THE ROLE OF MACROALGAE IN

CARBON SEQUESTRATION

Dorte Krause-Jensen

Department of Bioscience & Arctic Research Center,

Aarhus University, Denmark

8. NORDIC SEAWEED CONFERENCE

- SEAWEED AND SUSTAINABILITY

Grenå

10-11 October 2018

Photo: Greenland kelp forest by Scott Bennett

Photo: Dorte Krause-Jensen Photo: Núria Marbà

Seagrass meadow

Marine forests

Photo: Peter B. Christensen

Kelp/macroalgal forest

Mangrove Saltmarsh

Photo: Stein Frederiksen

OOceab

Seagrasses & macroalgae

• High biomass, slow

turnover

Similaries to forests on

land in terms of serving as

C-sink

Marine macrophytes as a global C-sink (Smith 1981, Science)

O

S

R

The “Blue Carbon” concept“Out of all the biological carbon (or green carbon) captured in the world, over half (55%)

is captured by marine organisms hence it is called blue carbon”

Seagrass, mangrove and saltmarsh cover <0.5% of the sea surface,

yet represent ~50% of C-burial in marine sediments

Nellemann, C., Corcoran, E., Duarte, C. M., et al. (Eds). 2009. United Nations Environment Programme, GRID-

Arendal, www.grida.no

Ecosystem services of marine forests

Gutierrez et al. 2011

McLeod et al. 2011 Front Ecol Environ

C burial of marine forests much faster than of land forests

… coastal wetlands including mangrove

forests, tidal marshes and seagrass

meadows

Photo Credit: Oceana / Juan Cuetos

And macroalgae ??

Macroalgal forests - the most

extended marine forests

mangroves

salt marshes

Ecosystem

Macroalgae

Seagrasses

Mangroves

Salt marshes

Area (mio km2)

3.4* (1.4-6.8)

0.2-0.6

0.14

0.02-0.4*Krause-Jensen & Duarte 2016, Duarte 2017 Biogeosciences

kelp forests Ste

ne

ck

et a

l. 2

00

2. E

nv.

Con

s.

seagrasses

Macroalgal forests – the globally most productive marine forests

Ecosystem

Macroalgae

Seagrasses

Mangroves

Salt marshes

Local net prim. prod.

(g C m-2 yr-1

91-522

394-449

394-1000

438-1100*Krause-Jensen & Duarte 2016. Nature Geoscience. Duarte 2017 Biogeosciences

Global net prim. prod.

(Pg C yr-1)

1.52* (0.13-2.9)

0.06-1.94

0.05-0.15

0.17-0.42

Fate of macroalgae

Duarte & Cébrian 1996. Limnol. Oceanogr.

Buried in algal bed

0.4%

Grazed

33.6%

Remineralised

37.3%Exported

43.5 %Ocean C

sinks?

Locations where macroalgal carbon

storage has been reported

Krause-Jensen & Duarte 2016

Macroalgae on the deep sea floor

Sargassum, Sargasso Sea, 5310 m depthSchoener & Rowe 1970. Deep-Sea Research

Sargassum in guts of deep sea fauna down to 6475 m Wolf 1962. Galathea Reports

Baker 2017. Deep-

Sea Research part II

Estimated

Sargassum biomass

in the deep

sea:0.07–3.75 g fw

m-2

Kra

use

-Je

nse

n &

Du

art

e 2

01

6 N

atu

re G

eo

scie

nce

Krause-Jensen & Duarte 2016

NPP

1521

1020-1961

Unit: TgC yr–1

Macroalgal C-flow

Krause-Jensen & Duarte 2016

NPP

1521

1020-1961

Unit: TgC yr–1

Macroalgal C-flow

Krause-Jensen & Duarte 2016

Unit: TgC yr–1

NPP

1521

1020-1961

Macroalgae support globally important C

sequestration

Ecosystem

Macroalgae

Seagrasses

Mangroves

Salt marshes

Global C-burial rate

TgC yr-1

173 (61-268)*

48.0-112

22.5-24.9

4.8-87.3Duarte et al. 2013. Nature Climate Change, *Krause-Jensen & Duarte 2016. Nature Geoscience.

