204 



ANALYSIS OF THE ENVIRONMENT 



with nearly neutral or alkaline water and 

 usually with 12 or more parts per million 

 (ppm) of bound carbonates. Two of the 

 species of Anodonta range into water with 

 as little as 3.2 ppm, and one of these lives 

 in even softer water. The Httle fingernail 

 clam Pisidium can Uve in lakes that have a 

 pH of 5.8 and as Httle as 1.5 ppm of bound 

 carbonates; shells of bivalves are thin and 

 may be quite flexible in such soft waters. A 

 few species of snails, both gilled and pul- 

 monate, thrive in exceedingly soft waters, 

 and the gilled snail Campeloma builds a 

 fairly large thick shell in lakes with a pH 

 of 5,8 and fixed carbonates of as Httle as 

 1.1 ppm. Tliis pH approaches the degree of 

 acidity at which it is theoretically impossi- 

 ble for animals to deposit lime shells (Jewell 

 and Brown, 1929), and it is puzzHng that 

 moUusks can withdraw sufficient calcium 

 from such Hme-poor, acid waters to build 

 their shells. It may be that they Hve in 

 habitat niches where conditions are more 

 favorable and so escape the full rigors of 

 the generalized habitat. The Wisconsin 

 lakes that are intermediate in hardness (10 

 to 20 ppm of bound carbonates) harbor the 

 greatest number of species, and the lakes 

 with the hardest waters (over 30 ppm), in 

 the region studied have the greatest abun- 

 dance of individuals (see also p. 341). 



SILICON, COPPER, AND OTHER 

 ELEMENTS 



SiHcon occurs in fresh water and in the 

 ocean in some form of soluble siHcate. It 

 may be present in colloidal form, especial- 

 ly in river waters. The silicon content of 

 lakes may be a limiting factor in the 

 growth and distribution of fresh-water 

 sponges, Spongillidae. Some species, like 

 Spongilla ingloviformis, are restricted to 

 waters low in mineral content, but are 

 largely indiflFerent to the amount of silicon 

 present. Others, like Ephydatia everetti, are 

 restricted to waters low in silicon as well as 

 in general mineral content. Others require 

 waters fairly rich in minerals; Ephydatia 

 mulleri is an example. Some species of 

 sponges can live in a wide range of mineral 

 content, although their skeletal develop- 

 ment may be much aflFected by the quan- 

 tity of available silicon. Jewell (1935) re- 

 ports: 



"In waters of Si02 content below 0.4 mgms. 

 per liter and of low conductivity and solids. 



Spongilla lacustris shows a progressive attenua- 

 tion of its spicules, eventually losing its micro- 

 spinal spicules (an important species char- 

 acter). These skeleton-poor forms appear no 

 less vigorous and thrifty than heavy-spiculed 

 specimens from more highly mineralized 

 waters. Similarly Tubella pennsylvanica shows 

 marked variations correlated with the degree of 

 mineralization of die water. These entirely 

 normal variations, in some cases, abrogate ac- 

 cepted generic criteria." 



Silicates of lakes and ocean have 

 been suggested as one of the limiting fac- 

 tors in the growth of marine and fresh- 

 water diatoms and of other organisms that 

 have a siliceous skeleton. SiHcates are pres- 

 ent in amounts ranging from a spring 

 maximum in the surface waters of the Eng- 

 Hsh Channel of 60 to 70 gm,/M' siHcate- 

 siHcon to a summer minimum of 12 

 gm./M^ There are larger amounts in the 

 unHghted depths. Harvey (1928) did not 

 believe that siHcon acted as a limiting fac- 

 tor in the sea in any of the cases observed 

 to that date. Sverdrup, Johnson, and Flem- 

 ing (1942, p. 769) comment that areas rich 

 in siHcon show a degree of utiHzation of 

 tins element by diatoms that exceeds the 

 total supply available in areas poor in siH- 

 con. 



In lakes, siHcon is present in epiHmnial 

 waters in quantities that are usually less 

 than 10 mg./L. Ricker (1937) determined 

 that siHca at 6 mg./L. per Hter in Cultus 

 Lake was about twenty-five times the 

 amount required by resident diatoms at 

 their maximum abundance. The indications 

 are that lake siHcon is not a limiting factor 

 for diatom populations either in vernal or 

 autumnal peaks of density. Since siHcon, 

 Hke so many other environmental factors in 

 lakes, is usually stratified, its gradient of 

 abundance may have indirect eflFects that 

 are not now apparent. 



Iron acts at times as a Hmiting factor 

 (Gran, 1931), as does also manganese 

 (Harvey, 1939). 



In addition to its effect upon oxidation- 

 reduction potentials (p. 196), iron is 

 thought to act as a catalyst in chlorophyll 

 formation and to be involved in respiratory 

 activities, as well as being a possible limit- 

 ing factor in phytoplankton growth. Iron is 

 usually stratified in lakes with a thermo- 

 cline; it is less than 2.0 mg./L. in the epi- 

 Hmnion and usually increases in the hypo- 

 limnion. Manganese may or may not serve 



