title: Low ratios of silica to dissolved nitrogen supplied to rivers arise from agriculture not reservoirs.
abstract: Coastal marine systems are greatly altered by toxic marine algae, eutrophication and hypoxia. These problems have been linked to decreased ratios of dissolved silica to inorganic nitrogen (Si : DIN) delivered from land. Two mechanisms for this decline under consideration are enhanced nitrogen (N) fertiliser losses from agricultural lands or Si sequestration in reservoirs. Here we examine these mechanisms via nutrient concentrations in impoundments receiving water from 130 watersheds in a landscape representative of the agriculture that often dominates coastal nutrient inputs. Decreased Si : DIN was correlated with agriculture, not impoundment. Watersheds with > 60% agricultural land yielded highest DIN, whereas Si was uncorrelated with agricultural intensity. Furthermore, eutrophic lakes were dominated by Cyanobacteria that use little Si, so reservoirs did not diminish Si : DIN. Instead, Si : DIN increased slightly as reservoir residence time increased. These data suggest that impoundments in agricultural watersheds may enhance the water quality of coastal ecosystems, whereas fertiliser losses are detrimental.
authors.affiliation: Minnesota Sea Grant College Program & Large Lakes Observatory, University of Minnesota Duluth, 31 W. College Street, Duluth, MN, 55812-1198, USA.
authors.forename: John A
authors.initials: JA
authors.lastname: Downing
authors.affiliation: School of Science, Technology, Engineering & Mathematics, University of Washington, Bothell, Washington, 98011, USA.
authors.forename: Christine T
authors.initials: CT
authors.lastname: Cherrier
authors.affiliation: Department of Earth and Environment, Boston University, 685 Commonwealth Ave., Boston, MA, 02215, USA.|Department of Biology, Boston University, 5 Cummington Mall, Boston, MA, 02215, USA.
authors.forename: Robinson W
authors.initials: RW
authors.lastname: Fulweiler
mesh_terms: D000383:Agriculture; D017753:Ecosystem; D055593:Geological Phenomena; D009584:Nitrogen / Q000737:chemistry*; D045483:Rivers / Q000737:chemistry*; D012822:Silicon Dioxide / Q000737:chemistry*
chemical_list: D012822:Silicon Dioxide; D009584:Nitrogen
keywords: Coastal; harmful algae blooms; landscape; marine ecology; nitrogen; reservoirs; silicon; watershed