The two agricultural practices that can lead to algal blooms are:
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Application of nutrients to crops - This often involves the use of fertilizers that contain nitrogen and phosphorus, which can run off into water bodies and promote algal growth.
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Animal waste accumulated in livestock production - Animal waste can also contain high levels of nutrients, which can lead to nutrient runoff into nearby water bodies, contributing to algal blooms.
Other practices listed, such as sediments from tilling, erosion from overgrazing, and withdrawals of water for irrigation, can have environmental impacts, but the direct link to algal blooms is primarily through nutrient enrichment from fertilizers and animal waste.