International Symposium on Isotopes in Hydrology, Marine Ecosystems, and

Climate Change Studies, 27 March - 1 April 2011, Monaco

Eutrophication in marine Environments as seen from an N-Isotope Perspective

Kay-Christian Emeis

for

Kirstin Dähnke, Astrid Deek (Johannsen), Vivian Mara, Jürgen Möbius,

Johannes Pätsch, Tim Schlarbaum, Alexandra Serna

2

The problem: Too much of a good thing….

Human well-being is closely

linked to the availability of

reactive nitrogen (rN).

Artificial sources now begin to

exceed natural sources of rN

on a global scale….

modified from

http://ian.umces.edu/loicz/

In spite of effective rN cycling in

natural environments in land- and

fresh water ecosystems, the

residue is transported to the coastal

ocean, leading to eutrophication

after Millennium Ecosystem

Assessment, 2005

3

d15N (‰ vs. air N2) in

surface sediments

(0-1 cm)

d15N (‰) ranges

Sediments in seas around Europe

bear an imprint of dominant sources

of rN:

agriculture/live stock farming,

industry

N2 fixation…(?, see below)

advection& assimilation of deep-

ocean nitrate

An isotopic fingerprint of eutrophication

4

Eider (1 kt NO3- y

-1)

Elbe (82 kt NO3- y

-1)

Weser (40 kt NO3- y

-1)

Ems (6 kt NO3- y

-1)

Rhine (220 kt NO3- y

-1)

North Sea/

German Bight

Case 1: Industrialised river catchments

cores

Johannsen et al., 2008

Method: Denitrifyer Method

to convert

nitrate/nitrite/DON to N2O

5 (Deek e

t a

l. s

ubm

., S

chla

rbaum

et

al. s

ubm

.)

Nitrate loads and isotopic fingerprints

summer

winter

High loads @ low d15N in

nitrate in winter

(leaching from soils,

mainly nitrified organic

fertilizer)

Low inputs @ high d15N of

nitrate in summer

(assimilation in rivers,

denitrification)

Annual load-weighted

average d15N of nitrate

~8.5 ‰ vs air N2

6

Relation to land use

% urban and agricultural land-use in catchment

An

nu

al f

lux-

wei

gh

ted

d

15N

–N

O3 (

‰)

(Johan

nse

n e

t al.,

2008)

Reactive nitrogen in river loads draining industrialised

catchment is enriched in 15N as a function of land use

r2 = 0.71

9

Dähnke e

t al., 2

010

Nitrate/d15NO3 winter 07;

The coastal ocean

d15N of surface sediments (0-1cm)

Sern

a e

t al., 2

010

d15N of surface sediments reflects the original d15N signature of nitrate

that has been assimilated by phytoplankton. Part of the particulate N

(and ist isotopic fingerprint) is transferred to the sediment and is

recorded…..

10

Using sediment d15N to reconstruct past inputs

Observation

δ15N (‰)

HE347

1995 D

epth

(cm

) Year A

D

1986

1995

HE327

δ15N (‰)

1986

1995

HE329

δ15N (‰)

Sern

a e

t al., 2

010

How far does the

influence of river

nitrate extend?

What was the history of

eutrophication?

11

5 6 7 81300

1400

1500

1600

1700

1800

1900

2000

Core GeoB 4801

d15

N (‰)

ag

e y

r A

D

The long view

1986

1995

HE327

δ15N (‰)

Observation

Sern

a e

t al., 2

010

Dd15N = >1.5‰

Dd15N

What were pristine

conditions (river loads,

isotope signature) like?

12

Modelling the history of nitrate inputs

Reduced loads assumed for 1860 AD

• 28% modern atmospheric loads

• 10% modern river loads

4

3

7

6

5

d15N river nitrate = 3‰

d15N river nitrate = 5‰

Päts

ch e

t al., 2

010;S

ern

a e

t al., 2

010

Sediment records of

oxbow lakes

in the Elbe valley suggest

that the d15N

of nitrate at 1860 AD

was correctly estimated!

13

Summary case 1

Recent river loads of nitrate are enriched in 15N; the modern

“industrialised“ estuaries have no attenuating effects on nitrate

discharges from land to the coastal ocean.

Sediments of marginal seas bordering “industrialised“ rivers exhibit

high d15N values.

Dated sediment records trace the history of eutrophication and

can be used as targets for modeling efforts to establish pristine

conditions.

14

Case 2: The eastern Mediterranean nutrient desert

Some Eastern Mediterranean

Sea puzzles….

• Oligotrophic (6-12 gC/m2/a)

• very high N/P (N*) ratio (>28) in

deep waters

d15N in nitrate, sinking particles and

surface sediments are all

unusually low

• Diazotrophic N2-fixation? But why

should it occur?

d15N (‰ vs. air N2) in

surface sediments

(0-1 cm)

15

The role of atmospheric transports

Atmospheric NOx

(mg/m2/a)

(EMEP)

Mara

et

al.,

2010;

Kro

m e

t al.,

2011

109 Mol NO3

The reactive N-cycle in the eastern Mediterranean Sea is

dominated by atmospheric inputs…

16

…and their isotopic composition

d15NO3 of atmospheric input

(flux-weighted d15N = -3.1‰)

is unusually low!

Mara

et

al.,

2010.

The isotope anomalies of the EMS may be accounted for by the mass

and unusual isotopic composition of atmospheric NOx inputs!

No need for N2 fixation!

monthly averages 2006/2007

17

d15NrN = 3.3 ‰

d15N-NO3 = 1.8‰

Evidence and processes

Em

eis

et al.,

2010

d15NrN = 1.5 ‰

d15N-NO3 = 2.4‰

d15NrN = 3.2 ‰

d15N-NO3 = 2.2‰

18

The modern N-cycle in the eastern

Mediterranean Sea is not a key to the past!

19

Available data suggest that unusually low d15N-NO3 of the deep

nitrate pool in the EMed reflects (continuing) atmospheric NOx input

This source is large and is transferred to deep waters (and into the

sediments) by rapidly sinking organic matter, which is to a large

extent ammonified and nitrified in intermediate (export to western

Mediterranean Sea) and deep waters

The N-cycle of the modern EMS is not in a pristine state!

Summary case 2

20

Stable N-isotope studies enable quantification of and attribution to

sources and processes in regional N-cycles

Hindsight studies and quantification of conditions prior to

observation are feasible due to preservation of the isotope signal in

sediment (or other) archives

Together, isotope data and models are a powerful tool in

establishing environmental baselines of pristine (pre-industrial)

conditions in estuarine and coastal ecosystems.

Many marine environments may not be what they seem to be!

Conclusions