M-/ 



MACROORGANISMS Those organisms visible to the unaided eye 

 (Macroinvertebrates) ^ wh±ch ^ retained on a u. S . standard 



sieve no. 30 (openings of 0.589 mm.). (See 

 Microorganisms ) 



MACROPHYTE 



Any plant that can be seen with the naked, 

 unaided eye; e.g., aquatic mosses, ferns, 

 liverworts, rooted plants, etc. 



MARL 



An earthy, unconsolidated deposit formed in 

 freshwater lakes, chiefly of calcium carbonate 

 mixed with clay or other impurities in 

 varying proportions. 



MARSH 



Periodically wet or continually flooded area 

 with the surface not deeply submerged. Covered 

 dominantly with emersed aquatic plants ; e.g., 

 sedges, cattails, rushes. 



MEDIAN TOLERANCE 

 LIMIT (TLm) 



The concentration of tested substance in 

 water at which just 50% of the test organisms 

 survive for a specified period of exposure. 

 (See Tolerance Limit) 



MEROMICTIC LAKES 



Lakes in which dissolved substances create 

 a gradient of density differences with depth; 

 this prevents complete mixing or circulation 

 of water masses. (See Chemical Stratification) 



