Nitrates promote the growth of algae (this is called algal bloom). This means less plants (that are not algae) can photosynthesise as the sunlight can not reach them. Therefore those plants die. Saprobiotic organisms (in other words decomposers) break down the dead plants. these decomposers therefore need to respire as they are breaking down the dead plants, this means they use the oxygen in the sea to respire. As the oxygen is taken up, less is dissolved in the water.