Iron and Poison Substances . 



Iron is found as an accoapanying phenomenon in "sour" waters, especially in sour 

 springs of a pH rate below 7. For this reason, the presence of iron denotes a poorly- 

 productive water, although some iron is needed for the grcfwth of plants. As soon as the 

 water becomes alkaline and contains sufficient oxygen, the Iron is precipitated as a red 

 iron hydrcfxide mud. One can therefore get rid of it through airing of the water and 

 introduction of chalk. 



Fi-om the amount of ferric mud at the bottom of the pond, one can draw conclusions 

 as to the iron content of the water. 



Iron is very easily deposited upon the alkaline gills of fishes and also upon the 

 eggs of trout, causing irritation and blocking of the respiratory channels. In other 

 words, it becomes occasionally quite noxious. It causes necrotic ^ite spots upon the 

 gills of trout fry. Hence, it is important that waters feeding trout ponds and hatcher- 

 ies be kept free — or freed — from larger deposits of iron. 



With regard to the presence of natural poisons in the watery we refer once again to 

 the elimination of Limnaea peregra . 



EbHling has shown that Thuja oil is leached out of the needles of arborvitae trees, 

 causing cramps in fish and often killing them. Arborvitae should therefore not be toler- 

 ated in the neighbourhood of ponds, 



Saponines, which are present in many plants (in barse chestnut trees, for instance) 

 kill fish in even so small amounts as 5 milligrams per liter (0,5 to 1 mg. of horse 

 chestnut), according to Schuring. 1925 and Ebeling. 1931, 



■i7aste waters can bring many toocic substances into ponds; the most frequent injuries 

 are caused through chlorine and phenols. Such substances cannot be detailed here, but 

 the operator should consult fish biologists and institutes. Spraying of arsenic dust 

 (from aeroplanes) has so far seldom caused death to fish, since the cowtaaoly used arsenic 

 compounds (with the exception of Hercynla Kli, Rli F and VH and calcium arsenlte) are hard 

 to dissolve in water, according to Bandt , They will become deadly to the most sensitive 

 fish only in amounts of 20 mg. AS2O3 per liter. 



The amounts, sprayed by aeroplanes are relatively small and a pond would have to 

 draw its vrater from a wide area, undergoing spraying, in order to bring about a toxic 

 concentration. 



In regard to gases developed in the mud of ponds, only carbon dioxide and hydrogen 

 sulphide are toocic (according to Bandt ). but not methane (marsh g&a). Altogether, gases 

 are rather negligible, and to be found only in poorly kept reservoirs. 



5. The Bottom of the Pond . 



The nature of the pond floor is of no lesser importance — from the productive'4)lological 

 viewpoint — than the water of the p<xid, and stands in closest relation to it. 



The normal bottom of older ponds is distinctly divided into two layers, to wit: The 

 chiefly mineral ground floor, i.e. the original bottom, and the overlaying mass of organic 

 mud, which is the result of metabolic processes within the pond. 



The task of the whole bottom is threefold, from the viewpoint of metaboli^mj 



(1) Qnission of nutritional matter from the underground into the water, 



(2) Fixation and chemical combination of nutritional matter — either produced 

 through metabolic processes or through Introduction from the outside — 

 and Trtiich by and by rejoin the metabolic cycle, 



(3) Offering shelter and food to bottom fauna, especially to mud dwellers. 



58 '-"«» 



