Biological Characteristics 



Meaningful functional attributes of a lacustrine system have been described by 

 Hutchinson (1967) and Wetzel (1975). A cross section of a typical moderately 

 deep mid-sized oligo-mesotrophic coastal zone lake is given in figure 7-3. 

 This diagram is based on biological function and is not to be confused with 

 the habitat classification system used by the NWl . Thus, the functional 

 littoral zone in Maine lakes is 10 to 15 m (33 to 50 ft) while the NWI 

 littoral zone is <2 m (7 ft). Major components are the littoral zone (where 

 emergent, floating, or submerged vegetation grow), and the limnetic (pelagial) 

 zone (where the bottom is at depths too great to possess enough light for 

 supporting macrophytes) . The littoral zone is trophogenic. The trophogenic 

 zone is separated from the tropholytic zone (deeper water) by the 

 "compensation level" where the rates of photosynthesis (determined by light 

 penetration) and respiration are equal. Depending upon weather conditions, 

 the depth of the compensation level may change somewhat from day to day. 



In clear, oligotrophic lakes the deepest attached plants are usually 

 nonvascular (algae or bryophytes). Below the compensation level, metabolism 

 is mostly heterotrophic and detritus-based. Because decomposition of organic 

 matter exceeds production of organic matter (by photosynthesis) there, the 

 deeper water of deep lakes is called tropholytic, in contrast to the near- 

 surface trophogenic zone where net production of organic matter occurs. 

 Eutrophic lakes have a shallow compensation level compared to clear, 

 oligotrophic lakes, because heavy plankton populations in eutrophic lakes 

 greatly reduce light penetration. Mesotrophic lakes have characteristics that 

 are intermediate between oligotrophic and eutrophic lakes. The littoral zone 

 has been described for only a few Maine lakes (Davis, unpublished ) and none of 

 these are in the coastal zone. 



The interaction of factors that ultimately determine the composition and 

 quantity of biota, the rates at which nutrients are recycled, and the general 

 productivity of lakes are highly complex (figure 7-4). For example, lake 

 morphology (lake depth, area, and contours, etc.) affects light penetration, 

 heat distribution, and oxygen distribution, all of which in turn affect the 

 trophic status of lakes and the biota present in them. 



ABIOTIC FACTORS AFFECTING THE LACUSTRINE SYSTEM 



Several major factors affect the productivity of Maine lakes. Some are 

 applicable to all lakes and some are particular to coastal zone lakes. Most 

 Maine lakes are infertile in comparison to average temperate zone lakes. 

 Climatic, other atmospheric, hydrological , geological, and other physical 

 factors that affect lakes, will be considered here. These factors work in 

 combination and their combined effects usually cannot be separated. 

 Topography may affect local climate (e.g., alters wind), climate may affect 

 edaphic factors (e.g., weathering and leaching of soils), people may alter 

 edaphic factors (e.g., soil erosion), etc. Lake hydrology, which has a major 

 influence on biological productivity, is determined by climate and other 

 factors including lake morphology and watershed size. Some of the complex 

 interrelations among these factors are illustrated in figure 7-4. 



7-13 



10-80 



