All of the Great Lakes are changing; however, the lower lakes, because of their increased productivity, are changing at a much faster rate. This condition is reflected in the overall nutrient and biomass content of the individual lakes. The nutrient concentrations of the lower lakes are 4-8 times higher than those in the upper lakes. The increase in phosphorus in the lower lakes has resulted in dramatic increases in phytoplankton biomass, as well as in shifts in the biomass to a greater percentage of bluegreen algae. Changes have occurred in the zooplankton communities. Changes in the biota of a lake can result from three major types of influences. The first is change in the physical environment. The second type of change is associated with direct toxicity. The third type of influence, and, probably, the most prevalent today, are changes resulting from altered food supplies.