Why can the addition of nutrients such as nitrates and phosphates result in a reduction of the amount of dissolved oxygen in the water?

The addition of nutrients like nitrates and phosphates can lead to algal blooms, where excessive algae growth occurs in aquatic environments. When these algae die and decompose, bacteria consume dissolved oxygen during the decomposition process, leading to a significant reduction in oxygen levels in the water. This decrease in dissolved oxygen can create hypoxic conditions, which can be detrimental to aquatic life, including fish and other organisms that rely on oxygen for survival.