Effects of the halocline on water quality and phytoplankton composition in a shallow brackish lake (Lake Obuchi, Japan) |
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Authors: | Shinji Ueda Kunio Kondo Yuki Chikuchi |
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Institution: | (1) Department of Radioecology, Institute for Environmental Sciences, 1-7 Rokkasho, Aomori 039-3212, Japan |
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Abstract: | The relationships of the halocline to both water quality and phytoplankton composition in Lake Obuchi, a shallow brackish
lake in northern Japan, were investigated from April 2001 to December 2004. The halocline in this lake became stronger in
summer (July–September, mean maximum density gradient 4.3–5.8 ρtm−1) but weaker in spring, fall, and winter (1.9–3.3 ρtm−1). Although the difference in water quality between the upper and lower layers separated by the halocline was high in summer,
nutrients (PO43−-P and NH4+-N) were eluted from the bottom sediment as levels of dissolved oxygen decreased in the bottom layer because of the strong
stratification caused by the halocline formed over the long term. Moreover, phytoplankton taxa composition also differed between
the upper and lower layers in summer, but was similar in other seasons. The dominant phytoplankton taxa in the upper layer
in summer were Skeletonema costatum and Cyclotella spp., whereas in the lower layer, Gymnodinium spp. (Dinophyceae) and Chlorophyceae, which prefer eutrophic and low dissolved oxygen conditions, dominated. This suggests
that the halocline was related to differentiations in both water quality and ecosystem components between the upper and lower
layers in the brackish lake water. |
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Keywords: | Halocline Gradient intensity Water quality Phytoplankton composition Brackish lake |
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