There are other ways water can loose its oxygen, which have nothing to do with people or pollution. The Black Sea, and some lakes in Africa, have no oxygen at all below the top layers. Water pumped up from below stinks of hydrogen sulfide, and only certain bacteria can live below the oxygenated zone. The third part of the graph, the red line, shows salinity, or the amount of salt dissolved in the water. You read the salinity off the next to lowest scale.
Salt is added to water by rivers dissolving rocks and carrying it to the ocean.
This process has been going on since the oceans formed. Water and salt come in, but only water evaporates away. Salinity may be less where, for example, there is heavy rainfall, or near melting icebergs. An important thing about salinity is that it is the major factor in determining how dense water is, and therefore at what level it is found with respect to other waters. The saltier water is, the denser it is, and the further it sinks.