COMMUNITY ORGANIZATION: METABOLISM 



505 



with a much greater ratio of silt and clay, 

 whereas those of northern Wisconsin had 

 bottoms with sand predominating. Conse- 

 quently, type of bottom partially controls 

 the amount of rooted hydrophytes and in- 

 directly the anabohsm of plant carbohy- 

 drate of the community. Also involved in 

 this general problem are numerous other 

 influences; for example, the "hardness" and 

 "softness" of water. It follows that the phys- 

 ical environment is an important Umiting 

 factor in total community metabolism, just 

 as it is in the metabohsm of the compo- 

 nent organisms. 



Lake community productivity has been 

 correlated with type of lake by Prescott 

 (1939). Ohgotrophic lakes, with sufficient 

 dissolved oxygen at all depths during sum- 

 mer and winter stagnation, have the 

 amount of phytoplankton, and attached hy- 

 drophytes of shore and bottom, relatively 

 reduced; autotrophic lakes, with little or 

 no dissolved oxygen in the hypolimnion 

 during summer stagnation, have a relatively 

 high yield of phytoplankton and attached 

 hydrophytes. In such autotrophic communi- 

 ties the yield of the rooted vegetation is 

 as much as 882 kg. per square meter for 

 52 per cent of the floor stratum. Conse- 

 quently oxygen supply, as well as floor 

 materials, influences the productivity in 

 aquatic, as well as in terrestrial, communi- 

 ties. 



The stream community diflFers physically 

 and biologically from the lake community. 

 Its phytoplankton, investigated by Tiffany 

 (1938), and other major aspects deserve 

 summarizing here. The great variation in 

 rate of flow over the course of a stream sys- 

 tem, from imperceptible movement in 

 ponded portions to turbulent rapids, ac- 

 companied by radical changes in character 

 of bottom, turbidity, and dissolved gases, 

 creates many habitat types in a relatively 

 short distance. Stream algae frequently are 

 adjusted to current. Many have holdfast 

 adjustments. These are found in Lemanea, 

 growing in waterfalls, and CladopJiora, 

 growing on submerged stones. 



Diatoms are plentiful and multiply as 

 they are carried downstream. Generally the 

 slower the current, the more numerous are 

 these free-floating individuals. Sluggish 

 streams may develop a "water bloom" of 

 diatoms, euglenoids, and blue-green algae. 



Streams differ taxonomically from lakes 



in their photosynthetic species, and such 

 fluviatile communities generally have a 

 smaller standing crop of phytoplankters 

 per unit of surface. 



Since streams lack a thermocline, there 

 is no summer stagnation, and when tur- 

 bidity does not interfere with photosyn- 

 thesis, stream algae multiply rapidly. This 

 is possible as rivers usually contain abun- 

 dant nitrates, so that the biotic potential 

 is high. 



Diatoms in rivers appear to be greatly 

 influenced by floods. High vernal peaks in 

 the stream diatom population usually fol- 

 low spring floods when the water is rich 

 in organic materials, nitrates, and sihcates. 

 This correlates well with the vernal pulse 

 of temperate lakes and seas. 



More annual studies of lake and stream 

 total plankton and rooted vegetation are 

 greatly needed to evaluate energy input 

 and productivity. Few direct answers are 

 available. An indirect answer is found in 

 discussions of the biological efficiencies of 

 the several trophic levels of the community 

 (p. 509), and a partial answer is available 

 in the result of such biochemical activity, 

 that is the weight of plant protoplasm pro- 

 duced per unit area, or plant biomass (p. 

 525). 



Photosynthetic efficiency is not great 

 in natural communities. It ranges from 0.1 

 to 0.4 per cent in lakes, and in artificially 

 maintained plantings of field corn it is as 

 high as 1.6 per cent (Table 43). This dif- 

 ferential, incidentally, is an interesting 

 datum with respect to man's eflFect upon 

 other organisms and communities. 



Manning and Juday (1941) have arrived 

 at an approximate photosynthetic produc- 

 tivity for seven lakes in northeastern Wis- 

 consin. Their results are in terms of the 

 production of glucose, using a clear day in 

 August as a basis of calculation. The high- 

 est production was 44 kg. of glucose per 

 hectare per day (ScaflFold Lake); the low- 

 est production was 14 kg. per hectare per 

 day (Helmet Lake). 



Much needs to be done on tropical lakes 

 and streams in general, and in regard to 

 bacterial and photosynthetic industries in 

 particular. 



Chlorophyll physiology for terrestrial 

 plants has been studied intensively. It is 

 known that photosynthesis takes place most 

 efficiently at either end of the visible spec- 



