Role of fish (Mugilidae) in the organic matter
transfer between tidal mudflats and coastal waters through biofilm consumption:
in situ experimentation on spatial, seasonal and ontogenic variability.
Alexandre
CARPENTIER
Post-doctorant,
ANR VASIREMI program, 2008-2009
University of
La Rochelle
Avenue Michel
Crépeau
17042 La Rochelle, France
seconded to
MNHN
– CRESCO (Centre de Recherche
et d’Enseignement sur les Systèmes COtiers)
38, rue du Port Blanc
35800 DINARD, France
alexandre.carpentier@univ-rennes1.fr
Scientific advisor:
Eric FEUNTEUN: Centre de Recherche et
d’Enseignement sur les Systèmes COtiers
(CRESCO) - UMR BOME 5178, MNHN, CNRS, IRD, University of Paris VI, 38,
rue du Port Blanc, 35800 Dinard, France
Christine
DUPUY: Centre
de Recherche sur les Ecosystèmes Littoraux Anthropisés – UMR6217 - Unité Mixte
de recherche CNRS, IFREMER, University of La Rochelle ; Avenue Michel
Crépeau, 17042 La Rochelle, France
Keywords: foraging
behaviour, organic matter transfer, mudflat food-web, Mugilidae, Liza
aurata, Liza ramada,
Scientific Context
© E. Feunteun
The primary productivity of
intertidal mudflats sustains sedentary invertebrates which are in their turn
predated by a number of transient species, including mammals, birds and fishes.
Fish invade salt marshes during flood and forage there during the short
submersion period (from few minutes to a few hours according to the location on
the mudflat and the duration of the flood). Then they leave the mudflat at ebb,
it is assumed that at least a part of the ingested food is exported and the non
assimilated OM is deposited in marine coastal waters as faeces. Therefore,
these transient species participate in energy fluxes between adjacent systems:
they act as biotic vectors of
Mullets are
among the most common species of all marine waters in the world (Nelson, 1994)
and account for a fundamental protein resource for a number of human
populations living in coastal areas. In Mont Saint-Michel’s bay (
Liza
ramada and L. aurata are considered as “marine
estuarine dependent” species since they only occurred in estuaries, tidal mud
flats and tidal salt marshes from the beginning of spring until the beginning
of autumn, at temperatures of 14-21°C. It is hypothesized that they migrate in
offshore warmer water during the cold season (Claridge & Potter 1983,
Aprahamian & Barr 1985, Rosecchi & Crivelli 1995). It has been shown
that these different habitats play an important nursery role for these species,
young of the year dominating the sampled populations, exploited food sustaining
an important part of their growth.
Mullets are able to select smallest items (Thomson 1954, 1966, Almeida
1996). Wood (1953) assumes that 99% of bacteria and dinoflagellids associated
to sediments (among the smallest available preys) are absorbed by mullets. The
diatoms which occur in the mullets’ stomachs are mainly small (<100µm)
whereas larger diatoms (> 100 µm) are available and dominate the diatom
community of the creeks sediment (Brosse 1996, Radureau et al. 1999).
Similar conclusions are maid for mineral particles: a majority of 20 µm
diameter grain are present in the stomach contents, whereas 30 to 50 µm
diameter particles dominate in the sediment available marsh. Nevertheless % of
particulate organic matter ingested strongly varies according to site, season
and fish behaviour.
© P. Richard
Despite very short submersion periods, mullets filter and ingest
large quantities of sediments, concentrate and assimilate organic matter
produced by these coastal habitats which therefore act as key trophic areas for
these species. More than all the other teleosts dwelling estuaries in temperate
areas, mullets probably contribute significantly to the ecological functioning
of these systems because they use directly the primary production and are
involved in energy fluxes in and between coastal marine systems (Odum 1970,
Laffaille et al. 1998, Lefeuvre et al. 1999). Spatial and
temporal variation in exploitation of these habitats may reduce trophic
competition between species and between stages that have similar diets and
consequently favour their growth.
Understanding i) the
function of tidal mudflat and especially their effects on abundance, movements
and growth of principal fish species (mullets) and ii) the role of these fish
in the organic matter transfer through biofilm consumption, appear essential
for management of entire ecosystems such as Marennes Oléron or Mont Saint
Michel bays.
Aims
The consumption of biofilm-associated
micro-organisms and their consumers (e.g. meiofauna) by mullets will be
assessed, and their export from the intertidal zone to coastal waters will be
estimated for the Brouage site of
i)
Quantify spatio-temporal movements of mullets between mudflats and subtidal
areas (number and mass of mullets per hour and per m of net) according to
season (Summer and Autumn), site (2 sampled bays) and
mesohabitat (3 progressive seaward distances).
ii)
Determine individual behavioural and life history traits of mullets according
to their displacements and populations structures (age determination by scale
analysis) using otolithometry (microstructure/scanning electron microscopy) and
microchemical analysis for individuals life history
(ICPMS and laser ablation).
iii)
Estimate the amount of organic matter transferred by the fish from intertidal
to coastal waters through stomach content analysis (diet composition analysis
for organic matter, diatoms, foraminifers and nematods), treatments being
conducted in collaboration with C. Dupuy (
Secondary objectives depending on
preliminary results and time available could be envisaged:
iv) Identify the contribution of this food consumption to
the growth of mullets using energy models applied in accordance with the
identified foraging patterns by a complementary approach aiming to investigate
the contribution of these diet items to mullet growth using stable isotope (13C
and 15N) analysis of fish and prey items. Stable isotope analysis will be
performed in collaboration with P. Richard (
In a second time, specific behaviour, functional role
and interspecific competition for food resources between different mullet species
(Liza ramada and Liza aurata) would be assessed, depending on the
possibility of taxonomic distinction (young of the year).
These in situ results will be compared and
completed with parallel experimental studies conducted by C. Lefrançois
(University of La Rochelle) on grazing pressure of mullets on reproduced
mudflats natural condition in mesocosms.
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