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...algae and cyanobacteria) to total microphytic epilithic biomass (Chl a concentration of epilithon samples) were estimated from biomarker carotenoid concentrations using CHEMTAX version 1.95 software (=-=Mackey et al., 1996-=-). This method was described in details in Majdi et al. (submitted). Feeding selectivity was expressed was expressed as D (Jacobs, 1974). D is defined as: ( r − p) D = ( r + p − 2rp) Where r is the pr...

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...given in Leflaive et al. (2008) and Majdi et al. (submitted) respectively. Meiofauna are by definition motile benthic invertebrates that pass through a 500 µm sieve but are retained on a 42 µm sieve (=-=Giere, 2009-=-). They are considered as influencing microphytobenthic biomass, bacterial activity and organic matter dynamics through bioturbation, grazing and excretion (Borchardt and Bott, 1995; Montagna et al., ...

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...n epilithon dominated by heterotrophs and/or organic detritus. Otherwise, 100 < AI < 400 indicates that the epilithic community is balanced between heterotrophs and autotrophs (Ameziane et al., 2002; =-=Biggs & Close, 1989-=-). Data analysis tel-00741853, version 1 - 15 Oct 2012 Mann-whitney U tests, Spearman rank tests and simple linear regressions were performed with STATISTICA software (version 8.0, Statsoft inc., Tuls...

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...e main site of primary production (Ameziane, Dauta & Le Cohu, 2003). Epilithon has been increasingly investigated in fluvial ecosystems because of its important influence on biogeochemical processes (=-=Battin et al., 2003-=-) and its role in secondary production (Fuller, Roelofs & Fry, 1986), decomposition (Ford & Lock, 1987) and nutrient retention (Sauvage et al., 2003; Teissier et al., 2007). Meiofauna are motile benth...

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...oved when meiofauna are considered (Schmid-Araya & Schmid, 2000). Nevertheless, most benthic studies in riverine environment remain focused on micro and/or macrofauna (e.g. Feminella & Hawkins, 1995; =-=Lock et al., 1984-=-), thus leading Giere (2009) to plead recently that “the meiofauna ecology of large rivers requires urgent investigation”. Despite the richness of biotic interactions occurring in lotic epilithon, ava...

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...gal biomass were important factors influencing positively nematode and rotifer densities during both sampling campaigns (Table 5 and Fig. 4). Due to their high content of polyunsaturated fatty acids (=-=Phillips, 1984-=-) and the lack of cellulose in their cell walls, diatoms represent a high-quality food resource for primary consumers (e.g. Buffan-Dubau & Carman, 2000a; Goedkoop & Johnson, 1996). Diatoms also produc...

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...mid-Araya et al (2002) point out the importance of meiofauna as intermediates in stream food webs. Consequently the accuracy of the benthic food web is greatly improved when meiofauna are considered (=-=Schmid-Araya & Schmid, 2000-=-). Nevertheless, most benthic studies in riverine environment remain focused on micro and/or macrofauna (e.g. Feminella & Hawkins, 1995; Lock et al., 1984), thus leading Giere (2009) to plead recently...

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...nto account meiofauna generally focus on interstitial, neglecting epibenthic meiofauna (Robertson, Rundle & Schmid-Araya, 2000). So far, only a few studies underlined meiofauna 187ecology in lentic (=-=Hillebrand et al., 2002-=-; Höckelmann, Moens & Jüttner, 2004; Peters et al., 2007) and lotic (Gaudes et al., 2006; Mathieu et al., 2007; Sabater et al., 2003) epilithon. The long-term dynamics of meiofauna in lotic epilithon ...

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...are predators and/or parasites of freshwater insects (Di Sabatino, Gerecke & Martin, 2000). It is also known that insect larvae peak in density during summer because of appropriate water temperature (=-=Morin & Dumont, 1994-=-). Furthermore trichopter larvae can strongly reduce epilithon biomass by grazing especially when water temperature is high (Feminella & Hawkins, 1995; Hillebrand, 2009). Therefore, it would seem that...

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...These habitats could, as for macrofauna (Rempel, Richardson & Healey, 1999), favour recolonization of meiofauna after flood disturbance following a ‘refuge as habitat’ strategy (Golladay & Hax, 1995; =-=Robertson et al., 1995-=-). However, the resilience times observed after spring snow-melt flood were longer than after stormflows (Table 4). A possible explanation for this difference might be that snow-melt flood periods sho...

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...hon (e.g. bacteria, ciliates) over microalgae. This would be partly consistent with previous studies indicating that bacteria which are abundant and diversified in the epilithon of the Garonne river (=-=Lyautey et al., 2005-=-) are crucial early colonizers for the establishment of the epilithon after disturbances (Lyautey et al., 2010). It would be also in agreement with Hillebrand et al. (2002) who show that 197macrofaun...

