RELATIONSHIP OF PII YTOPLANKTON TO LIMNOLO(JY 



73 



actually diji-ested. Tliis is certainly the 

 case with Artemia — it seems reasonable to 

 conclude that tlie same holds true for <'0]ie- 

 pods, Cladocera, mosquito larvae — and the 

 rest." 



On the other hand, some students, both 

 in Europe and this country, especially since 

 the published work of Putter (1909), con- 

 tend that fish food orji'anisms do not feed 

 directly on pliytoi)lankton, but use, rather, 

 dissolved or,uanic and colloidal substances. 

 Hence, larg-e aquatics as well as land flora 

 surrounding' a lake, contribute to the food 

 of w'ater animals as much as or more than 

 the phytoplankton. From the many studies 

 made in this connection we may cite the 

 work of Clarke and Gillis (1935) for evi- 

 dence. These workers found that the cope- 

 ]iod Cdhniiis survived and molted in larue 

 numbers when cultured in water containing- 

 bacteria and nannoplankton. AYhereas. cul- 

 tures in meml)rane-filtered water died 

 rapidly. Also it was shown that sterile 

 filtered Avater would not support Calanus. 

 Furthermore, it was found that the dis- 

 solved products of decay of oi-ganic matter 

 are not easily assimilated by animals nor 

 even by the bacteria. 



Convincing evidence is presented by vari- 

 ous Avorkers and data obtained liy their 

 laboratory experiments challenge the con- 

 ventioiui] concept of the place of phyto- 

 plankton in the food chain. However, the 

 interesting work of Krogh (1931) points out 

 that some fauna require more food than is 

 possible to be obtained from dissolved or- 

 ganic substances. (3ne study for instance, 

 showed that only 10 mg per liter of organic 

 substances to be present in the Avater with 

 an energy quotient of 40 Cal. It is con- 

 tended that this is far below the amount 

 required to support the micro-fauna con- 

 cerned. Also it has been shown in some 

 other studies that algae do not liberate any 

 carbohydrate material and hence cannot 

 contribute substances to the dissolved food 

 supply (Krogh I.e.). Where synthesized 

 matter has been found in algae cultures 

 there is no proof that it was not derived 

 from dead and decayed plants. 



The experimental work of Krizenecky 



shows that nnissels and tadpoles are able to 

 survive on dissolved organic matter, but 

 only when it is present in concentrations 

 greater than tlie occnn-ence iu nature. 

 However, it is worthy of note Ihat. in the 

 case of nuissels and some other organisms, 

 absorption of dissolved organic matter oc- 

 curs througli the intestines and not the gills. 



Using indirect evidence to support the 

 importance of dissolved organic matter as 

 food, we may turn to the studies of Huff 

 (1923) on Vadnais Lake in Minnesota. 

 Here it was shown that increases in zoo- 

 plankters and phytoplankters occurred 

 quite independently, and that a decrease in 

 the phytoplankton had no effect whatever 

 on the animal population, as would be ex- 

 liected if tlie fauna were dependent on phy- 

 toplankton for food. Huff concludes that 

 while phytojilankton may be important in 

 oxygenating the water or in furnishing food 

 for some zooplankters and bottom animals, 

 the micro-flora, as such, cannot be regarded 

 as ultimately basic in the food chain. 



Conflicting lines of reasoning may de- 

 velop from those studies Avhich indicate 

 that the fauna of a body of water require 

 more food than can be shown to be avail- 

 able from quantitative studies of the plank- 

 ton. HoAvever. in depths of the ocean AA-liere 

 dissolved substances are most abundant 

 there is a zoological desert. (Krogh ct al. 

 1931). This of course suggests the de- 

 pendency of micro-fauna on phytoplank- 

 ton. 



We remember that usually deductive 

 arguments are used to support the notion 

 that phytoplankton is of basic importance. 

 Its value is judged frequently by quantita- 

 tive studies of zooplankters before and after 

 the ]ieak of phytoplankton pulses. Those 

 not in agreement Avith the importance of 

 phytoplankton Avould claim, I suppose, that 

 the increases tliat do occur in the micro- 

 fauna after a phytoplankton pulse are due 

 to the increase in the food in the form of 

 substances created by the decay of phyto- 

 plankton and also by tlie accumulation of 

 organic matter from shore vegetation and 

 surface drainage. 



In connection Avith the food relationships 



