First of all, it’s never just one reason. Many factors, including human activity in the watershed, interact to affect the water quality of a lake. Just as fertilizing a lawn promotes growth, nutrients seeping into a lake increase aquatic plant growth rates. Agriculture and communities in the watershed may inadvertently add nutrients to the lake through stormwater runoff and over spraying. Normally lawn and agricultural fertilizers are not innately harmful. However, excess fertilizer can cause algae to "bloom" which, in turn, decreases light for other plants and animals. Water quality may worsen at the peak of the growing season when sunlight and nutrients are plentiful. Other sources which cause water quality degradation come from erosion and sediment washing in from disturbed shorelines or construction sites. The transported sediment contributes additional nutrients to the water. All that washed in sediment is what causes muddy water. Development on a lake shore usually means changes in the shoreline plant community (i.e., cultivated grass replaces bulrush). Nutrients and sediments wash into the water more readily without the natural filtering system.