Using too much fertilizer can add too much nitrogen to aquatic ecosystems. This can cause an algal bloom which provides lots of food for fish and birds blocks sunlight and starves things of oxygen makes the ecosystem healthier and able to

## Step 1<br />The problem is about the effects of excessive use of fertilizers on aquatic ecosystems. Fertilizers are rich in nutrients such as nitrogen, phosphorus, and potassium. When these nutrients are washed into water bodies, they can cause a variety of ecological issues.<br /><br />## Step 2<br />The first option, "provides lots of food for fish and birds", is not a direct consequence of excessive nitrogen from fertilizers. While nutrients can support the growth of aquatic plants, which in turn can be a food source for fish and birds, the question specifically refers to the effects of excess nitrogen, which primarily leads to algal blooms rather than directly providing food.<br /><br />## Step 3<br />The second option, "blocks sunlight and starves things of oxygen", is a direct consequence of excessive nitrogen leading to algal blooms. When there is an overabundance of nutrients, particularly nitrogen, in water bodies, it can lead to rapid growth of algae, known as algal blooms. These blooms can cover the water surface, blocking sunlight. This prevents the photosynthesis process in underwater plants, leading to a decrease in oxygen production. As the algae die and decompose, the decomposition process further consumes oxygen, leading to hypoxic conditions that can starve aquatic life of oxygen.<br /><br />## Step 4<br />The third option, "makes the ecosystem healthier and able to", is incomplete and does not provide a clear outcome of the excessive use of fertilizers. However, it is generally understood that excessive use of fertilizers, especially nitrogen-rich ones, does not lead to a healthier ecosystem. Instead, it often results in the problems mentioned above, such as algal blooms and oxygen depletion.