Microscopic green plants called algae or "phytoplankton" form the base of the food chain for fish. All green plants need proper temperature, light, and nutrients for growth. If sufficient light and proper temperature are present, the nutrients in chemical fertilizers (nitrogen, phosphorous and potassium) are readily assimilated by phytoplankton and their abundance increases. Manure contains the same nutrients which are released and become available to phytoplankton during and after decomposition. As phytoplankton assimilate fertilizer nutrients and reproduce to form dense communities’ pond water turns a greenish or brownish color. This is called a phytoplankton bloom.
Sudden death of phytoplankton or algal bloom, "bloom crash", may result from insufficient light (e.g. cloud cover) for photosynthesis, inadequate pond nutrients (a bloom too dense to be supported by available nutrients and oxygen) and/or bloom senescence (the plant cell line becomes too old to continue reproduction). Oxygen is consumed or depleted when dead phytoplankton/algae decay. During the nighttime hours, a dense phytoplankton bloom can remove all oxygen from the water for respiration (to breathe) alone. When a bloom crash occurs, the water appears to have become "black" or clear overnight.
As phytoplankton multiply they are eaten directly by some fish or by other mostly microscopic aquatic animals called "zooplankton." Phytoplankton and zooplankton (collectively called "plankton") also serve as food for larger aquatic organisms. Through a complexed chain of interactions, fertilizers increase production of natural food organisms eaten by fish. Different fish may have different food preferences. Some can filter plankton, others eat aquatic insects and others may feed on decomposing material. See figure 1. (Bocek, 2009)