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Eutrophication 2.1
Biogeochemical cycles
Alice NewtonP. Viaroli
Important nutrients N:P:Si~ Ratios N:P and N: Si are especially
important
~ N is usually limiting nutrient in coastal waters and estuaries. Can be fixed by cyanobacteria
~P most important in freshwater lakes.Anoxic sediments release P
~ Si important for phytoplankton composition (diatoms)
Redfield ratio N:P 16:1
~N limited when <16:1
~P limited when >16:1
~Range of 10 to 25 is “normal”
~N:P in sewage, manure and fertilizers is different from Redfield ratio.
N:Si 1:1~ Upstream eutrophication in rivers traps Si
in sediments before it reaches estuaries
~ Dams: trap Si
~ Si availability controls diatom growth
~ Decrease in Si relative to N & P linked to changes in phytoplankton community and HABs
Nitrogen cycle
-3 -1 0 +1 +2 +3 +5
(red-ox) N-cycle Biochemical pathways
AnAmOx
nitrification
Dissimilative Nitrate Reduction to Ammonium
denitrification
Assimilative reduction
N-fix
AAR
P. ViaroliP. Viaroli
Nitrification total: NH4+
+ 2O2 NO3- + H2O+ 2H+
NO3- NO2
- NH2OH .NO N2O N2
NH4+
Nitrification1:NH4
+ + 0.5 O2 NH2OH + H+
NH2OH + O2 NO2 - + H2O + H+
Nitrification2:NO2
- + 0.5 O2 NO3 -
Nitrogen fixation N2+6e-+ 6H+ 2NH3
Denitrification
(+OM, -O2)
Nitrification
(+O2)
P. ViaroliP. Viaroli
Ammonium-ammonia equilibrium NH3 + H3O+ NH4+ + H2O pKa =
4.75
NH4 +
NO2 -
NO3-
N2
N - phytoplankton N in zooplankton N-fish
N-detrital
N - sedimentary
Nitrogen cycling in pelagic waters
(plankton-dominated)
P. ViaroliP. Viaroli
N in benthos (zoo and phyto)
Biology of N
~ Gaseous N2 not useful to most photosynthesizers
~ N fixation~ eg Trichodesmium can produce NH4 from N2
~ Microbes and BG algae may form NO2 and NO3
~ NH4, NO2 and NO3 can be used as nutrients by photosynthesizers
~ Proteins in Organic matter are excreted or decompose as NH4
N org .NH4
+NO2
-NO
3-
Nitrification
Nitrosomonas, Nitrobacter
N2
ONO
N2
Ammonification
Denitrification
Pseudomonas,Thiobacillus
Anoxic horizon
oxic horizon
waterwater
DNRA
AtmosphereN2
Nitrogen fixation
Cyanobacteria
P. ViaroliP. Viaroli
N-cycle in shallow waters
dark dark
light
light
Denitrification (mmol m-2 h-1) in a Ruppia meadow
Denitrification from water nitrate Coupled nitrification-denitrification
Den
it. D
ark
(µm
ol m
-2h-
1)
0
100
200
600
800DnDw
Den
it. L
ight
(µm
ol m
-2h-
1)
0
100
200
Den
it. D
ark
(µm
ol m
-2h-
1)
0
60
120
180
DnDw
Den
it. L
ight
(µm
ol m
-2h-
1)
0
60
120
180
240
J F M A M J J A S O N D J F MJ F M A M J J A S O N D J F M
Bare sediment Sediment+Ulva
light
dark
Denitrification rates determined with dark and light incubations
Quantifying the N cycle~ N cycle:
~ Natural Sources of N~ Anthropogenic sources
Natural Sources
~ Lightening fixation 5-10Tg pa
~ Natural N fixation (non crop) 90-140 Tg p.a.
~ Marine fixation 30-300 Tg pa???
