Factors Governing Geographical Variation in Body Size of Diaptomus minutus (Copepoda, Calanoida)

Considerable variation in cephalothorax length of adult female Diaptomus minutus and D. oregonensis occurs among lakes of glaciated eastern North America during spring and early summer. Moderately significant Spearman rank correlations existed between D. minutus length, and Secchi transparency depth (negative) and true water color (positive). Similar significant correlations were not found for D. oregonensis. In these lakes water transparency is largely a negative function of color derived from humic substances. It is hypothesized that waters of low transparency provide a measure of protection to D. minutus against visually feeding fish predators. It can thus undergo a modest gain in size, thereby reducing vulnerability to invertebrate predation, increasing potential fecundity and (possibly) allowing it to feed on larger food particles.