Editing Downwelling (section)

== Effects on ocean biogeochemistry ==
[[Biogeochemical cycle|Biogeochemical cycling]] related to downwelling is constrained by the location and frequency at which this process occurs. The majority of downwelling, as described above, occurs in polar regions as deep and [[bottom water]] formation or in the center of [[Ocean gyre|subtropical gyres]]. Bottom and deep water formation in the [[Southern Ocean]] ([[Weddell Sea]]) and North [[Atlantic Ocean]] ([[Greenland Sea|Greenland]], [[Labrador Sea|Labrador]], [[Norwegian Sea|Norwegian]], and [[Mediterranean Sea]]s) is a major contributor towards the removal and [[Carbon sequestration|sequestration]] of anthropogenic [[carbon dioxide]], [[dissolved organic carbon]] (DOC), and dissolved oxygen.<ref>{{Cite journal |last1=Marinov |first1=I. |last2=Gnanadesikan |first2=A. |last3=Toggweiler |first3=J. R. |last4=Sarmiento |first4=J. L. |date=June 2006 |title=The Southern Ocean biogeochemical divide |url=https://www.nature.com/articles/nature04883 |journal=Nature |language=en |volume=441 |issue=7096 |pages=964–967 |doi=10.1038/nature04883 |pmid=16791191 |bibcode=2006Natur.441..964M |s2cid=4428683 |issn=1476-4687}}</ref><ref>{{Cite journal |last1=Tian |first1=Ru Cheng |last2=Vézina |first2=Alain F. |last3=Deibel |first3=Don |last4=Rivkin |first4=Richard B. |date=2003-10-03 |title=Sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a deep‐water formation region |url=http://dx.doi.org/10.1029/2002gb001969 |journal=Global Biogeochemical Cycles |volume=17 |issue=4 |page=1090 |doi=10.1029/2002gb001969 |bibcode=2003GBioC..17.1090T |issn=0886-6236|doi-access=free }}</ref><ref name="Emerson-2008">{{Cite book |last1=Emerson |first1=Steven R. |title=Chemical Oceanography and the Marine Carbon Cycle |last2=Hedges |first2=John |publisher=Cambridge |year=2008 |isbn=978-0-521-83313-4 |edition=4th |pages=205–206}}</ref> Dissolved gas [[solubility]] is greater in cold water allowing for increased gas concentrations.<ref name="Emerson-2008" />

The Southern Ocean alone has been shown to be the most important high-latitude region controlling pre-industrial atmospheric carbon dioxide by [[general circulation model]] simulations. Circulation of water into the Antarctic deep-water formation region is one of the main factors drawing carbon dioxide into the surface oceans. The other is the [[biological pump]], which is typically limited by [[iron]] in the Southern Ocean in areas with high [[nutrient]]s and low [[chlorophyll]] ([[High-nutrient, low-chlorophyll regions|HNLC]]). DOC can become entrained during bottom and deep water formation which is a large portion of biogenic carbon export. It is thought that the export of DOC is up to 30% of the biogenic carbon that makes it into the deep ocean. The intensity of the DOC flux to depth relies on the strength of winter convection, which also affects the microbial food web, causing variations in the DOC exported to depth. Dissolved oxygen is also downwelled at bottom and deep water formation sites, contributing to elevated dissolved oxygen concentrations below 1000 meters.

Subtropical gyres are typically limited in macro and micro nutrients such as [[nitrogen]], [[phosphorus]], and iron; resulting in [[Photosynthetic picoplankton|picophytoplankton]] communities that have low nutrient requirements. This is in part due to consistent downwelling, which transports nutrients away from the photic zone. These [[oligotroph]]ic areas are thought to be sustained by rapid [[Nutrient cycle|nutrient cycling]] which could leave little carbon remaining that could be sequestered. The dynamics of picophytoplankton's role in carbon cycling in subtropical gyres is poorly understood and is being actively researched.

Areas with the highest primary productivity play significant roles in biogeochemical cycling of carbon and nitrogen. Downwelling can either alleviate or induce anoxic conditions, depending on the initial conditions and location. Sustained periods of upwelling can cause deoxygenation which is relieved by a downwelling event transporting dissolved oxygen back down to depths. [[Anoxic waters|Anoxic]] conditions can also result from persistent downwelling after an [[algal bloom]] of high-biomass [[dinoflagellate]]s. The accumulation of dinoflagellates and other forms of biomass nearshore due to downwelling will eventually cause nutrient depletion and mortality of organisms. As the biomass decays, oxygen becomes depleted by [[heterotroph]]ic bacteria, inducing anoxic conditions.