Air Pollution
Since Istanbul is a very fast growing city, there is a big concentration of people and factories which contributed to the air pollution problems in the city. Istanbul is also a hub of Industry for Turkey so closing factories isn't a option. The government has taken several measures to help get rid of the pollution such, as greener energy sources, and studies from NIH, show it is working . There are still lots of smog in Istanbul, but if the trend of CO2 and SO2 going down keeps up, it might get brighter for Istanbul
Marine Population
Istanbul used to have huge marine population, but the the amount of Aquatic life is decreasing. Even though Istanbul isn't the only Country to border the black sea or the Mediterranean Sea, it is the country doing the most damage, since it is the biggest port. In fact, in the 20th century alone 6 million dolphins and porpoises were killed. Istanbul main problem isn't with the either sea, it is with the Bosphorus strait, aka the path made between the two sides of Istanbul. For more information on the damage done in the strait there is a video below.
Istanbul Strait NASA, wikipedia.web