DISSOLVED SALTS AS LIMITING FACTORS 



203 



ing continuously enriched by the death ot 

 many organisms. This results in the seasonal 

 maxinmm. Such annual cycles are basic m 

 thermally stratified lakes and, as we have 

 seen, have their counterpart, but on a much 

 vaster scale, in open ocean (Russell and 

 Yonge, 1928; Park, Allee, and Sheltord, 

 i9oyj. (Attention is directed to the Chap. 

 27, on Community Metabolism, for a fuller 

 integration with the ecology of communi- 

 ties.} The actual limiting influence of nitro- 

 gen and phosphorus on the production of 

 lake plankton is not yet thoroughly under- 

 stood; investigations demonstrate a correla- 

 tion in some cases and none in others. In 

 general, ohgotrophic lakes are low and 

 eutrophic lakes are high in nitrogen con- 

 tent. In the former, the dominant phyto- 

 plankters are desmids; as a rule, such lakes 

 have bottom sediments poor in organic 

 content. This partially explains the small 

 amount of nitrogen present. In eutrophic 

 lakes, phytoplankton is relatively rich in 

 quantity, and diatoms flourish. Frescott 

 (1939) found a direct correlation between 

 nitrogen content and the quantity of plank- 

 ton. He concludes that nitrogen is an impor- 

 tant determiner of both abundance and 

 distribution of phytoplankton and suggests 

 that the nitrogen demand by many blue- 

 green algae is so strong that their presence 

 may be used as an inaex of high nitrogen 

 concentration and organic wastes. 



Opposed, we have the work of Atkins 

 (1926), who found no evidence of correla- 

 tion between nitrogen deficiency and phy- 

 toplankton limitation in Wisconsin lakes. 

 Additional support for either view can be 

 found in the literature. 



To conclude the present discussion, Juday 

 (1942) records that phosphorus and nitro- 

 gen are thought to be limiting factors in 

 fresh waters. The growth of fish populations 

 in European ponds seems to be Hmited by 

 the concentration of these two sets of min- 

 eral nutrients. Although their role in limit- 

 ing the growth of fresh-water populations 

 is not yet fully known, greater emphasis is 

 usually placed on the limiting action of the 

 more dilute phosphorus salts alone than on 

 nitrogen salts alone (Welch, 1935; Ket- 

 chum, 1939). 



CALCIUM AND MAGNESIUM 



Calcium itself, or calcium plus magne- 

 sium, may be a hmiting factor in lakes. The 

 summer standing crop of phytoplankton 



and rooted vegetation in two calcium-poor, 

 soft- water lakes of Wisconsin (0.7 to 2.3 

 mg. Ca/L.) differs decidedly in quantity 

 and in species from that in two hard-water 

 lakes in the southern part of the same state 

 (21.2 to 22.4 mg. Ca/L.). Most of the 

 rooted plants in the soft-water lakes are 

 limited to that type of water. Lakes with 

 hard water were more productive per unit 

 area than were those with soft water. The 

 total plant crop weighs three to five times, 

 and the animal population, excluding fish, 

 two to tliree times, that of the sort-water 

 lakes. As might be expected from these 

 figures, the dissolved organic matter, largely 

 a degradation product, is also much larger 

 in the hard-water lakes (Juday, 1942). 



The amount of bound carbonates gives a 

 good measure of the hardness of water. In 

 fresh waters the carbonates are mainly 

 combined ("bound") with calcium and 

 magnesium. For practical purposes the 

 quantity of calcium may be taken as a 

 measure of the "hardness" of lake water. 

 Ohle (1934) suggested that lakes with 9 

 mg. or less per liter of water are to be re- 

 garded as poor in calcium and may be 

 known as soft-water lakes; those with 10 to 



25 mg. per Hter are inteiTnediate; those wdth 



26 mg. or more are hard-water lakes. In 

 general, soft-water lakes show Httle calcium 

 stratification (Juday, Birge, and Meloche, 

 1938), but a sfight increase occurs in the 

 deep hypoUmnial area; intermediate cal- 

 cium lakes have a decided increase in the 

 hypoliinnial calcium. This is a generalization 

 to which exceptions are known, but its basic 

 application is important. 



Calcium is circulated through thermally 

 stratified lakes during vernal and autumnal 

 overturns. This redistribution is the more 

 needed since this substance enters into inti- 

 mate relations with plants and animals in 

 many ways. Besides being much used in 

 shell formation, it is essential in plant and 

 animal metabolism and helps regulate per- 

 meability to water. Calcium has important 

 general relations, as we have seen, with car- 

 bon dioxide and, through its carbonates, 

 with the H ion concentration. 



The eflFectiveness of hardness of water as 

 a limiting factor in distribution can be 

 tested by its correlations with environmen- 

 tal relations of mollusks. Of the bivalve 

 moUusks, all members of the family Union- 

 idae found in an extensive study of Wis- 

 consin waters were restricted to habitats 



