246 WETZEL AND RICH 



1969; Wetzel, Rich, Miller and Allen, 1972). Detritus is here defined 

 functionally as all nonpredatory losses of organic carbon from any trophic level 

 (including egestion, secretion, excretion, etc.) or inputs from sources external to 

 the ecosystem that enter and cycle in the system (allochthonous organic 

 carbon). DOC is functionally the most important component of the detrital- 

 carbon trophic structure and is detritus in the truest sense of the definition. This 

 definition removes the highly artificial "particulate" constraint from existing 

 definitions of detritus. Further, discriminating between detritus and its 

 associated flora and fauna provides needed functional applicability to more 

 general conditions when associated bacteria serve other system functions, such as 

 regeneration of nutrients (e.g., C0 2 , N, P) or when detritus is utilized in other 

 ways (e.g., algal heterotrophy, losses by adsorption, and precipitation with 

 CaC03 and other inorganic particulate matter). 



Any attempt to analyze trophic detrital dynamics necessitates a functional 

 separation between the dissolved and particulate phases, both in the annual 

 treatment of carbon biomass and in understanding functional pathways of 

 sources, utilization, and losses. Over the past 5 years, we have attempted to 

 develop a carbon analysis on this functional basis for a hard-water lake, 

 Lawrence Lake, southwestern Michigan. Major pools and pathways of carbon 

 flux are illustrated diagrammatically in Fig. 1. Conspicuous in this treatment of 

 detrital organic carbon are: (1) the central dominant pool of DOC; (2) three 

 major sources of POC, allochthonous, the littoral zone, and the pelagic zone; and 

 (3) the major areas of detrital metabolism, the benthic region, where a large 

 majority of POC is decomposed in many lakes, and pelagic sedimentation and 

 decomposition. 



Lawrence Lake is a typical hard-water lake of small size (4.9 hectares), 

 moderate depth (12.6 m), and relatively low productivity (see Wetzel, 1971). We 

 will summarize only the salient points of the carbon budget and fluxes here. 

 Details of annual cycles and experimental analyses have been presented in 

 Wetzel, Rich, Miller, and Allen (1972) and in an array of papers discussed 

 therein. Only the mean annual budgetary figures and flux values are given here 

 for illustration of rates of carbon cycling in this freshwater system and potential 

 extrapolation to others of differing structure and productivity. 



The rates of net primary production for Lawrence Lake have been 

 investigated in some detail, and they serve to illustrate the commonly ignored 

 contribution of the sessile producers to total autotrophic production (Table 1). 

 The littoral zone and its flora are rather poorly developed in this lake; the values 

 in no way exaggerate their importance. Needless to say, the relative contribution 

 of these components is highly variable among freshwaters and is enormously 

 complicated by differences in lake morphometry and edaphic factors. If we 

 scrutinize existing data for lakes carefully, however, certain generalities appear in 

 relation to increasing nutrient fertility of lakes (Fig. 2). At a given latitude, 

 maximum growth of phytoplankton increases progressively with increasing 

 fertility, and the intensity spectrum of simultaneously interacting nutrient 



