Final answer:
Fluctuations in the phosphorus cycle, often exacerbated by excessive phosphorous input from human activities like fertilizer use, have significant effects on aquatic ecosystems. This excess phosphorous leads to microorganism overgrowth, oxygen depletion, and 'dead zones' detrimental to aquatic life. However, a natural phosphorous cycle also exists, driven by slow geological processes.
Step-by-step explanation:
Fluctuations in the phosphorus cycle in aquatic ecosystems can lead to significant changes in aquatic growth rates and overall ecosystem health. When excess phosphorous, often stemming from fertilizer runoff or sewage, enters these ecosystems, it causes an excessive growth of microorganisms. This in turn depletes dissolved oxygen, creating uninhabitable 'dead zones' within bodies of water and killing aquatic life such as shellfish and finfish.
Phosphorus in nature occurs as the phosphate ion. Apart from human activity, natural phosphorous movement can occur when phosphate-containing rocks undergo weathering, sending phosphates into rivers, lakes, and oceans. This creates a natural, albeit very slow, phosphorous cycle of its own.
Learn more about Phosphorus Cycle