Because they are recycled in standing water, nutrients are difficult to control in lake ecosystems. Rather than leaving the ecosystem, the nutrients cycle among the water column, algae and plant tissues, and the bottom sediments. For example, algae may temporarily remove all the nitrogen from the water column, but the nutrients will return to the water column when the algae die and are decomposed by bacteria. Therefore, gradual inputs of nutrients tend to accumulate over time rather than leave the system. In a water quality context, sedimentation usually refers to soil particles that enter the water column from eroding land.
Sediment consists of particles of all sizes, including fine clay particles, silt, sand, and gravel.
Sedimentation and siltation can severely alter aquatic communities. Sediment may clog and damage fish gills, suffocate eggs and aquatic insect larvae on the bottom, and fill in the space between bottom cobbles where fish lay eggs. Suspended silt and sediment interfere with recreational activities and aesthetic enjoyment at water bodies by reducing water clarity. Sediment may also carry other pollutants into water bodies. Nutrients and toxic chemicals may attach to sediment particles on land and ride the particles into surface waters where the pollutants may settle with the sediment or detach and become soluble in the water column.