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...l up to the seventh-order. During low-water periods, a high epilithic biomass can be found on pebbles and cobbles, favoured by low water velocities on the river bed (< 0.5 m s -1 ) and low turbidity (=-=Boulêtreau et al., 2006-=-; Leflaive et al., 2008). The study site was situated on one of these cobble bars located at 36 km upstream the city of Toulouse (01°17’53”E, 43°23’45”N; elevation 175 m a.s.l., sixth-order) (Fig. 1)....

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...of polyunsaturated fatty acids (Phillips, 1984) and the lack of cellulose in their cell walls, diatoms represent a high-quality food resource for primary consumers (e.g. Buffan-Dubau & Carman, 2000a; =-=Goedkoop & Johnson, 1996-=-). Diatoms also produce exopolymeric substances which consolidate the mucus matrix of the mat and stimulate bacterial growth (Underwood, Paterson & Parkes, 1995). Our study did not permit to separate ...

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...stion remains, if meiofauna partly disappeared by being ingested together with the epilithon, or by escaping knowing that meiofauna can actively drift between sediment and water (Smith & Brown, 2006; =-=Swan & Palmer, 2000-=-). Trichopter density peaked also during October (particularly in 2005 and 2008). However, in October, the potential related impact on epilithon and its associated meiofauna appeared less important th...

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... are retained on a 42 µm sieve (Giere, 2009). They are considered as influencing microphytobenthic biomass, bacterial activity and organic matter dynamics through bioturbation, grazing and excretion (=-=Borchardt and Bott, 1995-=-; Montagna et al., 1995; Traunspurger et al., 1997; Hakenkamp and Morin, 2000). Rotifers are a permanent and important component of meiofauna associated to epilithic periphyton in the Garonne river (M...

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..., only a few studies underlined meiofauna 187ecology in lentic (Hillebrand et al., 2002; Höckelmann, Moens & Jüttner, 2004; Peters et al., 2007) and lotic (Gaudes et al., 2006; Mathieu et al., 2007; =-=Sabater et al., 2003-=-) epilithon. The long-term dynamics of meiofauna in lotic epilithon are particularly poorly understood as available field studies investigated rather limited time periods. Thus, the purpose of this st...

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...af litter accumulate. These habitats could, as for macrofauna (Rempel, Richardson & Healey, 1999), favour recolonization of meiofauna after flood disturbance following a ‘refuge as habitat’ strategy (=-=Golladay & Hax, 1995-=-; Robertson et al., 1995). However, the resilience times observed after spring snow-melt flood were longer than after stormflows (Table 4). A possible explanation for this difference might be that sno...

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...cing microphytobenthic biomass, bacterial activity and organic matter dynamics through bioturbation, grazing and excretion (Borchardt and Bott, 1995; Montagna et al., 1995; Traunspurger et al., 1997; =-=Hakenkamp and Morin, 2000-=-). Rotifers are a permanent and important component of meiofauna associated to epilithic periphyton in the Garonne river (Majdi et al., submitted). Contrarily to monogonont rotifers which feed on a la...

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...ropriate water temperature (Morin & Dumont, 1994). Furthermore trichopter larvae can strongly reduce epilithon biomass by grazing especially when water temperature is high (Feminella & Hawkins, 1995; =-=Hillebrand, 2009-=-). Therefore, it would seem that during July, the abundance increase of trichopter larvae together with a temperature-dependant increase of their metabolism would likely constitute a shortterm ‘extrem...

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...e on biogeochemical processes (Battin et al., 2003) and its role in secondary production (Fuller, Roelofs & Fry, 1986), decomposition (Ford & Lock, 1987) and nutrient retention (Sauvage et al., 2003; =-=Teissier et al., 2007-=-). Meiofauna are motile benthic invertebrates defined as passing through a mesh of 500 µm and retained on a mesh of 44 µm. Meiofauna can be divided in two functional groups depending on life cycle cha...

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...an AI > 400 indicates an epilithon dominated by heterotrophs and/or organic detritus. Otherwise, 100 < AI < 400 indicates that the epilithic community is balanced between heterotrophs and autotrophs (=-=Ameziane et al., 2002-=-; Biggs & Close, 1989). Data analysis tel-00741853, version 1 - 15 Oct 2012 Mann-whitney U tests, Spearman rank tests and simple linear regressions were performed with STATISTICA software (version 8.0...

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...or this difference might be that snow-melt flood periods showed numerous higher flood peaks than stormflow events. In the Garonne, snow-melt floods can deeply scour and submerge proximate floodplain (=-=Chauvet & Décamps, 1989-=-), thus reducing the availability of eventual refuges for meiofauna. Another possible explanation for this difference might be that an additional type of disturbance retards meiofaunal resilience afte...

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... de l’article Introduction et objectifs tel-00741853, version 1 - 15 Oct 2012 Le biofilm epilithique phototrophe de la Garonne (France) est composé en proportion importante de microalgues benthiques (=-=Leflaive et al., 2008-=-; Majdi et al, submitted). La contribution des cyanobactéries, des microalgues vertes et des diatomées varie en fonction des perturbations saisonnières auxquelles cet écosystème est soumis, même si le...

