<Li> Nurdles, small ubiquitous waterborne plastic pellets . See plastic pollution and microplastic pollution . </Li> <Li> Shipwrecks, large derelict ships . </Li> <P> Thermal pollution is the rise or fall in the temperature of a natural body of water caused by human influence . Thermal pollution, unlike chemical pollution, results in a change in the physical properties of water . A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers . Elevated water temperatures decrease oxygen levels, which can kill fish and alter food chain composition, reduce species biodiversity, and foster invasion by new thermophilic species . Urban runoff may also elevate temperature in surface waters . </P> <P> Thermal pollution can also be caused by the release of very cold water from the base of reservoirs into warmer rivers . </P>

The main cause of water pollution in cities is