The use of Eco-Exergy in Oceanology: Application to Posidonia oceanica meadows

THE USE OF ECO-EXERGY IN

OCEANOLOGY:

APPLICATION TO POSIDONIA

OCEANICA MEADOWS

Dorothée Pête, Branko Velimirov & Sylvie

Gobert

Introduction

 Exergy = « Useful work a system can perform when

brought into equilibrium with its environment » (Szargut et al., 1988)

= distance from thermodynamic equlibrium

 Applying this theory to understand ecosystems and

to detect environmental perturbations

What a mystification!

It’s metaphysics!

Are you crazy?

Thermodynamic theory for Ecosystems

(S. E. Jørgensen)

Thermodynamic

equilibrium = Inorganic soup

Take energy in: Matter Storage in biochemical constituents

Thermodynamic theory for Ecosystems

(S. E. Jørgensen)

 Trends to keep away from thermodynamic equilibrium when becoming more complex (Prigogine, 1980)

Loose energy:

Matter Maintenance Trophic webs

Exergy index or eco-exergy: a practical

way to apply the exergy theory to

ecosystems

• Ex (kJ/volume or surface)

= distance between the system and the thermodynamic equilibrium

  when the ecosystem is moving away from thermodynamic equilibrium

 when the ecosystem is getting closer from its climax, its ecological optimum

= « work capacity possessed by organisms and ecological networks of organisms due to biomass and information embodied in their genome and the amino acid sequence of proteins » (Jørgensen et al., 2010)

i n i i

C

Ex







.

-β-factor of the ith organism

-defined on a genetic basis: enzymes and proteins, defined by DNA, are driving life processes (Jørgensen et al., 2005)

- kind of approximation of organisms complexity

- higher for « specialised » organisms

- expressed relatively to detritus (no genetic information, only free energy of the organic matter, ≅ 18,7 kJ.g-1)

- ex: β = 1 for detritus, 8,5 for bacteria and 133 for nematods

Exergy index or eco-exergy

Formula:

Specific exergy





C

Ex

Ex

Ex = informations on the capacity of the ecosystem

to develop and get more complex

Use of Ex and Ex

in Oceanology

• 2 main uses:

- Modeling of ecosystems development (plankton dynamics)

Can we use them to detect a

perturbation in a marine ecosystem

early?

Basis for food webs Spawning and breeding

ground Hydrodynamic protection Stabilizatio n of the bottom Trapping of suspended particules

• Focus ecosystem = Posidonia oceanica meadow - What?

Posidonia oceanica = endemic seagrass of the Mediterranean Sea

• Focus ecosystem = Posidonia oceanica meadow

- Why?

P. oceanica = descriptor of the quality of the Mediterranean

coastal zone

University of Liege:

- Tradition of marine research (Biology, chemistry, physics, modeling)

- Research station in Calvi Bay, Corsica: STARESO (STAtion

de REcherche Sous-marine et Océanographique)

- Years of experience in the Mediterranean Sea with

a special focus on the Posidonia oceanica ecosystem

Can we use them to detect a

perturbation in a marine ecosystem

early?

In Calvi Bay, pristine and perturbated meadows are well known.

Can we use them to detect a

perturbation in an ecosystem early?

 Posidonia oceanica meadow has a low turnover.

 Sediment = final container of pollutants (sedimentation)

 Microbenthic loop: organic matter (OM), microphytobenthos (microscopic algae), meiofauna (microscopic animals), bacteria

 Important sub-system in P. oceanica meadows  High turnover

Goals

 Clarification and validation of the use of Ex and Exsp as

descriptors of anthropogenic perturbations in P. oceanica beds

 Effects of nutrients and organic matter inputs which are the main perturbations in the Mediterranean coastal zone

 New method to measure and detect perturbations affecting P. oceanica meadows

Sampling

What?

- sediment cores (vertical profile)

- Biomass determination for

every component of the

microbenthic loop.

- sediment and environment parameters

How to validate an index and a method?

• Spatial heterogeneity at small scale

• Comparison between a pristine and a perturbated site • In situ experiments

Sampling sites

Spatial heterogeneity

 3 grids

 March, June,

November 08,

March 09

 12 nodes/grid

(uniform random)

 3 cores/node

Results : DIVA analysis

Spatial heterogeneity : Estimation of Ex &

Ex

• Important heterogeneity especially for the 1st _{cm of the}

sediment

Most dynamic slice, exchanges with the water column

BUT probably the most affected by environmental perturbations

• The less heterogenous slice is the 5-10 cm

Less dynamic slice and no exchanges with the water column Anoxic conditions for most samples

 BUT « old » sediment

Spatial heterogeneity : Ex & Ex

• Important heterogeneity in spite of the choice • No real seasonal variability

Seems stable along the year

STARESO vs. Fish farm:

• Awaited results for November 2008 only

 Not an estimation…

Median ± range

• No difference in Ex_sp

 No difference in biomass « quality » between sites

• This estimation is not able to catch the difference in EX between sites.

• In November 2008, Ex STARESO>Ex Fish farm

STARESO is closer from the ecosystem climax than the fish farm.

• No difference in Ex_sp.

No difference in the « complexity » of organisms living in the ecosystem.

 The ecosystem is able to adapt itself to this perturbation

In situ experiments: Sediment alteration

 Site: STARESO, 10 m depth.

 Duration: 3 months (from end of May to end of August 2009).  Alteration (mimic pollution by fish farms or dredging):

- 500 ml of sediment were added once a week on 21 marked points in a 3x3 m frame.

In situ experiments: Shading

 Shading ( in turbidity because of in nutrients concentration, fish farms, sewages, land farms):

- 3 nets (3x1 m, mesh size: 0,5 mm2) about 50 cm from the

canopy.

- Light extinction: 52 ± 1,6 %

In situ experiment : Ex

• No difference between periods Estimation?

 Too short experiment?

In situ experiment : Ex

• No real difference between periods

Estimation?

Conclusions

 Spatial variability

• Important heterogeneity BUT less important in the 5-10 cm sediment depth zone

Choice of the 5-10 cm sediment horizon to compare samples even if it is maybe less precise

 Fish farm vs. STARESO

• Ex STARESO>Ex Fish farm in November 2008 for the 5-10 cm horizon

Ex seems able to dicriminate both sites

 In situ experiment

• No difference along the experiment.

• Use of Ex and Exsp as a tool to detect perturbations in the

Mediterranean coastal zone is not easy to validate in this part of P. oceanica ecosystem.

• Important to link the results with environmental parameters to understant why it works or not.

• Work in progress…

Thank you!

Tanks to Loïc Michel, Renzo Biondo, Gilles Lepoint, Sylvie Gobert, Branko Velimirov, people of the STARESO, students, cleaning team, spreading team, repairing team,…