Algal bloom is caused by eutrophication. Eutrophication refers to the excessive nutrient enrichment in a body of water, usually from agricultural runoff or sewage discharge. These nutrients, such as nitrogen and phosphorus, act as fertilizers for the growth of algae. When there is an excess of nutrients, algae can reproduce rapidly, forming dense populations known as algal blooms.

Option 1, global warming, is not directly responsible for algal bloom, as the main cause is nutrient enrichment. However, climate change can indirectly influence algal blooms by affecting water temperature and nutrient availability.

Option 2, salination, does not directly cause algal bloom. Salination refers to the increase in salt concentrations in soil or water bodies, which can negatively impact plant and algal growth.

Option 4, biomagnification, is not related to algal bloom. Biomagnification refers to the increasing concentration of toxins or pollutants in the bodies of organisms as they move up the food chain.

Therefore, the correct answer is option 3, eutrophication.