Eutrophication is the process of nutrients like phosphates and nitrates being excessively added to a waterway. Eutrophication can happen naturally over long periods of time, but human activity such as agriculture accelerates this process because fertilisers (which are rich in phosphates) among other stuff wash into the rivers and waterways.
Algal blooms are a consequence of eutrophication, the algae and bacteria rapidly grow & dominate the waterway increasing the BOD (biochemical oxygen demand), so the concentration of oxygen decreases and fish die of suffocation etc.
Hope that answers your question.