title: Regime shift from phytoplankton to macrophyte dominance in a large river: Top-down versus bottom-up effects.
abstract: The lower Ebro River (Catalonia, Spain) has recently undergone a regime shift from a phytoplankton-dominated to a macrophyte-dominated system. This shift is well known in shallow lakes but apparently it has never been documented in rivers. Two initial hypotheses to explain the collapse of the phytoplankton were considered: a) the diminution of nutrients (bottom-up); b) the filtering effect due to the colonization of the zebra mussel (top-down). Data on water quality, hydrology and biological communities (phytoplankton, macrophytes and zebra mussel) was obtained both from existing data sets and new surveys. Results clearly indicate that the decrease in phosphorus is the main cause of a dramatic decrease in chlorophyll and large increase in water transparency, triggering the subsequent colonization of macrophytes in the river bed. A Generalized Linear Model analysis showed that the decrease in dissolved phosphorus had a relative importance 14 times higher than the increase in zebra mussel density to explain the variation of total chlorophyll. We suggest that the described changes in the lower Ebro River can be considered a novel ecosystem shift. This shift is triggering remarkable changes in the biological communities beyond the decrease of phytoplankton and the proliferation of macrophytes, such as massive colonization of Simulidae (black fly) and other changes in the benthic invertebrate communities that are currently investigated.
authors.affiliation: IRTA Aquatic Ecosystems, Carretera Poble Nou, Km 5.5, 43540 St. Carles de la Ràpita, Catalonia, Spain. carles.ibanez@irta.cat
authors.forename: Carles
authors.initials: C
authors.lastname: Ibáñez
authors.forename: Carles
authors.initials: C
authors.lastname: Alcaraz
authors.forename: Nuno
authors.initials: N
authors.lastname: Caiola
authors.forename: Albert
authors.initials: A
authors.lastname: Rovira
authors.forename: Rosa
authors.initials: R
authors.lastname: Trobajo
authors.forename: Miguel
authors.initials: M
authors.lastname: Alonso
authors.forename: Concha
authors.initials: C
authors.lastname: Duran
authors.forename: Pere J
authors.initials: PJ
authors.lastname: Jiménez
authors.forename: Antoni
authors.initials: A
authors.lastname: Munné
authors.forename: Narcís
authors.initials: N
authors.lastname: Prat
mesh_terms: D000818:Animals; D002734:Chlorophyll / Q000032:analysis; D049875:Dreissena; D017753:Ecosystem; D004777:Environment; D005618:Fresh Water / Q000032:analysis; D010839:Phytoplankton; D011156:Population Density; D045483:Rivers; D013030:Spain; D014874:Water Pollutants, Chemical / Q000032:analysis; D060753:Water Quality
chemical_list: D014874:Water Pollutants, Chemical; D002734:Chlorophyll