title: Can humic water discharge counteract eutrophication in coastal waters?
abstract: A common and established view is that increased inputs of nutrients to the sea, for example via river flooding, will cause eutrophication and phytoplankton blooms in coastal areas. We here show that this concept may be questioned in certain scenarios. Climate change has been predicted to cause increased inflow of freshwater to coastal areas in northern Europe. River waters in these areas are often brown from the presence of high concentrations of allochthonous dissolved organic carbon (humic carbon), in addition to nitrogen and phosphorus. In this study we investigated whether increased inputs of humic carbon can change the structure and production of the pelagic food web in the recipient seawater. In a mesocosm experiment unfiltered seawater from the northern Baltic Sea was fertilized with inorganic nutrients and humic carbon (CNP), and only with inorganic nutrients (NP). The system responded differently to the humic carbon addition. In NP treatments bacterial, phytoplankton and zooplankton production increased and the systems turned net autotrophic, whereas the CNP-treatment only bacterial and zooplankton production increased driving the system to net heterotrophy. The size-structure of the food web showed large variations in the different treatments. In the enriched NP treatments the phytoplankton community was dominated by filamentous >20 µm algae, while in the CNP treatments the phytoplankton was dominated by picocyanobacteria <5 µm. Our results suggest that climate change scenarios, resulting in increased humic-rich river inflow, may counteract eutrophication in coastal waters, leading to a promotion of the microbial food web and other heterotrophic organisms, driving the recipient coastal waters to net-heterotrophy.
authors: Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden. agneta.andersson@emg.umu.se
authors: Agneta
authors: A
authors: Andersson
authors: Iveta
authors: I
authors: Jurgensone
authors: Owen F
authors: OF
authors: Rowe
authors: Paolo
authors: P
authors: Simonelli
authors: Anders
authors: A
authors: Bignert
authors: Erik
authors: E
authors: Lundberg
authors: Jan
authors: J
authors: Karlsson
mesh_terms: D000818:Animals; D057231:Climate Change; D005068:Eutrophication / Q000502:physiology*; D020387:Food Chain; D005618:Fresh Water / Q000382:microbiology*; D052836:Heterotrophic Processes; D006812:Humic Substances; D010839:Phytoplankton; D045483:Rivers; D012623:Seawater / Q000382:microbiology*
chemical_list: D006812:Humic Substances
references: Appl Environ Microbiol. 1998 Sep;64(9):3246-55
references: 9726867
references: Microb Ecol. 2001 Oct;42(3):372-382
references: 12024262
references: Sci Total Environ. 2011 Apr 15;409(10):1739-45
references: 21345482
references: Nature. 2009 Jul 23;460(7254):506-9
references: 19626113
references: FEMS Microbiol Ecol. 2008 May;64(2):219-29
references: 18336557
references: Nature. 2008 Sep 18;455(7211):387-90
references: 18716617