3. 3 The nutrient in the coastal seawater The main addition of Nand p compounds to the marine environment are considered to be sewage wastes, either directly discharged to the sea or via rivers into which treated waste are discharged Other sources are agricultural runoff and wastes from food processing industries but these are normally considered to be small compared to the input of sewage◼ The main addition of N and P compounds to the marine environment are considered to be sewage wastes, either directly discharged to the sea or via rivers into which treated waste are discharged. ◼ Other sources are agricultural runoff and wastes from food processing industries,but these are normally considered to be small compared to the input of sewage. 3.3 The nutrient in the coastal seawater