RELATIONSHIP OK PIT YTOI'LANKTON TO LIMNOLOGY 



71 



usually accompany temperature changes, 

 but not always. 



For instance, some studies show that phy- 

 toplankton may increase in amount during 

 the periods of longer daylight, although, for 

 a time at least, the temperature may be 

 falling off rather than rising (Atkins 1924). 

 However, rises in temperature accompany 

 excessive growths of many species of Myxo- 

 phyceae and certain Heterokontae, such as 

 Botryococcns Braunii, and temperature 

 seems to be the important factor in the 

 development of ' ' waterblooms. ' ' 



We have the instance of a lake which has 

 a phytoplankton in the winter only, while 

 zooplankton is dominant during the warm, 

 summer period (Griffiths 1936). 



An interesting correlation exists between 

 phytoplankton pulses and lunar cycles. 

 From various observations we may cite 

 those of Allen (1920) who found a rise of 

 chlorophyll-bearing organisms at the first 

 quarter, with a climax at the time of full 

 moon, seven days, later, after which there 

 was a rapid decline in numbers. The decline 

 in this case was explained by the unusual 

 abundance of Entomostraca, presumably 

 feeding on the phytoplankton. 



It is not out of order here to remind that 

 the influence of temperature on plankton 

 distribution, as well as that of other factors, 

 is manifested in the response of species. 

 That is, the results take specific expression. 

 This cannot be overemphasized, and to dis- 

 cuss the effects of environmental factors in 

 this or that group of organisms, or this or 

 that genus, leads only to meaningless gen- 

 eralities. It is necessary to recognize that 

 ecological and limnological studies of phyto- 

 plankton should be based upon species adap- 

 tations only. Much of our literature is not 

 as helpful as it might be on account of the 

 failure to reduce phytoplankton ecology to a 

 study of specks. 



Currents 



There is space only for the most general 

 remarks concerning the effects of currents 

 on phytoplankton. When lake plankters are 

 carried awaj- by streams there are marked 

 quantitative and qualitative changes. These 



changes, involving a decrease in total 

 amount and numbers of plankton species, 

 are in keeping with clianges in chemistry 

 and gas content of the water. 



The increased amount of sediment, the 

 screening-out action of larger vegetation, 

 and dilution of vital salts by tributaries, 

 all function physically and chemically in re- 

 ducing the plankton or modifying its com- 

 position. Periodicity, seasonal pulses or 

 maxima, and dominant species present en- 

 tirely different pictures than they do in 

 lakes. 



Furthermore, the degree to which phyto- 

 plankton is altered in streams depends 

 largely upon the basic chemical nature of 

 the stream and the place in the stream 

 (Chandler 1937). It has been shown that 

 the quantity of phytoplankton is inversely 

 proportional to the slope of the river (Eddy 

 1931). Pliytoplankton changes near the 

 headwater of a stream seem to be due en- 

 tirely to sedimentation and the screening- 

 out action of larger vegetation. Further 

 down stream even more marked changes are 

 caused by variations in the relative amounts 

 of dissolved nutrients. At the same time, 

 water which was originally low in nutrients 

 at or near the source, might come to sup- 

 port a greater phytoplankton population as 

 it "ages" down stream, with the building 

 \\Y> of necessary concentrations of nutrients, 

 particularly nitrogen compounds. 



Regarding currents in another sense, i.e., 

 currents set up within a lake, we find many 

 interesting relationships. The availability 

 of the nutrients on the bottom to the bulk 

 of water in the lake is determined by two 

 factors: depth, that is, the bulk of water 

 above the bottom and not in contact with 

 it, and vertical currents. To these might 

 be added a third factor, i.e., that which has 

 to do with the possible distribution of bot- 

 tom nutrients by rooted vegetation, to be 

 mentioned below. 



Where depth is such as to provide for a 

 seasonal overturn of water, the plankton in 

 the upper levels will obviously profit by 

 the distribution of dissolved materials that 

 have been formed by the decay of organic 

 matter on the bottom. 



