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Optical Changes in a Eutrophic Estuary During Reduced Nutrient Loadings

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Abstract

Loss of water clarity is one of the consequences of coastal eutrophication. Efforts have therefore been made to reduce external nutrient loadings of coastal waters. This paper documents improvements to water clarity between 1985 and 2008–2009 at four stations in the microtidal estuary Roskilde Fjord and find significant relationships to freshwater nutrient loadings. The paper then investigates to which extent changes in phytoplankton biomass (chlorophyll a (Chl a)), non-algal particulate organic matter (POM*), and residual attenuation in the water (K b), respectively, can account for this optical improvement. Vertical light attenuation (K d) declined, on average, by 34 %, accompanying a 71 % reduction of Chl a and an 80 % reduction of POM*. Residual attenuation declined by 26 % over the period in accordance with a measured 34 % decline of dissolved organic nitrogen. Analysis of simultaneous changes in light attenuation and Secchi depth also suggested a reduction of the scatter-to-absorption ratio over time. Considering the stronger reductions of particle concentrations than dissolved organic matter, the contribution of residual attenuation to vertical attenuation increased from 54 to 74 % in 1985 to 78 to 85 % in 2008–2009. Overall, efforts to reduce nutrient loading and improve water clarity appeared to have had a larger impact on POM* than on Chl a and colored dissolved organic matter concentrations in the estuary, which can account for the decrease in the scatter-to-absorption ratio. These optical changes lead to larger improvements of Secchi depth than of vertical light attenuation. The consequence of this is an overestimation (0.45–1.48 m) of the predicted increase of potential seagrass depth limits when based on Secchi depth rather than K d.

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Acknowledgments

TMP, KSJ, and SLN were supported by grant 2104-07-0011 (“REELGRASS”) from The Danish Strategic Research Council. The study also received support from the IMAGE-project (2104-09-67259) from the Danish Strategic Research Council. We thank Dr. Gary T. Banta for linguistic corrections to the manuscript.

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Authors and Affiliations

  1. Department of Environmental, Social and Spatial Change, Roskilde University, Universitetsvej 1, 4000, Roskilde, Denmark

    Troels Møller Pedersen & Søren Laurentius Nielsen

  2. Freshwater Biological Laboratory, University of Copenhagen, Universitetsparken 4, 2100, København Ø, Denmark

    Kaj Sand-Jensen

  3. Department of Bioscience, Aarhus University, Frederiksborgvej 399, 4000, Roskilde, Denmark

    Stiig Markager

Authors
  1. Troels Møller Pedersen
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  2. Kaj Sand-Jensen
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  3. Stiig Markager
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  4. Søren Laurentius Nielsen
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Correspondence to Søren Laurentius Nielsen.

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Communicated by Kenneth Dunton

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Pedersen, T.M., Sand-Jensen, K., Markager, S. et al. Optical Changes in a Eutrophic Estuary During Reduced Nutrient Loadings. Estuaries and Coasts 37, 880–892 (2014). https://doi.org/10.1007/s12237-013-9732-y

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  • DOI: https://doi.org/10.1007/s12237-013-9732-y

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