Editing Mediterranean Sea (section)

==Biogeochemistry==
[[File:Cycling of marine phytoplankton.png|thumb|alt=A detailed description of the cycling of marine phytoplankton in the ocean's photic zone. Phytoplankton's role in photosynthesis, oxygen production, and marine food webs highlighted.|Cycling of marine phytoplankton]]
In spite of its great [[biodiversity]], concentrations of [[chlorophyll]] and nutrients in the Mediterranean Sea are very low, making it one of the most [[oligotrophic]] ocean regions in the world. The Mediterranean Sea is commonly referred to as an [[Low-nutrient, low-chlorophyll region|LNLC (Low-Nutrient, Low-Chlorophyll) area]]. The Mediterranean Sea fits the definition of a [[desert]] in which its nutrient contents are low, making it difficult for plants and animals to develop.

There are steep gradients in nutrient concentrations, chlorophyll concentrations and primary productivity in the Mediterranean. Nutrient concentrations in the western part of the basin are about double the concentrations in the eastern basin. The [[Alboran Sea]], close to the [[Strait of Gibraltar]], has a daily [[primary productivity]] of about 0.25 g C (grams of carbon) m<sup>−2</sup> day<sup>−1</sup> whereas the eastern basin has an average daily productivity of 0.16 g C m<sup>−2</sup> day<sup>−1</sup>.<ref name="Uitz Stramski Gentili DOrtenzio 2012 p. ">{{cite journal | last1=Uitz | first1=Julia | last2=Stramski | first2=Dariusz | last3=Gentili | first3=Bernard | last4=D'Ortenzio | first4=Fabrizio | last5=Claustre | first5=Hervé | title=Estimates of phytoplankton class-specific and total primary production in the Mediterranean Sea from satellite ocean color observations | journal=Global Biogeochemical Cycles | volume=26 | issue=2 | date=2012 | issn=0886-6236 | doi=10.1029/2011GB004055 | page=| bibcode=2012GBioC..26.2024U }}</ref> For this reason, the eastern part of the Mediterranean Sea is termed "ultraoligotrophic". The productive areas of the Mediterranean Sea are few and small. High (i.e. more than 0.5 grams of [[chlorophyll a|Chlorophyll ''a'']] per cubic meter) productivity occurs in coastal areas, close to the river mouths which are the primary suppliers of dissolved nutrients. The [[Gulf of Lion]] has a relatively high productivity because it is an area of high vertical mixing, bringing nutrients to the surface waters that can be used by [[phytoplankton]] to produce Chlorophyll ''a''.<ref name="Bosc, E. 2004">{{cite journal | last1=Bosc | first1=E. | last2=Bricaud | first2=A. | last3=Antoine | first3=D. | title=Seasonal and interannual variability in algal biomass and primary production in the Mediterranean Sea, as derived from 4 years of SeaWiFS observations | journal=Global Biogeochemical Cycles | volume=18 | issue=1 | date=2004 | issn=0886-6236 | doi=10.1029/2003GB002034 | page=| bibcode=2004GBioC..18.1005B }}</ref>

Primary productivity in the Mediterranean is also marked by an intense seasonal variability. In winter, the strong winds and precipitation over the basin generate [[Convective mixing|vertical mixing]], bringing nutrients from the deep waters to the surface, where phytoplankton can convert it into [[Biomass (ecology)|biomass]].<ref>{{cite journal | last1=Lebeaupin Brossier | first1=Cindy | last2=Béranger | first2=Karine | last3=Deltel | first3=Charles | last4=Drobinski | first4=Philippe | title=The Mediterranean response to different space–time resolution atmospheric forcings using perpetual mode sensitivity simulations | journal=Ocean Modelling | publisher=Elsevier BV | volume=36 | issue=1–2 | year=2011 | issn=1463-5003 | doi=10.1016/j.ocemod.2010.10.008 | pages=1–25| bibcode=2011OcMod..36....1L }}</ref> However, in winter, light may be the limiting factor for primary productivity. Between March and April, spring offers the ideal trade-off between light intensity and nutrient concentrations in surface for a [[spring bloom]] to occur. In summer, high atmospheric temperatures lead to the warming of the surface waters. The resulting [[density]] difference virtually isolates the surface waters from the rest of the water column and nutrient exchanges are limited. As a consequence, primary productivity is very low between June and October.<ref>{{cite journal | last1=D'Ortenzio | first1=F. | last2=Ribera d'Alcalà | first2=M. | title=On the trophic regimes of the Mediterranean Sea: a satellite analysis | journal=Biogeosciences | publisher=Copernicus GmbH | volume=6 | issue=2 | date=5 February 2009 | issn=1726-4189 | doi=10.5194/bg-6-139-2009 | doi-access=free | pages=139–148| bibcode=2009BGeo....6..139D }}</ref><ref name="Bosc, E. 2004"/>

