Editing Urbanization (section)

=== Water quality ===
[[Urban runoff]], polluted water created by rainfall on impervious surfaces, is a common effect of urbanization. Precipitation from rooftops, roads, parking lots and sidewalks flows to [[storm drain]]s, instead of [[Infiltration (hydrology)|percolating]] into [[groundwater]]. The contaminated [[stormwater]] in the drains is typically untreated and flows to nearby streams, rivers or coastal bays.<ref name="USGS-runoff">{{cite web |url=https://water.usgs.gov/edu/runoff.html |title=Runoff (surface water runoff) |author=<!--Not stated--> |date=2018-06-06 |website=USGS Water Science School |publisher=U.S. Geological Survey (USGS) |location=Reston, VA}}</ref>

[[Eutrophication]] in water bodies is another effect large populations in cities have on the environment. When rain occurs in these large cities, it filters CO<sub>2</sub> and other pollutants in the air onto the ground. These chemicals are washed directly into rivers, streams, and oceans, making water worse and damaging ecosystems in them.<ref name=":0">{{cite journal|last1=Jiang|first1=Leiwen|last2=Hoepf Young|first2=Malea|last3=Hardee|first3=Karen|year=2008|title=Population, Urbanization, And The Environment|journal=World Watch|volume=21|issue=5|pages=34–39}}</ref>

Eutrophication is a process which causes low levels of oxygen in water and algal blooms that may harm aquatic life.<ref>{{Cite web|url=https://www.wri.org/our-work/project/eutrophication-and-hypoxia/about-eutrophication|title=About Eutrophication {{!}} World Resources Institute|website=wri.org|date=12 September 2013|access-date=2018-11-18|archive-date=19 November 2018|archive-url=https://web.archive.org/web/20181119010905/https://www.wri.org/our-work/project/eutrophication-and-hypoxia/about-eutrophication|url-status=live}}</ref> [[Harmful algal bloom]]s make dangerous toxins. They live best in nitrogen- and phosphorus-rich places which include the oceans contaminated by the aforementioned chemicals.<ref>{{Cite web|url=https://www.epa.gov/nutrientpollution/harmful-algal-blooms|title=Harmful Algal Blooms|website=US Environmental Protection Administration|date=3 June 2013|access-date=2018-11-18|archive-date=4 February 2020|archive-url=https://web.archive.org/web/20200204072812/https://www.epa.gov/nutrientpollution/harmful-algal-blooms|url-status=live}}</ref> In these ideal conditions, they choke surface water, blocking sunlight and nutrients from other life forms. Overgrowth of algal blooms makes water worse overall and disrupts the natural balance of aquatic ecosystems. Furthermore, as algal blooms die, CO<sub>2</sub> is produced. This makes the ocean more acidic, a process called acidification.<ref name=":12">{{Cite web|url=http://www.nutrientchallenge.org/sites/default/files/documents/files/FINAL%20Ocean%20Acidification%20policy%20brief%20spread_press%20quality_print.pdf|title=Eutrophication and Ocean Acidification|last1=Ramesh|first1=R|last2=Lakshmi|first2=A|date=2013|last3=Purvaja|first3=R|last4=Costanzo|first4=S.D|last5=Kelsey|first5=R.H|last6=Hawkey|first6=J|last7=Datta|first7=A|last8=Dennison|first8=W.C|access-date=18 November 2018|archive-date=21 April 2021|archive-url=https://web.archive.org/web/20210421212836/http://www.nutrientchallenge.org/sites/default/files/documents/files/FINAL%20Ocean%20Acidification%20policy%20brief%20spread_press%20quality_print.pdf|url-status=live}}</ref>

The ocean's surface can absorb CO<sub>2</sub> from the Earth's atmosphere as emissions increase with the rise in urban development. In fact, the ocean absorbs a quarter of the CO<sub>2</sub> produced by humans.<ref name=":03">{{Cite web|url=https://nca2014.globalchange.gov/report/our-changing-climate/ocean-acidification|title=National Climate Assessment|website=National Climate Assessment|access-date=2018-11-18|archive-date=18 November 2018|archive-url=https://web.archive.org/web/20181118145133/https://nca2014.globalchange.gov/report/our-changing-climate/ocean-acidification|url-status=live}}</ref> This helps to lessen the harmful effects of greenhouse gases. But it also makes the ocean more acidic.<ref>{{Cite journal|last1=Feely|first1=Richard A.|last2=Alin|first2=Simone R.|last3=Newton|first3=Jan|last4=Sabine|first4=Christopher L.|last5=Warner|first5=Mark|last6=Devol|first6=Allan|last7=Krembs|first7=Christopher|last8=Maloy|first8=Carol|date=August 2010|title=The combined effects of ocean acidification, mixing, and respiration on pH and carbonate saturation in an urbanized estuary|journal=Estuarine, Coastal and Shelf Science|volume=88|issue=4|pages=442–449|doi=10.1016/j.ecss.2010.05.004|issn=0272-7714|bibcode=2010ECSS...88..442F}}</ref> A drop in pH the prevents the proper formation of calcium carbonate, which sea creatures need to build or keep shells or skeletons.<ref>{{Cite web|url=https://www.fisheries.noaa.gov/insight/understanding-ocean-acidification|title=Understanding Ocean Acidification {{!}} NOAA Fisheries|last=Fisheries|first=NOAA|date=2018-09-09|website=fisheries.noaa.gov|access-date=2018-11-18|archive-date=19 November 2018|archive-url=https://web.archive.org/web/20181119053040/https://www.fisheries.noaa.gov/insight/understanding-ocean-acidification|url-status=live}}</ref><ref name=":03" /> This is especially true for many species of molluscs and coral. However, some species have been able to thrive in a more acidic environment.<ref>{{Cite web|url=https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification|title=Ocean Acidification|website=Smithsonian Ocean|date=30 April 2018 |access-date=2018-11-18|archive-date=9 November 2020|archive-url=https://web.archive.org/web/20201109050311/https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification|url-status=live}}</ref>