title: Variable climatic conditions dominate recent phytoplankton dynamics in Chesapeake Bay.
abstract: Variable climatic conditions strongly influence phytoplankton dynamics in estuaries globally. Our study area is Chesapeake Bay, a highly productive ecosystem providing natural resources, transportation, and recreation for nearly 16 million people inhabiting a 165,000-km(2) watershed. Since World War II, nutrient over-enrichment has led to multiple ecosystem impairments caused by increased phytoplankton biomass as chlorophyll-a (chl-a). Doubled nitrogen (N) loadings from 1945-1980 led to increased chl-a, reduced water clarity, and low dissolved oxygen (DO), while decreased N loadings from 1981-2012 suggest modest improvement. The recent 30+ years are characterized by high inter-annual variability of chl-a, coinciding with irregular dry and wet periods, complicating the detection of long-term trends. Here, we synthesize time-series data for historical and recent N loadings (TN, NO2 + NO3), chl-a, floral composition, and net primary productivity (NPP) to distinguish secular changes caused by nutrient over-enrichment from spatio-temporal variability imposed by climatic conditions. Wet years showed higher chl-a, higher diatom abundance, and increased NPP, while dry years showed lower chl-a, lower diatom abundance, and decreased NPP. Our findings support a conceptual model wherein variable climatic conditions dominate recent phytoplankton dynamics against a backdrop of nutrient over-enrichment, emphasizing the need to separate these effects to gauge progress toward improving water quality in estuaries.
authors.affiliation: Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, California 90095, United States.
authors.forename: Lawrence W
authors.initials: LW
authors.lastname: Harding
authors.affiliation: Interstate Commission on the Potomac River Basin, United States Environmental Protection Agency Chesapeake Bay Program Office, 410 Severn Avenue, Annapolis, Maryland 21403, United States.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Mallonee
authors.affiliation: Statistics Consultant, 2000 Kings Landing Road, Huntingtown, Maryland 20639, United States.
authors.forename: Elgin S
authors.initials: ES
authors.lastname: Perry
authors.affiliation: US Naval Research Laboratory, 4555 Overlook Ave., SW, Washington, D.C. 20375, United States.
authors.forename: W David
authors.initials: WD
authors.lastname: Miller
authors.affiliation: Marine Science Program, University of Hawaii at Hilo, 200 W. Kawili Street, Hilo, Hawaii 96720, United States.
authors.forename: Jason E
authors.initials: JE
authors.lastname: Adolf
authors.affiliation: Smithsonian Environmental Research Center, 647 Contees Wharf Road, Edgewater, Maryland 21037, United States.
authors.forename: Charles L
authors.initials: CL
authors.lastname: Gallegos
authors.affiliation: Institute of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell Street, Morehead City, North Carolina 28557, United States.
authors.forename: Hans W
authors.initials: HW
authors.lastname: Paerl
mesh_terms: D061006:Bays; D018533:Biomass; D002734:Chlorophyll / Q000032:analysis; D000077194:Chlorophyll A; D002980:Climate; D005068:Eutrophication; D008396:Maryland; D009585:Nitrogen Dioxide / Q000032:analysis; D009589:Nitrogen Oxides / Q000032:analysis; D010839:Phytoplankton; D012623:Seawater / Q000032:analysis; D014877:Water Pollution, Chemical / Q000032:analysis; D060753:Water Quality
chemical_list: D009589:Nitrogen Oxides; D002734:Chlorophyll; C031701:nitrogen trioxide; D009585:Nitrogen Dioxide; D000077194:Chlorophyll A