Eutrophication is caused by excess loads of mainly nitrogen and phosphorus from point and diffuse sources. Most contributing sources are point loads from urban areas and industries, and diffuse emissions from agricultural activity, related to excess fertilizer application.

A stationary state between photosynthesis and respiration (figure 1a).

-vertical segregation of
photosynthetic and respiratory functions

-in surface waters nutrients become exhausted by photosynthesis

-the decomposition (respiration) of organism-produced particles (detritus) after settling leads to enrichment in the deeper water layers with nutrients and a depletion of dissolved oxygen (figure 1b).

An increase in C, N, P concentrations due to external inputs will destabilize the balance between photosynthesis and respiration. This leads to an increase of primary production.