Figure 3: Oxygen saturation
percentage measured inside and just above a litter accumu-lation (40 cm thick) using 2 Op-todes during 3 consecutive
very calm days in August 2011. For Optode methodology ap-plied in this seagrass meadow, see Champenois & Borges, Limnology & Oceanography (2012), 57(1), 347-361. C o n ce n tr at io n s (µ M ) Oxy ge n s at u ra tio n (%) 0.0 0.2 0.4 0.6 0.8 1.0 -120 -90 -60 -30 0 30 60 90 120 3 4 5 J J A S O N D J F M A M J J A S O N D 2010 2011 Saturation Water column C o n ce n tr at io n s (µ M ) Oxy ge n s at u ra tio n (%) 0.0 0.2 0.4 0.6 0.8 1.0 -120 -90 -60 -30 0 30 60 90 120 3 4 5 J J A S O N D J F M A M J J A S O N D 2010 2011 Saturation
Water just above the litter
C o n ce n tr at io n s (µ M ) Oxy ge n s at u ra tio n (% ) 0.0 0.2 0.4 0.6 0.8 1.0 -120 -90 -60 -30 0 30 60 90 120 10 20 30 Nitrates
Ammonium Phosphates Oxygen J J A S O N D J F M A M J J A S O N D 2010 2011 Saturation Litter water C o n ce n tr at io n s (µ M ) 0 2 4 6 8 10 20 40 60 J J A S O N D J F M A M J J A S O N D 2010 2011 Interstitial water O xy ge n s at u rat io n ( % ) 0 50 100 150
Water in the litter
Water just above the litter
2011.08 .15 15 h00 2011.08 .16 00 h00 2011.08 .17 00 h00 2011.08 .18 00 h00 Figure 2: Oxygen saturation percentage and nutrient concentrations measurement in the water column, just above and inside the litter accumulation and in the interstitial water under Posidonia oceanica litter.
Oxygen have been measured using a Winkler methods adapted for small volumes (10 ml) and nutrient have been measured by spectrophotometry using SKALAR analytic chain with method and tubing adapted for oligotrophic seawater (R. Biondo) (n = 12 per point, except Interstitial water, n = 6).
Figure 1: Posidonia oceanica litter accumulation outside the seagrass meadow at 10 m depth in the Revellata Bay (Corsica,
Mediterranean Sea). a. Large patch (about 200 m2) of fragmented dead leaf and uprooted green Posidonia shoots; b. Larger dead leaves; c. water sampling using tri-dented syringe inside the litter for O2 and nutrient
concentrations measurements; d. Litter monster. a
b
c d
Take home message:
P. oceanica phytodetritus form large ephemeral accumulations outside the seagrass meadow,
These accumulations, heterogeneous in their composition, are present in all season, but are important in autumn,
Water inside the litter encompassed a large range of O
2and nutrient concentrations, but may be depleted in O
2and
enriched in nutrient compared to water column,
This particular environment could be an interesting natural model to study hypoxic effect on biodiversity.
Aim:
By repetitive sampling in different litter accumulations occurring in the Revellata Bay (Calvi, Corsica) (2009-now), this preliminary study aimed todescribe the cycle of litter accumulation occurrence, to determine their composition, to describe oxygen and nutrient concentrations inside the litter.
Acknowledgments: This study is financed by a FRS-FNRS collaborative project (FRFC FRFC 2.4511.09 ). G.L and A.V.B. are Research Associate at FRS-FNRS. T.M. and F.R. are granted by PhD scholarships from FRIA. We are grateful to STARESO crew for their welcome and to Renzo Biondo for his technical assistance and methods adaptation for O2 and nutrient measurements. Corresponding authors E-mail : G.Lepoint@ulg.ac.be. Authors bibliography is available on
institutional repository: http://orbi.ulg.ac.be/
Introduction:
*The natural life cycle of the seagrass Posidonia oceanica involves senescing of leaf biomass and seasonal massive leaf fall,
*Such phytodetritus exported outside the seagrass meadow may constitute big submarine accumulations (fig. 1),
*This particular habitat is colonized by various organisms which
participate to the degradation and to the seagrass carbon transfer trough detritus foodweb,
*These accumulations are subject of intense remineralisation and degradation processes.
A descriptive study of physico-chemical characteristics of
Posidonia litter accumulation
Gilles Lepoint, Alberto V. Borges
2, François Darchambeau
2, Patrick Dauby
3, Thibaud
Mascart
1, François Remy
1, Willy Champenois
21
MARE center, Laboratory of Oceanology, University of Liège, Belgium,
2
MARE Centre, Chemical Oceanography Unit, University of Liège, Belgium
3
MARE Centre, Animal Systematics and Diversity Laboratory, University of Liège, Belgium
ISBW10 Buzos
(Rio de Janeiro) November 2012
Results I: Litter accumulation occurrence and composition
Big accumulation of dead leaves were only found in fall (average thickness: 30-50 cm), but persisted till winter (average thickness: 5 - 10 cm),
Litter accumulation may be found in all seasons, but in spring and early summer, macroalgae wracks and uprooted P. oceanica shoots contributed significantly to
accumulation biomass,
Photosynthetic active material was found in litter accumulations (macroalgae as wrack or epiphytes and diatoms as epiphytes).
Results II: Oxygen and nutrient concentrations
*oxygen: Wateri nside the litter showed high variability in oxygen concentrations
(from saturation to anoxy) (fig. 2) ; oxygen depletion was not measured in the water just above the litter (fig. 2) ; anoxia appeared rapidly during the night and period of low water movement (fig. 3),
*nutrients: Litter was generally enriched in NH4+ and PO4 and depleted in nitrates
when compared to oligotrophic water column (fig. 2) ; nutrient enrichment was not measured in the water just above the litter (fig. 2).
Figure 4: Conceptual model
Results III: Conceptual model
When and where water movements are low,
litter accumulations act as a permeable barrier between sediment and water column (fig. 4) and:
Nutrient (and DOC) coming from sediment or produces by remineralisation inside the litter are
tem-porarily trapped inside (and consumed by associated heterotrophic and autotrophic organism),
Nitrification, oxidation of reduced compound (e.g. H2S) and respiration by decomposing organisms
and litter fauna consume oxygen, driving sometimes to anoxy,
Photosynthesis occurs due to the presence of macro and micro epiphytes and of active P. oceanica
and macroalgae wrack, but it is not enough to compensate respiration due to decomposition,
Passage to oxic and low nutrient condition inside the litter are due to physical disturbance of litter
