Effects of liming on plankton and young-of-the-year bluegill growth in Flat Top Lake, West Virginia
Coahran, David Alan
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The responses at three trophic levels (phytoplankton, zooplankton, young-of-the-year bluegill) were compared between a limed and unlimed arm in a moderately fertile, circumneutral reservoir that was sensitive to acidification, but had not yet shown signs of damage. The east arm (25.5 ha) of Flat Top Lake was treated with 28.8 dry metric tonnes of calcite using slurry box technology between July 13-20, 1987. The design was confounded because calcite dissolution products diffused into the unlimed (west) arm after the treatment which made total alkalinity similar between the arms within 1 month. Phytoplankton gross productivity and chlorophyll a was similar between the arms for the pre- and post-treatment samples as well as for the period after the treatment when total alkalinity was relatively higher in the limed arm. Zooplankton biomass was higher in the unlimed than limed arm in the pre-treatment samples but was similar between the arms in the post-treatments samples; however, the changes in zooplankton biomass after treatment in the limed arm could not be attributed to the treatment. Phytoplankton community composition for the common netplankton and nannoplankton was similar between the limed and unlimed arms for the combined pre- and post-treatment samples: however, Gemellicystis spp., Gloeocystis spp., and Dinobryon spp. showed an order of magnitude higher density in the limed relative to the unlimed arm on the initial post-treatment sample. This difference, as well as the observed post-treatment differences in zooplankton community composition between the two arms could not be attributed to the treatment because there were minimal impacts on nutrient levels (e.g., CO2, total phosphorus) and zooplankton-phytoplankton interactions. Lake transparency was significantly deeper in the limed arm relative to the unlimed arm for the post-treatment samples; however, a corresponding lower dissolved organic carbon concentration was not measured in the limed arm. Young-of-the-year bluegill (e.g., primarily those 15-20 d old) showed significantly higher growth rates in the unlimed arm during the period when total alkalinity was higher in the limed arm, apparently due to a higher density of suitable pelagic zooplankton in the unlimed arm during this period. In conclusion, no significant positive or negative responses to the treatment were detected because of trophic level interactions. The high pre-treatment pH (7.07), small change in post-treatment pH, low dissolution percentage (8-10%), the inability of the sediment dose to neutralize the acidic sediments and induce phosphorus release, and phosphorus rather than carbon dioxide limited primary productivity were the main factors why no significant post-treatment biological responses were detected.
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