52 



Nature of Aquatic Environment 



Waters that are rich in calcium salts, especially in 

 calcium carbonate, maintain, as a rule, a more abundant 

 life than do other waters. Especially favorable are 

 they t« > the gr< >wth of those organisms which use much 

 lime for the building of their hard parts, as molluscs, 

 Stoneworts, etc. There are, however, individual pref- 

 erences in many of the larger groups. The crustaceans 

 for example, prefer, as a rule, calcium rich waters, but 

 one of them, the curious entomostracan, Holopedium 

 gibberum, (Fig. 10) is usually found in 

 calcium poor waters, in lakes in the 

 Rocky Mountains and in the Adiron- 

 < lacks, in waters that flow off 

 archaean rocks or out of silic- 

 e< ais sands. The desmids 

 with few exceptions are more 

 abundant in calcium poor 

 waters. The elegant genus 

 Mierasterias is at Ithaca espec- 

 ially abundant in the peat- 

 stained calcium-poor waters 

 of sphagnum bogs. 



Other minerals in the water — The small quantities of 

 other mineral substances required for plant growth are 

 furnished mainly by a few sulphates, phosphates and 

 chlorids: sulphates of sodium, potassium, calcium 

 and magnesium; phosphates of iron, aluminum, cal- 

 cium and magnesium, and chlorids of sodium, potas- 

 sium, calcium and magnesium. Aluminum alone of 

 the elements composing the above named compounds, 

 is not always requisite for growth, although it is very 

 often present. Silica, likewise, is of wide distribution, 

 and occurs in the w r ater in considerable amounts, and 

 is used by many organisms in the growth of their hard 

 parts. As the stoneworts use lime for their growth, 

 some 4 r ; of the dry weight of Chara being CaO, so 



f 



Fig. io. A gelatinous-coated mi- 

 crocrustacean, Holopedium gib- 

 ber um, often found in waters 

 that are poor in calcium. 



