118 



PROBLEMS OF LAKE BIOLOGY 



cies of the fifth type are like the fourth in 

 their distribution. 



It appears then that tlie habitat is re- 

 fleeted in the vegetation and an under- 

 standing of the relationships which exist 

 between the lake, the vegetation, and the 

 animal life in the lake is of great value to 

 the applied scientist. Here in the "growth 

 "forms" there appears to exist a rough in- 

 dex to the productivity of a lake. From 

 the above, an inference may be made point- 

 ing to a static nature of the aquatic plant 

 community. When observed more closely, 

 and with reference to the hydrographic 

 age and drainage of the lakes in which these 

 types grow the dynamic quality is very 

 obvious. 



- It will be noted that all five "growth 

 forms" occur in medium hard- water lakes, 

 and reference to Fig. 1 shows that these 

 are either drainage lakes or seepage lakes 

 in youth. This luxuriant vegetation is dia- 

 gramatically shown in A of Fig. 11. All 

 "growth forms" occur here, and in the suc- 

 ceeding diagrams B, C, and D it is indi- 

 cated how, by development, the seepage 

 lakes lose species belonging to types one, 

 four, and five. Types two and three persist 

 longer, but eventually only type three, 

 which has floating leaves and horizontal 

 stems, is found to live in the lakes, as they 

 near old age and extinction. A fuller dis- 

 cussion of these problems of the Wisconsin 

 lakes will be published. 



On page 119 is a diagram of the aquatic 

 succession in Muskellunge Lake in Vilas 

 County, Wisconsin. This lake is t^^pical 

 of the region and might be considered 

 in early maturity and of the intermittent 

 drainage type. The diagram summarizes 

 the relationships which exist among the 

 various aquatic communities, and shows 

 iow they are related to the substratum. 



Productivity Studies 



In 1926 Klugh suggested that the abun- 

 dance of the larger and rooted hydrophytes 

 might be taken as an index for the produc- 

 tivity of lakes. He cited attempts at deter- 

 mining such an index for lakes, and sug- 

 gested that upon investigation the hydro- 



phytes might prove to be tlie simplest and 

 most useful method of determining the pro- 

 ductivity of a lake in terms of fish. 



Manj^ rough observations have been made 

 upon the abundance of certain species of 

 aquatic plants, and upon the entire crop in 

 lakes. These have value in that they allow 

 general statements to be made concerning 

 the relative abundance of the vegetation, 

 but more exact studies should be under- 

 taken. 



The number of published studies that 

 have been made with reference to the 

 weight of the total crop of rooted hydro- 

 phytes are very few and entirely inade- 

 quate for any reliable correlations with the 

 density of the fish population, or with other 

 factors. In this country published studies 

 have been made in only five lakes, all of 

 which are in Wisconsin (Rickett 1922, 

 192-t; Wilson 1935, 1937). In each, sam- 

 ple quadrates were studied along profiles 

 in various habitats. The plants were re- 

 moved from the quadrates and separated 

 into species. Dry weights were determined, 

 and the entire crop in the lake was esti- 

 mated from these samples. 



A comparison of the estimations is most 

 interesting, for it shows how varied the 

 quantity of aquatic plants may be in dif- 

 ferent regions. The work of Rickett was 

 done in Southern Wisconsin and that of 

 the writer, in Northern Wisconsin. 



Eegion 



Total crop 

 (kilograms) 



Average crop 

 per sq m 

 (grams) 



Southern Wisconsin 



Lake Mendota 2,100,000 202 



Green Lake 1,527,900 178 



Northern Wisconsin 



Little John Lake Ill 52 



Muskellunge Lake 882 45 



Silver Lake 17 08 



The explanation for the great difi^erence 

 in the abundance of the aquatic plants in 

 the two regions appears to be the follow- 

 ing: (1) sandy soils predominate in the 

 north, while silt and clay are more abun- 

 dant in the southern lakes, (2) there is 



