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Long-term changes and impacts of hypoxia in Danish coastal waters

Conley, Daniel LU ; Cartensen, J; Ærtebjerg, G; Christensen, PB; Dalsgaard, T; Hansen, JLS and Josefson, AB (2007) In Ecological Applications 17(5). p.165-184
Abstract
A 38-year record of bottom water dissolved oxygen concentrations in coastal marine ecosystems around Denmark (1965-2003) and a longer partially reconstructed record of total nitrogen (TN) inputs (1900-2003) were assembled to describe long-term patterns in hypoxia and anoxia. Interannual variations in bottom water oxygen concentrations were analyzed in relation to various explanatory variables (bottom temperature, wind speed, advective transport, TN loading). Reconstructed TN loads peaked in the 1980s with a gradual decline to the present, commensurate with a legislated nutrient reduction strategy. Mean bottom water oxygen concentrations during summer have significantly declined in coastal marine ecosystems, decreasing substantially during... (More)
A 38-year record of bottom water dissolved oxygen concentrations in coastal marine ecosystems around Denmark (1965-2003) and a longer partially reconstructed record of total nitrogen (TN) inputs (1900-2003) were assembled to describe long-term patterns in hypoxia and anoxia. Interannual variations in bottom water oxygen concentrations were analyzed in relation to various explanatory variables (bottom temperature, wind speed, advective transport, TN loading). Reconstructed TN loads peaked in the 1980s with a gradual decline to the present, commensurate with a legislated nutrient reduction strategy. Mean bottom water oxygen concentrations during summer have significantly declined in coastal marine ecosystems, decreasing substantially during the 1980s and were extremely variable thereafter. Despite decreasing TN loads, the worst hypoxic event ever recorded in open waters occurred in 2002. For estuaries and coastal areas, bottom water oxygen concentrations were best described by TN input from land and wind speed in July-September, explaining 52% of the interannual variation in concentrations. For open sea areas, bottom water oxygen concentrations were also modulated by TN input from land, however, additional significant variables included advective transport of water and Skagerrak surface water temperature and explained 49% of interannual variations in concentrations. Reductions in benthic species number and alpha diversity were significantly related to the duration of the 2002 hypoxic event. Gradual decreases in diversity measures (species number and alpha diversity) over the first 2-4 weeks show that the benthic community undergoes significant changes before the duration of hypoxia is severe enough to cause the community to collapse. Enhanced sediment-water fluxes of NH4+ and PO43- occur with hypoxia, increasing nutrient concentrations in the water column, and stimulating additional phytoplankton production. Repeated hypoxic events have changed the character of benthic communities and how organic matter is processed in sediments. Our data suggest that repeated hypoxic events lead to an increase in susceptibility of Danish waters to eutrophication and further hypoxia. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
sediment-water fluxes, nutrient enrichment, thresholds, estuaries, benthic communities, diversity, hypoxia, anoxia
in
Ecological Applications
volume
17
issue
5
pages
165 - 184
publisher
Ecological Society of America
external identifiers
  • Scopus:34250652569
ISSN
1051-0761
DOI
10.1890/05-0766.1
language
English
LU publication?
yes
id
68056b4e-a75d-40b8-887d-c0c9d75e337b (old id 829697)
date added to LUP
2008-02-05 11:27:28
date last changed
2016-10-13 04:34:17
@misc{68056b4e-a75d-40b8-887d-c0c9d75e337b,
  abstract     = {A 38-year record of bottom water dissolved oxygen concentrations in coastal marine ecosystems around Denmark (1965-2003) and a longer partially reconstructed record of total nitrogen (TN) inputs (1900-2003) were assembled to describe long-term patterns in hypoxia and anoxia. Interannual variations in bottom water oxygen concentrations were analyzed in relation to various explanatory variables (bottom temperature, wind speed, advective transport, TN loading). Reconstructed TN loads peaked in the 1980s with a gradual decline to the present, commensurate with a legislated nutrient reduction strategy. Mean bottom water oxygen concentrations during summer have significantly declined in coastal marine ecosystems, decreasing substantially during the 1980s and were extremely variable thereafter. Despite decreasing TN loads, the worst hypoxic event ever recorded in open waters occurred in 2002. For estuaries and coastal areas, bottom water oxygen concentrations were best described by TN input from land and wind speed in July-September, explaining 52% of the interannual variation in concentrations. For open sea areas, bottom water oxygen concentrations were also modulated by TN input from land, however, additional significant variables included advective transport of water and Skagerrak surface water temperature and explained 49% of interannual variations in concentrations. Reductions in benthic species number and alpha diversity were significantly related to the duration of the 2002 hypoxic event. Gradual decreases in diversity measures (species number and alpha diversity) over the first 2-4 weeks show that the benthic community undergoes significant changes before the duration of hypoxia is severe enough to cause the community to collapse. Enhanced sediment-water fluxes of NH4+ and PO43- occur with hypoxia, increasing nutrient concentrations in the water column, and stimulating additional phytoplankton production. Repeated hypoxic events have changed the character of benthic communities and how organic matter is processed in sediments. Our data suggest that repeated hypoxic events lead to an increase in susceptibility of Danish waters to eutrophication and further hypoxia.},
  author       = {Conley, Daniel and Cartensen, J and Ærtebjerg, G and Christensen, PB and Dalsgaard, T and Hansen, JLS and Josefson, AB},
  issn         = {1051-0761},
  keyword      = {sediment-water fluxes,nutrient enrichment,thresholds,estuaries,benthic communities,diversity,hypoxia,anoxia},
  language     = {eng},
  number       = {5},
  pages        = {165--184},
  publisher    = {ARRAY(0x94b03f0)},
  series       = {Ecological Applications},
  title        = {Long-term changes and impacts of hypoxia in Danish coastal waters},
  url          = {http://dx.doi.org/10.1890/05-0766.1},
  volume       = {17},
  year         = {2007},
}