LAKE DYNAMICS 17 



soluble than the bicarbonates, but in the presence of carbon dioxide 

 or carbonic acid they readily change into the more soluble bicarbon- 

 ates. Plants can utilize some of the carbon dioxide from bicarbonates. 

 In doing so they change the bicarbonates back to the less soluble car- 

 bonates, which sometimes precipitate, thus forming the extensive marl 

 deposits common in northern lakes. Carbonates are so important that 

 the fertility of a lake can often be estimated from the amount present 

 in the water. 



The many important food elements all pass from one stage to an- 

 other, eventually ending in the body of some aquatic plant or animal, 

 which dies and decomposes, liberating these elements back into the 

 water to be used over again. This cycle is a repetition of the one that 

 takes place in the farmer's field when crops are plowed under in order 

 to maintain the fertility of the soil. 



It is interesting to find that the production of food in a lake in actual 

 pounds per acre is far greater in areas where the surrounding soil con- 

 sists of fertile farm lands. Some of the lakes in central Minnesota carry 

 as high as 80 pounds (dry weight) of animal food per acre in the shallow 

 waters. The lowest number of pounds of food per acre is found in the 

 lakes that are in areas of rocky and thin infertile soils. Some of the 

 rocky lakes along the Gunflint Trail carry only two-tenths of a pound 

 (dry weight) of food per acre. This correlation is partly due to the fact 

 that the lake obtains its original supply of fertile elements from the 

 drainage waters of the surrounding areas and relies on this source to 

 replenish the elements it loses through ground seepage or outlets. A 

 lake can produce only as many pounds of fish per acre as there are 

 available agents of fertility and in general is no more fertile than the 

 surrounding soils. 



Equal in importance to the dissolved contents of the water are the 

 types of bottoms present in a lake. Plants and food organisms have 

 distinct preferences as to types of bottoms. It is impossible for most 

 species of plants to grow in any abundance on solid rock bottoms, and 

 similarly few forms of animal life can find subsistence on bare rocks. 

 Shifting sands likewise are too unstable for the existence of plants or 

 food organisms. On the other hand soft muds, though they foster 

 luxuriant plant growth, may smother many animals. The best type of 

 bottom for food production is a relatively firm but fertile bottom, such 

 as one of mud and sand. 



The contour, or shape, of the bottom of a lake may have an im- 

 portant effect on the total amount of food produced. The conditions of 

 warm, fertile lakes usually make only the bottoms above the 20- or 

 30-foot level suitable for food production. In these lakes the maximum 

 production of food is above the 15-foot level, which is the usual limit 

 of sufficient light penetration for most plant growth. Deep, fertile lakes 