Oceanographic expeditions uncovered a characteristic feature of the Mediterranean Sea biogeochemistry: most of the chlorophyll production does not occur on the surface, but in sub-surface waters between 80 and 200 meters deep.<ref>{{cite journal | last1=Moutin | first1=T. | last2=Van Wambeke | first2=F. | last3=Prieur | first3=L. | title=Introduction to the Biogeochemistry from the Oligotrophic to the Ultraoligotrophic Mediterranean (BOUM) experiment | journal=Biogeosciences | publisher=Copernicus GmbH | volume=9 | issue=10 | date=8 October 2012 | issn=1726-4189 | doi=10.5194/bg-9-3817-2012 | doi-access=free | pages=3817–3825| bibcode=2012BGeo....9.3817M }}</ref> Another key characteristic of the Mediterranean is its high nitrogen-to-phosphorus ratio (N:P). [[Alfred C. Redfield|Redfield]] demonstrated that most of the world's oceans have an average N:P ratio around 16. However, the Mediterranean Sea has an average N:P between 24 and 29, which translates a widespread phosphorus limitation.{{clarify|date=August 2019}}<ref>{{cite journal | last1=Berland | first1=Br | last2=Bonin | first2=Dj | last3=Maestrini | first3=Sy | title=Azote ou phosphore ? Considérations sur le paradoxe nutritionnel de la mer méditerranée | journal=Oceanologica Acta | publisher=Gauthier-Villars | volume=3 | issue=1 | date=1 January 1980 | issn=0399-1784 | pages=135–141 | url=https://archimer.ifremer.fr/doc/00121/23257/ | language=fr | access-date=1 May 2024 | archive-date=1 May 2024 | archive-url=https://web.archive.org/web/20240501212014/https://archimer.ifremer.fr/doc/00121/23257/ | url-status=live }}</ref><ref>{{cite journal | last1=Bethoux | first1=J.P. | last2=Morin | first2=P. | last3=Madec | first3=C. | last4=Gentili | first4=B. | title=Phosphorus and nitrogen behaviour in the Mediterranean Sea | journal=Deep Sea Research Part A. Oceanographic Research Papers | publisher=Elsevier BV | volume=39 | issue=9 | year=1992 | issn=0198-0149 | doi=10.1016/0198-0149(92)90053-v | pages=1641–1654| bibcode=1992DSRA...39.1641B }}</ref><ref>{{cite journal | last1=Kress | first1=Nurit | last2=Herut | first2=Barak | title=Spatial and seasonal evolution of dissolved oxygen and nutrients in the Southern Levantine Basin (Eastern Mediterranean Sea): chemical characterization of the water masses and inferences on the N:P ratios | journal=Deep Sea Research Part I: Oceanographic Research Papers | publisher=Elsevier BV | volume=48 | issue=11 | year=2001 | issn=0967-0637 | doi=10.1016/s0967-0637(01)00022-x | pages=2347–2372| bibcode=2001DSRI...48.2347K }}</ref><ref>{{cite journal | last1=Krom | first1=M.D. | last2=Thingstad | first2=T.F. | last3=Brenner | first3=S. | last4=Carbo | first4=P. | last5=Drakopoulos | first5=P. | last6=Fileman | first6=T.W. | last7=Flaten | first7=G.A.F. | last8=Groom | first8=S. | last9=Herut | first9=B. | last10=Kitidis | first10=V. | last11=Kress | first11=N. | last12=Law | first12=C.S. | last13=Liddicoat | first13=M.I. | last14=Mantoura | first14=R.F.C. | last15=Pasternak | first15=A. | last16=Pitta | first16=P. | last17=Polychronaki | first17=T. | last18=Psarra | first18=S. | last19=Rassoulzadegan | first19=F. | last20=Skjoldal | first20=E.F. | last21=Spyres | first21=G. | last22=Tanaka | first22=T. | last23=Tselepides | first23=A. | last24=Wassmann | first24=P. | last25=Wexels Riser | first25=C. | last26=Woodward | first26=E.M.S. | last27=Zodiatis | first27=G. | last28=Zohary | first28=T. | title=Summary and overview of the CYCLOPS P addition Lagrangian experiment in the Eastern Mediterranean | journal=Deep Sea Research Part II: Topical Studies in Oceanography | publisher=Elsevier BV | volume=52 | issue=22–23 | year=2005 | issn=0967-0645 | doi=10.1016/j.dsr2.2005.08.018 | pages=3090–3108 | bibcode=2005DSRII..52.3090K |display-authors=4}}.</ref>

Because of its low productivity, plankton assemblages in the Mediterranean Sea are dominated by small organisms such as [[picophytoplankton]] and [[bacteria]].<ref>{{cite journal | last1=Sammartino | first1=M. | last2=Di Cicco | first2=A. | last3=Marullo | first3=S. | last4=Santoleri | first4=R. | title=Spatio-temporal variability of micro-, nano- and pico-phytoplankton in the Mediterranean Sea from satellite ocean colour data of SeaWiFS | journal=Ocean Science | publisher=Copernicus GmbH | volume=11 | issue=5 | date=25 September 2015 | issn=1812-0792 | doi=10.5194/os-11-759-2015 | doi-access=free | pages=759–778| bibcode=2015OcSci..11..759S }}</ref><ref name="Uitz Stramski Gentili DOrtenzio 2012 p. "/>