HIgHLy VArIABLE BEnTHIC-PELAgIC COuPLIng In A LArgE FLuVIAL LAkE rEVEALED By CArBOn ISOTOPES a

– Stable carbon isotopes are increasingly used to estimate the relative contribution of phytoplankton and periphyton towards secondary production in aquatic food webs. This approach, developed mostly through studies of deep stratified lakes, remains to be evaluated in the context of large rivers, where factors such as short water residence time, little stratification, and variable turbidity may affect carbon fractionation by primary producers. We used invertebrate filterers and grazers as integrators of isotopic ratios of phytoplankton and periphyton, respectively, in twelve stations from a large (35 km) fluvial lake of the Saint-Lawrence river, Québec, Canada. We found strong but variable differences in Δδ13C, ranging from 1 to 7 ‰, between grazers and filterers collected at the same sites. Following an experimental approach we then showed that the light regime, known for influencing isotopic fractionation from dissolved inorganic carbon, accounted for very little variability in the isotopic ratio of periphyton. On the other hand, field data showed that the isotopic difference between filterers and grazers was strongly related to spatial variation in the relative abundance of phytoplankton and periphyton within the lake. This study showed that the basis of benthic food web production in a fluvial lake is spatially highly variable and can be efficiently quantified with carbon isotopes. Our results revealed the existence of a complex spatial mosaic ranging from an almost complete trophic dependence on phytoplankton to an important assimilation of non-sestonic sources. STABLE ISOTOPES PHyTOPLAnkTOn PErIPHyTOn BEnTHIC InVErTEBrATES FrACTIOnATIOn FLuVIAL LAkE PELAgIC-BEnTHIC COuPLIng ST.-LAWrEnCE rIVEr 288 A. PArIS, g. CABAnA, J. J. FrEnETTE Vie milieu, 2016, 66 (3-4) ascribed to two physical processes ultimately affecting the δ13C of algae. First, compared to phytoplankton, periphytic algae grow in a benthic environment with relatively low turbulence, resulting in a thicker boundary layer near the cell surface. This limits the diffusion of C into the cell resulting in a decreased isotopic discrimination during photosynthesis (Smith & Walker 1980, Heckey & Hesslein 1995). A second important process leading to more negative δ13C in phytoplankton in the pelagic zone is the recycling of respiratory dissolved inorganic carbon (DIC) accumulated in the deeper strata of the lake. This depleted carbon can be in part assimilated by phytoplankton and contributes to increasing the isotopic differentiation between pelagic planktonic and littoral periphytic algae (rau 1978). Diffusion of very negative δ13C originating from hypolimnetic methane emission followed by oxidation may also contribute to the depletion in phytoplankton 13C observed in some lakes (ravinet et al. 2010). Thus isotopic discrimination between food chains based on benthic vs pelagic primary producers is commonly observed in stratified lakes (France 1995a, Sierszen et al. 2006) and this pattern has been exploited to evaluate the relative importance of benthic vs pelagic production in these ecosystems (Heckey & Hesslein 1995, Vander Zanden & Vadeboncoeur 2002) and applied to evaluate the flow of contaminants associated to these alternate trophic paths (Eagles-Smith et al. 2008). However, the applicability of this isotopic approach to the study of food webs in large rivers can be a priori questioned on the grounds of three observations. First, large shallow rivers typically have a well-mixed water column resulting in a vertical mixing of δ13C of DIC. unlike in deeper lakes, there is no pelagic zone distanced from the littoral zone and consequently phytoplankton and periphyton should be using the same source of DIC with no difference in δ13C. Secondly, while current velocity may be reduced in certain nearshore areas and embayments of rivers, the occurrence of flowing waters in rivers near bottom may contribute to alleviate the effect of the boundary layer on fractionation by periphyton hypothesized to occur in lakes (Hecky & Hesslein 1995). This would then result in a more negative periphyton δ13C in rivers, closer to phytoplankton δ13C, opposite to what is observed in lakes. Indeed, field as well as experimental work has shown that water velocity greatly increases C isotopic discrimination from DIC by periphyton (Finlay et al. 1999, Trudeau & rasmussen 2003). Thirdly, diminished isotopic discrimination between phytoplankton and periphyton could also be caused by variable fractionation in periphyton as a function of light levels. Turbidity related to suspended organic or inorganic matter, a common situation in rivers, decreases the quantity of light available for the photosynthesis of benthic algae. This causes a reduction in the C demand, resulting in an increase in isotopic fractionation from DIC (Laws et al. 1995). under light stress, periphyton could therefore acquire a more negative δ13C closer to that of phytoplankton. The aim of this study is to examine if δ13C can be used to distinguish phytoplankton-based from periphytonbased food chains in a large fluvial lake. To achieve this goal, we used the δ13C of grazers and filterers as integrators of the δ13C signal of periphyton and phytoplankton, respectively (Post 2002). We show that the isotopic differentiation between those consumers is highly variable within the fluvial lake. This result could be brought by at least two mechanisms: first, variable fractionation by primary producers from their carbon source (s) and transfer of this signal up the food chain and secondly, pelagicbenthic coupling which would lead to the mixing of food sources. To investigate these two mechanisms, we used (i) an experimental approach to determine if the light regime, known for influencing isotopic fractionation from DIC (MacLeod & Barton 1998), accounted for the variability in the isotopic ratio of periphyton, and (ii) field data to determine if isotopic difference between filterers and grazers was related to spatial variation in the relative abundance of phytoplankton and periphyton within the lake.