CARBON IN FRESHWATER SYSTEMS 249 



limitations decreases (Wetzel, 1971). A point is reached where population 

 densities become self-limiting by self-shading effects beyond which growth can 

 be increased further only by greater turbulence and light availability than occur 

 under natural conditions (Wetzel, 1966). 



Submersed macrophytes usually assume increasingly greater importance to 

 the total autochthonous production of lakes (Fig. 2) until the fertility of the 

 system reaches a point of severe light attenuation generally associated with 

 biogenic turbidity of phytoplankton (Wetzel and Hough, 1972). Algae attached 

 to the substrata or to macrophytes of less fertile sites commonly constitute a 

 major, even dominant, site of organic-carbon synthesis. These sessile populations 

 exhibit a population-growth stability not common among the phytoplankton. 



As the emergent macrophytes assume greater dominance in lakes and 

 eventually encompass a majority of the lake basin, an exceedingly productive 

 combination of littoral macrophytes and attendant microflora develops (Fig. 2). 

 Attached algae and algae loosely associated with the macrophytes (eulittoral 

 algae; tychophytoplankton) develop strongly in association with the emergent 

 flora. 



The transitional stages of dominance in autotrophic productivity by 

 phytoplankton, sessile submersed macroflora and microflora, phytoplankton, 

 and then emergent macrophytes and associated microflora, as depicted in Fig. 2 

 and subsequent figures, are not intended to imply a succession of stages in the 

 ontogeny of freshwaters. Some small lakes do evolve in relatively short periods 

 through this general sequence; others do not (see Wetzel and Allen, 1972). 

 Rather, within a nearly infinite spectrum of lakes, these general relationships 

 among sites of relative primary productivity and increasing fertility of the 

 systems are found. Many exceptions certainly exist. 



The quantities of organic carbon released extracellularly by the primary 

 producers of Lawrence Lake were relatively low (Table 2). Although the rates of 

 secretion of DOC fluctuated to a marked degree spatially and temporally over an 

 annual cycle, the mean percentage was about 5% of that carbon fixed in 

 photosynthesis. Higher values have been reported in the literature, but 

 practically no detailed annual means are available for comparison. High 

 extracellular release of DOC during algal and macrophytic photosynthesis has 

 been shown to be a function of C0 2 limitation, both high and low population 

 densities, and both high and very low light intensities (Wetzel, Rich, Miller, and 

 Allen, 1972). Greater secretion at the extreme ends of light and pH spectra 

 indicate the operation of light and dark mediated secretion under different 

 pathways (Fogg and Watt, 1966; Hough and Wetzel, 1972). Dark C0 2 fixation 

 and secretion occur by numerous carboxylations, e.g., Wood— Werkman and 

 Utter reactions, in which carbon enters four-carbon acids in the mitochondria 

 rather than phosphoglyceric acid in the chloroplast, as in light-mediated 

 Calvin-cycle fixation and secretion. This assumes no C4 photosynthesis, which is 

 probably true for both algae (Hatch et al., 1971) and submersed angiosperms 

 (Wetzel and Hough, 1972). 



