SUMMARY 



Important factors in aquatic habitats are 

 pressure, density, light, current, temperature, oxygen, 

 carbon-dioxide and other gases, dissolved solids, and 

 hydrogen-ion concentration. Of special importance in 

 many lakes is the occurrence of a thermocline that 

 divides the water into an epilimnion and a hy- 

 polimnion. The hypolimnion retains a low tempera- 

 ture throughout the year, and in some lakes becomes 

 deficient in oxygen in late summer. These differences 

 in temperature and oxygen greatly affect local and 

 seasonal occurrence of organisms. Lakes are classified 

 several ways on the basis of physical characteristics ; 

 biologically, only two distinct communities, the oligo- 

 trophic and eutrophic lake biocies, are distinguish- 

 able. 



The life-forms of lake organisms are chiefly plank- 

 ton, benthos, and nekton. Zooplankton exhibits diel 

 movements to greater depths in the daytime and gen- 

 eral dispersal, including movements toward the sur- 

 face, at night. Peak populations are commonly 

 reached in late spring, and again in autumn ; low 



points occur in summer and winter. Benthos de- 

 creases in abundance from the littoral to the profundal 

 zone. Profundal animals in eutrophic lakes are ad- 

 justed in various ways to survive the low oxygen 

 late summer stagnation period. Nekton includes 

 aquatic birds as well as fish. 



The lake is a closely knit ecosystem, largely inde- 

 pendent of the rest of the world except for its solar 

 energy, inflowing water, and mineral salts. The base 

 of food-chains is composed of detritus, bacteria, and 

 phytoplankton, then zooplankton and small benthic 

 organisms, and finally fish and birds. All dead or- 

 ganisms, as well as their excreta during life, decom- 

 pose so that their nutrient substances start the food- 

 cycle over again. The biomass and productivity of 

 the three life-forms usually rank, from high to low : 

 plankton, benthos, and nekton. 



The life-cycle and behavior of lake organisms are 

 closely adjusted to the various environmental situa- 

 tions available. Control or management of fish pro- 

 duction by man is difficult, except in lakes of small 



78 Habitats, communities, succession 



