ZOOPLANKTON IN IMETABOLISM OF LAKES 



85 



tious ill tlie salmon run result in, U' in vary- 

 ing' amounts of fertilizer added by the dead 

 salmon. In a year of niiusiially large abun- 

 dance of salmon the duration of the plank- 

 ters' period of abundanee was found to be 

 correspondinjily increased. Younp' (1935) 

 also found the life of Flathead Lake not 

 constant. In 1893 Forbes reported the zoo- 

 plankton as consisting' mainly of Daphuia 

 longispina, Avhile recently it was found that 

 this form was far inferior in numbers to 

 Cyclops and usually to Biapioymis. Forbes 

 noted Epischura as common, while today it 

 is unknown. Epischura lacnstris was also 

 common in Lake Michigan 40 years ago, but 

 is absent now. The remainder of the life 

 of Lake Michigan was found by Eddy 

 (1927) to be about the same. Other lakes 

 in which changes have been noted are Eed 

 Lake near Luzern (Bachman 1931) and 

 Devils Lake, North Dakota. The last two 

 variations were undoubtedly caused by 

 chemical changes in the lake, but in many 

 other cases no such explanation is avail- 

 able. 



An interesting accompaniment of seasonal 

 changes is the change in body form which 

 many claclocerans and some rotifers exhibit. 

 In most cases this cyclomorphosis takes the 

 form of added spines or an increase in 

 length of spines, or helmets, as the sum- 

 mer approaches, but in some rotifers there 

 is a reduction in the length of the projec- 

 tions at this time. The change is compara- 

 tively rapid, usually occurring in tAvo or 

 three weeks and takes place as a eiimulative 

 addition in length in each succeeding gen- 

 eration and not as a process of groAvth in 

 one individual. Various factors have been 

 assigned as the cause of this unique phe- 

 nomenon, the most probable explanation 

 being that it is caused by the combination 

 of change in viscosity of the water and a 

 change in the factors in the environment, 

 such as food relations. 



The present status of our knowledge of 

 zooplankton distribution is well summed up 

 by Wortliington and Ricardo (1936) who 

 state that "Much work on the measurement 

 of controlling factors in the field and ex- 

 perimental work on the behavior of dif- 



ferent oi-ganisms must l)e pei-formed and 

 correlated before a coiii])lete and general 

 explanation can be given of the distribu- 

 tion and movements of plankton." 



Plankton Productivity 



The limnological measurement of the use- 

 fulness of a lake is based upon its produc- 

 tion of plankton, and lakes have been classi- 

 fied by Naumaiin and Thienemaini (1929 

 and 1931) on this basis. Thus we have the 

 highest producers known as eutropliic, the 

 lowest as oligotrophic, and in between meso- 

 trophic. Within the last few yeai's, how- 

 ever, several modifications have been made 

 in this classification and lakes have been 

 found which do not exactly fit into the 

 original scheme. Hutchinson (1938) has 

 suggested that some system of lake typology 

 should be formed which would take into 

 consideration both the trophic conditions 

 in their original sense (Naumann 1929) 

 and factors due to the morphology of the 

 lake basin. These two factors vary inde- 

 pendently. Lundbeck (1934) distinguishes 

 between primary, or edaphic, oligotrophy 

 and secondary, or metamorphic. A shallow^ 

 lake with a poor supply of nutrient mate- 

 rials would be relatively oligotrophic, but 

 the hypolimnion might be so small as to 

 produce an oxygen deficit in the bottom 

 water. Also, in eutrophic regions a deep 

 lake may have such a large hypolimnion 

 that no oxygen deficit may be formed. In 

 secondarily oligotrophic lakes the volume 

 of water is so great that it requires a long 

 time to accumulate enough salts for the lake 

 to become productive. Also, accumulation 

 of sediment may cause high productivity in 

 immature lakes, such as in Green Lake, 

 Wisconsin. In alpine lakes in Europe no 

 amount of reduction of the hypolimnion 

 would cause secondary eutro]ihy due to tlie 

 extreme lack of nutrient salts, and in ex- 

 tremely productive lakes, such as some lakes 

 in the tropics, an enormous hyiiolimnion 

 would be necessary to rediu-e the oxygen 

 deficit appreciably. Thus in the Philippines 

 it was found (Woltereck and Tressler un- 

 published) that no oxygen existed below 10 

 meters in most of those lakes which were 