(Teragrams = 1 million metric tonnes)
Anthropogenic sources of N~ Industrial fixation inc Fertilizer 80Tg of N pa (NH3 & N2O)
2020 projection 134 Tg pa ~ Agricultural Legume Fixation
32-53 Tg pa~ Fossil fuels 20 Tg pa (NO & NH3)
2020 projection 46Tg pa~ Forests Burning 40 Tg pa
(NO, N2O & NH3 )~ Loss of wetlands (denitrifying)
10 Tg pa~ Land clearing for crops 20 Tg pa~ Domestic and Animal Waste
32 Tg pa (NH3)
1996 Total annual anthropogenic N inputs ~140Tg (Teragrams = 1 million metric tonnes)
see text below
Human alteration to N cycle~ N has doubled in 50 years
(C has only increased 10%)~ 80 Tg of N pa applied as fertilizer ~ 174 kg/ha/pa Xs ~ Impacts include:
~ Increase N2O, (nitrous oxide, a greenhouse gas), due to burning of fossil fuels
~ Increased NO (nitric oxide, photochemical smog formation)
~ Acidification of soils and freshwater~ Erosion & leaching of N to estuaries and coast
(Teragrams = 1 million metric tonnes)
Changes in N cycleRevised Kates et al. (1990).
Human-Caused Global N- Emissions
Atmospheric deposition
NOx and NHx in the Atmosphere
Origins~Domestic combustion ~Industrial processes~Traffic~Agricultural sources
~Animal housing ~Spreading of manure
Atmospheric Deposition of N in the North Atlantic Ocean
~ AD-N to the NAO basin arises from pollution sources in North America and Western Europe
~ Sources have increased drastically (5-10-fold) since the Industrial Revolution and continue to increase in both geographic and depositional magnitude.
~ AD-N flux (11.2 Tg N yr-1 ) accounts for 46-57% of the total "new" or anthropogenic nitrogen flux to the NAO.
(Teragrams = 1 million metric tonnes)
Transfers of nitrogen~ N fixed in industrial areas~ N transported to agricultural areas
~ N applied to fields, some retained in crops~ N loss to atmosphere and water~ Crops transported to livestock producing
areas and cities
~ Crops consumed in cities and N enters sewage
~ Animal feed crops consumed in livestock farms ~ Livestock transported to cities~ Manure spread on fields, enters atmosphere
and water
Industrial areas
Agricultural areas
Atmosphere
Aquatic environment
Livestock area
Cities
Natural transfers
~ Sea Birds and guano
~ Salmon migration and death
N input into Aquatic
SystemsModified from Howarth et al. (1996)
NO3 in major EU
rivers 1980-95
Nitrate concentrations have been largely unchanged since 1980
EEA
NO3 in EU coastal waters 1985-98
P-Cycle~P most important in freshwater lakes
~P limitation has been documented in coastal waters and estuaries:
~Apalachicola (Gulf of Florida)~some Dutch estuaries~Tropical systems with carbonate sands
~ P is released from anoxic sediments
~N-fixing cyanobacteria proliferate when P is abundant, e.g. in the Baltic sea
P-Cycle
P.Viaroli
Primary producers
P-Refractory
External load
burial/early diagenesis
PO4
Clay
Fe P
Ad
-so
rpti
on
De-so
rptio
n
Assimilation Decomposition
Organic Detritus
Precipitation(es:apatite, hydroxiapatite)
Assimilation
Ca P
P-cycle
P. ViaroliP. Viaroli
Human alteration to P cycle
~ 600 Tg applied as fertilizer from 1950 to 1995
~ ~250 Tg of P harvested as crops
~ ~50 Tg used as feed crops returned to soil as manure
~ Net XS addition 400 Tg in 45 years, ~10 Tg p.a.
(Teragrams = 1 million metric tonnes)
Sources of P in EU
1988-96
P sources in the UKEA UK
P in major EU rivers 1980-95
Phosphorus concentrations in some EU rivers have fallen since the mid-1980s, particularly in the largest and most polluted rivers. EEA.
P in coastal waters 1985-98. EEA
NO3 & PO4 in EU coastal waters, 1985-1996Nutrient concentrations in coastal waters show little overall improvement