Organic matter exploitation in a highly turbid environment: Planktonic food web in the Charente estuary, France

Estuaries are highly dynamic systems where multiple organic matter sources coexist and where complex biogeochemical processes greatly affect their fate. Although zooplankton plays a key role of in the energy fluxes between primary sources and exploited macrofauna, there is still a critical lack of field information concerning the spatio–temporal variability of the trophic pathways supporting its high biomasses in estuaries. From January 2007 to January 2008, suspended matter, microphytobenthos and zooplankton were sampled along the salinity gradient of the Charente estuary to determine their carbon and nitrogen stable isotope composition. The relative homogeneity of the δ13C values of particulate organic matter (POM) all along the estuary (−23.6 to −26.5‰ except in March and June, ∼ −28.5‰) was attributed to physical mixing of marine and terrestrially derived organic matter with the great load of tidally resuspended particles. The five zooplankton taxa analysed displayed a wide range of δ13C (from −34.9 to −17.4‰) and δ15N values (3.4–15.2‰) over the year, providing strong evidence for high selectivity toward different organic matter sources and reinforcing the idea that a spatio–temporal succession of species assemblages lead to multiple trophic pathways and may stabilize the estuarine trophic network. The high δ15N values of Eurytemora affinis in the maximum turbidity zone were believed to reflect a higher carnivorous tendency as a functional response to the strong decrease of phytoplankton availability. Conversely, Acartia spp. appeared unable to change their diet in the same way and was thus unable to colonize upstream areas. Stable isotope analysis also revealed that Mesopodopsis slabberi mostly relied on fresh phytoplankton and microphytobenthos while Neomysis integer presented a clear carnivorous tendency toward copepods, at least during the warm period. Additionally evidence was provided for passive (downstream advection of freshwater cladocerans with river runoffs) and active (coast-estuary seasonal migration of M. slabberi) longitudinal movements in estuarine organisms.