Eutrophication 2.1 Biogeochemical cycles Alice Newton P. Viaroli

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)


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)


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


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


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.


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


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.


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

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Eutrophication 2.1 Biogeochemical cycles Alice Newton P. Viaroli