the health of an aquatic ecosystem is degraded when the ecosystem's ability to absorb a stress has been exceeded. a stress on an aquatic ecosystem can be a result of physical, chemical or biological alterations of the environment. physical alterations include changes in water temperature, water flow and light availability. chemical alterations include changes in the loading rates of biostimulatory nutrients, oxygen consuming materials, and toxins. biological alterations include over-harvesting of commercial species and the introduction of exotic species. human populations can impose excessive stresses on aquatic ecosystems. there are many examples of excessive stresses with negative consequences. consider three. the environmental history of the great lakes of north america illustrates this problem, particularly how multiple stresses, such as water pollution, over-harvesting and invasive species can combine. the norfolk broadlands in england illustrate similar decline with pollution and invasive species. lake pontchartrain along the gulf of mexico illustrates the negative effects of different stresses including levee construction, logging of swamps, invasive species and salt water intrusion.