What changes can occur in an aquatic ecosystem as a result of nutrient loading?

Yes, definitely algal bloom is a significant change. What is more important is how the bloom affects other aquatic life. The bloom forms a dense "blanket" of growth, which as it grows rapidly and dies, decomposes. Bacteria feed on this rich organic stuff and use up the dissolved oxygen in the water. As the oxygen levels drop, fish and aquatic insects die. This is the result of the algal bloom.