hard. 



Scirpus validus, and possibly related spe- 

 cies are common on "hard" bottoms, except 

 bedrock. The waters ranged from acid to 

 alkaline. 



Zizania aqnatica grows in clear water 

 with a range of hardness of 6 to 17 grains 

 per gallon, and a pH of 8.0 to 8.6. The 

 soils were soft, fluid, organic mud, or peat, 

 thin peaty mud over calcareous clay, and 

 clayey inorganic calcareous nuid. 



Sagitfaria and Pontedaria are most abun- 

 dant in shallow alkaline or hard water over 

 the more clayey (inorganic) mud or in 

 peaty sands. A few were recorded on acid 

 peats. 



Typlia hit i folia is most abundant on the 

 more clayey muds and on a thin covering 

 of black alkaline organic soil over clay or 

 calcareous sand and silt. The associated 

 waters were alkaline. A few plants were 

 observed on coarse acid peat. 



Erwcaulon septangidare grows over 

 either peat, peaty sand, or clean non-cal- 

 careous sand. This plant appeared to be 

 a very reliable indicator of soft, and nearly 

 neutral or slightly acid water. 



Pearsall (1920, 1921) and Wilson (1935, 

 1937) have shown that there are important 

 relationships between the sedimentation of 

 soils and the succession of the rooted 

 aquatic plants, but this will be discussed 

 under the heading of dynamic ecology. 



Dynamic Ecology 

 The fact that plant formations are not 



static features has been known a long time. 

 That they evolve along certain trends is 

 observable wherever plants grow. The 

 aquatic plant communities are no exception, 

 but attempts have been made to classify 

 them in such a way that may lead one to 

 suspect that they are static. Pearsall 

 (1918) has discussed the classification of 

 aquatic plant communities and has shown 

 that one must consider them as stages or 

 steps in a single development reaching from 

 the early stages of lake colonization to the 

 closed swamp or moor. These stages go 

 hand in hand with sedimentation. 



The dynamic factors affecting aquatic 

 vegetation in the English lakes have been 

 set forth in papers by Pearsall (1921, 

 1929), and these may be generally applied 

 to the lakes of this country. He states that 

 two sets of factors which govern the de- 

 velopment of aquatic vegetation are (1) 

 those affecting the lake as a whole, and (2) 

 the forces altering the shore. 



Pearsall (1920) found that where silting 

 is rapid the soils are richer in available 

 potash and usually in phosphates and ni- 

 trates. Also they are more abundant in the 

 finer silts. He, therefore, assumes that the 

 variation in the quality and quantity of 

 silts very largely controls the distribution 

 of submerged vegetation. Figs. 2 and 3 

 (Pearsall 1921) show diagramatically the 

 aquatic vegetation associated Avith the 

 quality and quantity of sediments in shal- 

 low and deep water in the English lakes. 



PLLP UMTFR COMMUMITirS 



increosf mobunjancfof^/lt^ accumulation of organic matt? r 



e 



8c; 



2-70' 



Juncus fluiTans^V^^^er TuDf 

 R Pfrfolmrw ► IPnaTons 



I3-J0\ I 60-90 



I 

 I 

 I 

 I 

 I 



^*5 5-20 v'lO-2S \IO-il' .2(5-45- I 



g"f. Naios-^LmMr — ^P.obTuiifolius-^SMrflamum ! 



|"f|Naias-^LmMr -^P.obTuiifolius-^Sparjanium jUfer 

 y • I J is-« „,„,„„„ I II 



r- 20-JO I gjj".^* 



L.sThwaiTf Ty pf 



leaved asiocws 



10-15 



PearsaW '20) 



Pig. 2. Diagram of plant succession among com- 

 munities of plants below the zone of erosion. The 

 figures given indicate the percentage of organic 

 content of the soil. 



