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Who would want to live in there? A history of Posidonia oceanica detritus

accumulations, the associated invertebrate

community, and its food web…

F. Remy, L. Michel, F. Darchambeau, T. Mascart, M. De Troch,

N. Sturaro, G. Lepoint

[email protected]

Herbivory

Exported

litter

in situ litter

Foliar primary

production

General framework

50-95 %

« banquettes »

What is exported P. oceanica litter ?

Exported litter

accumulation

General framework

1m

Exported litter

accumulation

What lives in P. oceanica litter?

> 10 cm > 500 µm > 38 µm

General framework

Main objective

General objective: characterize the exported P. oceanica litter macrofauna community, assess

its dynamics and trophic ecology in Calvi Bay

Community characterization

• General patterns

• Seasonal variation

• Environmental parameters

Trophic web characterization

• General patterns

• Seasonal variation

• GUT + ISOTOPES

© A. Abadie

Characterization of the

exported P. oceanica litter

macrofauna community, its

dynamics and trophic

ecology

1. Community

General technical aspects

Community characterization:

2010-2012: seasonal sampling at 2 sites

Standardized sampling

Identification to the specific level

General community pattern:

~20.000 identified individuals

115 species (1/3x meadow)

High abundance (up to 5100 ind.m-2 2x meadow)

Arthropods (especially amphipods) are highly

dominant

Important dominance : 19 species = 90%

One species to remember : Gammarella fucicola

(~40%)

Re

lati

ve

ab

un

da

nc

e (

%)

0

5 0

1 0 0

A rth ro p o d a

A n n e lid a

M o llu sc a

E c h in o d e rm a ta

N e m e r te a

P la ty h e lm in th e s

C h o rd a ta

Habitat dynamics influence the macrofauna


General community pattern:

0,00

0,50

1,00

1,50

2,00

2,50

3,00

3,50

4,00

4,50

5,00

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

SPRING SUMMER AUTUMN WINTER SPRING SUMMER AUTUMN WINTER SPRING

2010 2011 2012

ind

.gD

M-1

Arthropoda

Annelida

Mollusca

Echinodermata

Nemertea

Platyhelminthes

Chordata

Temporal variations


0

500

1000

1500

2000

2500

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

HA

RB

OR

OS

CE

HA

RB

OR

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HA

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OR

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HA

RB

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HA

RB

OR

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CE

SPRING SUMMER AUTUMN WINTER SPRING SUMMER AUTUMN WINTER SPRING

2010 2011 2012

gD

M.m

-2

A variable habitat: litter biomass


Water column

Water Just Above Litter

Water Inside Litter

Interstitial Water

O2 Nutrients Variability Oxygen and nutrients

O2 is not the only influencing parameter, but is a very

interesting one


Summary

Seasonal variations : community AND environment

Community not diverse but abundant and dynamic

Litter biomass / composition seasonal pattern

Seasonal changes in O2 concentration inside the litter

Litter accumulations are highly dynamic habitats

supporting an abundant and variable community

of macro-invertebrates

© A. Abadie

Characterization of the

exported P. oceanica litter

macrofauna community, its

dynamics and trophic

ecology

2. The food web

What about dietary habits?

Eating detritus? Hey…what did you expect?


Trophic ecology: two different approaches.

Gut content examination

• Pros: easy, fast, visual observation

• Cons: snapshot, ingestion ≠ assimilation, identification

Stable isotopes

• Pros: long term integration, assimilation, trophic level

• Cons: time (and money) consuming, isotopic separation, need the

sources

δ13C (‰)

+0-1‰

+0-1‰

More negative Less negative


What use in trophic ecology? “You are what you eat,

plus (or minus) a few

permill…” (DeNiro & Epstein 1976)

Fractionation

C = source tracer

N = trophic level


Food web characterization:

2011-2012: seasonal sampling at the 2 sites

Non-standardized hand sampling (50L sealed bags)

Identification to the specific level

Individual

Gut content Acidification

Drying

EA-IRMS

SIBER SIAR

Gut contents speciesResemblance: D1 Euclidean distance

SPECIESGF

GA

MH

PX

AN

GI

HL

IB

SL

LH

NGU

GSPP

LN

NS

2D Stress: 0,05Increased

Animal

proportion

Algal

dominance

Dead seagrass

dominance

Gut contents Gammarella fucicola

Melita hergensis

Gammarus aequicauda

Nototropis guttatus

Nebalia strausi

Athanas nitescens

Palaemon xiphias

Gobius spp.

Galathea intermedia

Food sources dynamics influence the macrofauna

nm-MDS

+

ANOSIM

-25 -20 -15 -10

-20

24

68

δ13C

δ1

5N

Gf

Ga

Mh

Ngu

Ib

Sl

Ac

An

Px

Pe

Hl

Ml

Ln

Lh

Gi

Ns

Pspp

Br

GsppDead

leaves

Living

leaves Epiphytes

/algae

SPOM

SIBER ellipses Gammarella fucicola

Melita hergensis

Gammarus aequicauda

Nototropis guttatus

Nebalia strausi

Athanas nitescens

Palaemon xiphias

Gobius spp.

Galathea intermedia

Bittium reticulatum


SIAR model summary

DLLLESPOMRMA C

MIXED consumers(GFMH)

DL consumers(GA)

OMNI consumers

Predator 1(PX+PE+LN)

Predator 2(Gspp)

Global food web:

Different trophic levels

Different dietary preferences

Dead leaves « signal » transferred

to the top of the food web

Litter macrofauna are

litter fragmenters,

consumers and assimilators


Synthesis

Macrofauna food web 3-4 trophic levels with different diets

Detritus are a non-negligible food source

BUT…

Isotopic compositions of are highly variable seasonally

Sometimes linked to diet modification (sometimes only baseline shift…)

Impact of food source availability


Thank you for your attention

The authors warmly thank the STARESO field station staff for their

support during the sampling campaign.

This study was conducted with funding from a F.R.I.A. grant of the FRS-

FNRS.

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