**Duarte & Krause-Jensen 2017 Export from seagrass meadows.

Contribution by export to C-sinks outside the vegetated habitat

➢ Double the previous C-burial estimate of marine forests

+

30%**

+?

+?

Many shades of macroalgal Blue Carbon

The evolutionary diversity of macroalgae affect

functions and fate of macroalgal C

Traits increasing the likelihood of C-sequestration:

• Recalcitrance

e.g. kelps export 82% of PP vs average 43%

• Low light requirements -> deep, extended pop.

• Large global production

• Capacity for long-distance transport (boyancy)

Biology Letters. 2018

• Macroalgae in BC strategies remain controversial

• De facto recognized - as C-donors to angiosperm BC habitats

• China & Korea have included macroalgae in BC

• Criteria for including macroalgal C in BC✓ Extensive and actionable

✓ Verified Carbon Standard (VCS): GHG emission reduction/removal

must be ‘real’, ‘measurable’, ‘permanent’, ‘unique’ and ‘additional’.

• Key challenges identified

Review (21 papers on macroalgae in BC context)

- e.g. as buffers against ocean acidification

Erling Svensen

Climate change adaptation potentials

Delille et al. 2000

Macrocystis forest, Southern Ocean

Inside/outside seaweed habitat

High Arctic - Disko Bay (24:0) Jun 16-25, 2014

Frida Lindwall

Slope ~10 ppm d-1

0

50

100

150

200

250

300

350

400

12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00

10

12

14

16

18

20

pC

O2

O2

con

cen

trat

ion

(m

g l-1

)KELP FOREST - 24 H PHOTOPERIOD

pCO2

O2

Slope0.015 units d-1

0

100

200

300

400

500

600

700

800

900

7.95

8

8.05

8.1

8.15

8.2

8.25

8.3

8.35

12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00

Ligh

t (µ

mo

l ph

ot

m-2

s-1)

pH

Light

pH

Krause-Jensen et al. 2016

Marine forests are under pressure

Ecosystem

Macroalgae

Seagrasses

Mangroves

Salt marshes

Tropical forests

Global loss rate

(% yr-1)

0.018*

0.9

1-3/0.07-1.17

1-2

0.5

Krumhansl et al. 2016 Waycott et al. 2009

Rates of loss and gain

SeagrassKelp forest

Duarte et al. 2013 and refs therein, *Krumhansl et al. 2016

Potentials for Climate Change

Mitigation and Adaptation by Seaweed

Aquaculture

Photo: Prof. Wu

SEAWEED FARMING AND BLUE CARBON

MITIGATION VIA: ADAPTATION TO:Ongoing processes:

Food production with reduced CO2 foot print

C-sequestration via export of “unseen” production

Future potentials:

Bioenergy productionsubstituting fossil fuels

Reduction of methane emission via seaweed feed additive to ruminants

Stimulation of land-based production

via seaweed biochar soil amelioration & seaweed prebiotic health benefits to

livestock

Climate benefit of circular nutrient management

Via avoidance of CO2 emissions for synthetic fertiliser production

Ocean AcidificationHigh daytime pH in seaweed to

the benefit of calcifiers

Increased storminess

and sea level riseShoreline protection via

dissipation of wave energy

Oxygen inputs to coastal

watersAvoiding ocean deoxygenation

with warming

Duarte, Wu, Xiao, Bruhn, Krause-Jensen. 2017

Conclusion & perspectives

• Macroalgal forests are hot spots of productivity

and key coastal habitats

• They support globally important C-sequestration

and their inclusion doubles previous Blue Carbon

C-sequestration estimates

• Macroalgal forests also have potentials in climate

change adaptation – e.g. by increasing pH

• Sustainable management of macroalgal habitats

support ecological quality and offers opportunities

to mitigate and adapt to climate change