70 



PROBLEMS OF LAKE BIOLOGY 



nitrog'en and phosphorus in respect to the 

 effects of the substances on plankton pro- 

 duction. 



At the same time, it appears from these 

 and other studies that the maximum require- 

 ments of phytoplankton for phosphorus are 

 very minute. 



Culture studies also, as well as correlations 

 resulting from field work, support the belief 

 that phosphorus is of critical importance. 

 Its abundance in water containing rich 

 plankton growths, particularly in the Arctic, 

 and its seasonal depletion with the increase 

 in both marine and fresh- water plankton, 

 have been established many times. Atkins 

 (1923) found that tropical waters in which 

 phytoplankton was poorly represented, were 

 low in phosphorus, even in the winter. 

 Arctic waters, however, with a high phos- 

 phorus content, sustain a ph^^toplankton 

 flora even greater in amount than is found 

 in the temperate zone in sunnner. 



I have found positive correlations between 

 phosphorus content and productivity of 

 plankton in Iowa lakes (Prescott 1931) and 

 there is some evidence that the difference 

 between one eutrophic lake and another in 

 respect to its flora might be due to the dif- 

 ference in phosphate content. For example, 

 East Okoboji Lake, a shallow eutrophic 

 type, was found to have an average dis- 

 solved phosphorus content of 0.031 ppm, 

 even when tlie Myxophyceae population Avas 

 enormous. At the same time Spirit Lake, 

 another eutrophic body of water, had an 

 average phosphorus content of 0.0055 ppm. 

 Ph.ytoplankton quantity was much less by 

 hundreds of thousands of individuals per 

 liter in the latter lake. Lake West Okoboji, 

 with some oligotrophic characteristics, had 

 an average phosphorus-content of 0.008 ppm. 

 Here the phytoplankton production was 

 comparatively low and it was predominantly 

 Chlorophycean at most seasons. 



Nitrates 



The well-known demand for nitrates by 

 phytoplankton, particularly by many of the 

 Myxophyceae species, helps to explain the 

 successful support of these plants by 

 eutrophic lakes. Bodies of water, well sup- 



plied with nitrates, or sources of nitrates, 

 have enormous phytoplankton productivity. 

 In fact, the connection is so strong that ex- 

 cessive growths of certain Myxophyceae 

 species such as Aphanizomenon flos-aquae or 

 Anahaena circinalis, may be used as indica- 

 tors of the presence of organic wastes and 

 high nitrogen content. The turning of a 

 clear, virgin lake into a cesspool of decaying 

 vegetation is repeated over and over where 

 bodies of water come under the influence of 

 human habitation and the cultivation of 

 land. 



I have found direct correlation between 

 nitrogen and plankton quantity; and my 

 own culture studies and those of many others 

 all establish nitrogen as an important deter- 

 miner in phytoplankton distribution and 

 production. As previously mentioned, 

 oligotrophic lakes are low or are lacking in 

 nitrogen and it is of some significance that 

 desmids dominate the phytoplankton in such 

 nitrogen-poor lakes. The lake of this type 

 Avith a desmid flora is frequently found to 

 lie over ancient geological formations of 

 hard rock and to possess a bottom sediment 

 low in organic content ; hence it is one which 

 is j)oor in nitrogen (Fig. 1). 



Light and Temperature 



Light is obviously a very critical factor 

 in phytoplankton production and one Avhich 

 is modified crucially by suspended matter, 

 color, depth, and latitude. Algal periodic- 

 ity is of course greatly effected by latitude 

 because of the attending lengths of daylight 

 periods. Productivity is, however, not 

 diminished in northern latitudes for there 

 is an abundance of phytoplankton in certain 

 arctic and subarctic lakes. 



To Avhat extent temperature and other 

 factors attending latitude effect quality of 

 phytoplankton aa'C are perhaps unable to 

 state at present. Many species are, hoAV- 

 ever, knoAA'n only from arctic regions. 

 While temperature is a Avell-knoAvn factor 

 in determining not only distribution but 

 periodicity, Ave discover that increases or 

 decreases in plankton sometimes attributed 

 to temperature changes are actually due to 

 light fluctuations or A^ariations, AA'hich 