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...g that bacteria which are abundant and diversified in the epilithon of the Garonne river (Lyautey et al., 2005) are crucial early colonizers for the establishment of the epilithon after disturbances (=-=Lyautey et al., 2010-=-). It would be also in agreement with Hillebrand et al. (2002) who show that 197macrofaunal grazing impact epilithic microalgae while it tend to favour the growth of ciliates and bacteria during fiel...

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...its important influence on biogeochemical processes (Battin et al., 2003) and its role in secondary production (Fuller, Roelofs & Fry, 1986), decomposition (Ford & Lock, 1987) and nutrient retention (=-=Sauvage et al., 2003-=-; Teissier et al., 2007). Meiofauna are motile benthic invertebrates defined as passing through a mesh of 500 µm and retained on a mesh of 44 µm. Meiofauna can be divided in two functional groups depe...

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...estigated in fluvial ecosystems because of its important influence on biogeochemical processes (Battin et al., 2003) and its role in secondary production (Fuller, Roelofs & Fry, 1986), decomposition (=-=Ford & Lock, 1987-=-) and nutrient retention (Sauvage et al., 2003; Teissier et al., 2007). Meiofauna are motile benthic invertebrates defined as passing through a mesh of 500 µm and retained on a mesh of 44 µm. Meiofaun...

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...cting epibenthic meiofauna (Robertson, Rundle & Schmid-Araya, 2000). So far, only a few studies underlined meiofauna 187ecology in lentic (Hillebrand et al., 2002; Höckelmann, Moens & Jüttner, 2004; =-=Peters et al., 2007-=-) and lotic (Gaudes et al., 2006; Mathieu et al., 2007; Sabater et al., 2003) epilithon. The long-term dynamics of meiofauna in lotic epilithon are particularly poorly understood as available field st...

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...e range (Giere, 2009). Meiofaunal organisms are abundant in streams (Hakenkamp & Morin, 2000). With a high production/biomass ratio, they may contribute substantially to benthic secondary production (=-=Bergtold & Traunspurger, 2005-=-; Stead, Schmid-Araya & Hildrew, 2005). Meiofauna are expected to regulate microalgae, bacterial activity and organic matter dynamics through bioturbation, grazing and excretion (Borchardt & Bott, 199...

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... (bacteria, fungi, algae, heterotrophic protozoans, meiofauna and macrofauna) embedded in a mucous matrix of exopolymeric substances together with particulate organic matter and sediment (Lock, 1993; =-=Gaudes et al., 2006-=-; Majdi et al, submitted). Epilithic periphyton grows on any hard substrate submerged in lotic or lentic freshwater. During an annual cycle, the epilithic periphyton of the Garonne river is subjected ...

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...rofauna tel-00741853, version 1 - 15 Oct 2012 For each sampling, the organic fraction of four epilithon suspensions were separated from the mineral fraction, using a differential flotation technique (=-=Hodda & Abebe, 2006-=-) with LUDOX HS-40 (Sigma-Aldrich corp., St. Louis, MO, U.S.A.). Then, LUDOX was discarded (sieved on 63 µm for C1 samples and on 40 µm for C2 samples). Epilithon samples collected on sieve were then ...

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...s were performed 140within months following samplings. Sampled epilithon areas were photographed and scrubbed surfaces were calculated by computer image analysis, using ImageJ software version 1.38 (=-=Rasband, 1997-=-). 2.3. Rotifer dry mass tel-00741853, version 1 - 15 Oct 2012 Rotifer dry masses were assessed from microscopic pictures. For each sample, all rotifers were counted and 100 individuals were randomly ...

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...vae were considered as temporary meiofauna. Trichopter larvae were considered as macrofauna. The resilience of permanent meiofauna—time required for population densities to reach pre-flood densities (=-=Schmid-Araya, 1994-=-)—was also estimated during C2 after 2 stormflows (23 January 2009 and 15 January 2010, both MDD = 462 m 3 s -1 ) and after the last flood of the spring snow-melt flood period (12 April 2009, MDD = 33...

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... during July. The question remains, if meiofauna partly disappeared by being ingested together with the epilithon, or by escaping knowing that meiofauna can actively drift between sediment and water (=-=Smith & Brown, 2006-=-; Swan & Palmer, 2000). Trichopter density peaked also during October (particularly in 2005 and 2008). However, in October, the potential related impact on epilithon and its associated meiofauna appea...

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...ithon areas scraped were photographed and scrubbed surfaces were calculated using ImageJ software version 1.38 (Rasband, 1997). It has to be noticed that as C1 supplied other studies about epilithon (=-=Boulêtreau, 2007-=-; Leflaive et al., 2008), AFDM was recorded at each of the 44 samplings. Algal pigments were measured for 24 of the 44 samplings. Meiofauna were determined for 17 189of the 44 samplings. During C2, A...