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Ceratophyllum demersum and Myriophyllum spicatum are widespread nuisance aquatic weed species. They are harmful weeds in the Iranian freshwater systems especially in the Anzali lagoon.
When massive growths of these aquatic weeds in the late winter to spring occur, they can have an influence on water quality because oxygen is depleted by plant respiration overnight, often causing fish kills (Mitchell,
1974, Aldrich, 1984, Riemer, 1984, Murphy & Barrett, 1990). Their ecology is regarded as similar. Both species
have been previously classified in terms of established-phase survival strategy as "competitive disturbance- tolerant" species.
Experimental data are presented to show that although this broad categorization of strategy is probably correct for these two species, it is possible to demonstrate significant differences in terms of response to
disturbance and competition. Less difference was discernible in their comparative response to stress. The
results help explain reports of variable success in attempting to manage these two species using
disturbance-based weed control measures, and suggest that Ceratophyllum is even less susceptible to such measures than Myriophyllum.