Confused Editorial Follow-Up, Ocean Acidification

In my  previous post, I made an argument against an editorial which proposed that CO2 was not a pollutant. In that brief post I did not mention recent observations on the gradual change in pH of ocean water (ocean acidification). The above graph (a little fuzzy) shows the increase in atmospheric CO2 (red line) since the 1950s as measured at the same location in Hawaii, The green shows increasing amounts of CO2 in seawater near Hawaii and the red shows the decline in pH (more acid). The decline is small but remember the pH scale is logarithmic. While there is not a lot of solid information on the current effect of declining pH there is plenty of concern for the future based on our knowledge of how shrimp, crabs, oysters, corals etc. react physiologically to lower pH.
It seems pretty logical to me that if a gas (CO2), at unusually high concentration,  causes a decrease in the pH of water it is a pollutant.