What is a common source of phosphorus enrichment?

Phosphorus enrichment commonly occurs due to sewage and fertilizer runoff, making it the correct answer. This enrichment happens when excess phosphorus from agricultural fertilizers and untreated or inadequately treated sewage enters water bodies. The phosphorus can lead to algal blooms and eutrophication, which depletes oxygen in the water and harms aquatic life. Fertilizers used in agriculture often contain high levels of phosphorus, and when it rains, runoff can carry this fertilizer into nearby streams, rivers, and lakes.

Other processes like atmospheric nitrogen fixation, natural mineral weathering, and groundwater recharge do not primarily contribute to phosphorus enrichment in the same manner. Atmospheric nitrogen fixation relates more to nitrogen than phosphorus, while natural mineral weathering is a slow process that typically contributes less to immediate phosphorus loads in water bodies. Groundwater recharge may also play a role in carrying nutrients but is less direct compared to the runoff from fertilizers and sewage that translates into rapid and significant phosphorus pollution.