COMMUNITY ORGANIZATION: METABOLISM 



503 



The diatom response to these two critical 

 factors is rapid and spectacular. 



Summer 



By middle May to early June the diatom 

 population has reached a maximum and be- 

 gins a rapid decline. This decrease con- 

 tinues through the summer into early 

 autumn. This is paradoxical, since the in- 

 tensity of daylight is maximal in the sum- 

 mer (7000 to 10,000 foot-candles at the 

 surface); consequently this season has the 

 highest potential photosynthesis. The rapid 

 decline in diatom abundance is a conse- 

 quence of the delicate balance of influences 

 operating in the photic strata of the com- 

 munity. 



There is a great loss of diatoms through 

 their consumption by herbivores. Second, 

 the weak marine thermocline, now at its 

 strongest, separates the relatively warm 

 photic layer from the cold aphotic layer. 

 This strong temperature differential, with 

 its associated electrical properties, prevents 

 rapid up-welling of nitrates and phosphates 

 by diffusion and reduces mixing by con- 

 vection currents. From the viewpoint of 

 community metabolism, the thermocline 

 acts like a semipermeable membrane sep- 

 arating the upper and lower strata of the 

 community. Were it not for this tempera- 

 ture and density barrier, the diatoms might 

 have suflRcient salts for protein synthesis 

 and, with the favorable light intensity, 

 would continue to increase in numbers. The 

 summer diatom population is several times 

 larger than the winter population, but only 

 one-fifth to one-sixth as large as the spring 

 pulse. 



Autumn 



The surface layer of the photic zone be- 

 gins to cool in correlation with the de- 

 creasing air temperature. This surface cool- 

 ing causes a reduction of the discontinuity 

 layer, which gradually disappears as the 

 upper and lower layers approach each 

 other in temperature. With loss of the ther- 

 mocline, there is an upward diffusion of nu- 

 trient salts. This upwelling is aided by the 

 churning forces of autumnal gales. With 

 sufficient amounts of critical salts for pro- 

 tein synthesis, there is an increase of the 

 diatoms; this increase is not spectacular, 

 since the light intensity is now decreasing, 

 and photosynthesis is reduceo 



This combination of influences causes 

 the autumnal pulse of diatoms. It is about 

 twice the summer density of population but 

 about one-half that of the spring pulse. 

 With onset of winter weather, the diatom 

 population density returns to the minimal 

 condition. 



This general account of diatom produc- 

 tivity again demonstrates the delicate 

 balance between physical and biological 

 factors in a community. It should be re- 

 membered that this basic photosynthetic 

 key industry is related to, and dependent 

 upon, the bacterial key industry. The abun- 

 dance of diatoms is a result of this balance. 

 In turn, these minute phytoplankters, with 

 their allies, and the bacteria are at the base 

 of the food web of the marine community. 



Obviously, photosynthesis in the sea is 

 largely the work of the phytoplankton. At- 

 tached algae and the higher plants of lit- 

 toral areas play a relatively small part in 

 the over-all industry, though perhaps im- 

 portant in the immediate zones occupied 

 by them. 



In fresh-water communities, the role of 

 the macroflora is relatively greater in car- 

 bohydrate anabolism than it is in the 

 marine littoral. The higher plants become 

 progressively more important as ponds and 

 lakes fill up, and the growth of pond weeds 

 and their ecological equivalents restricts 

 the open water where phytoplankton may 

 carry on photosynthesis. 



The green "producers" of fresh-water in- 

 clude, among other forms, the blue-green 

 algae (Myxophyceae), green algae (Chlor- 

 ophyceae), diatoms (Bacillarophyceae), 

 the peridinians (Dinoflagellata), the eugle- 

 nas (Euglenoidina), and the Volvocales. 

 This producer plankton is similar ecologi- 

 cally to its marine counterpart. The bibliog- 

 raphies of Fritsch (1935), Smith (1938), 

 and Tiffany (1938) will open the subject 

 for more intensive study. Rapid vernal mul- 

 tiplication results in "pulses" or "water 

 bloom;" they are dominated by algae or 

 the algae-like flagellates. Both sets of pro- 

 ducers depend upon light for photosynthe- 

 sis, and on dissolved inorganic salts for pro- 

 tein synthesis; both are fundamental in 

 community metabolism. 



Fresh-water algae, as a group, comprise 

 six seasonal categories: spring annuals, win- 

 ter annuals, perennials, summer annuals, 

 autumn annuals, and ephemerals. In large 



