Five-year Changes in Pond Sediment Nutrients and Phosphorus Adsorption-Desorption at Minor Clark Fish Hatchery, Kentucky

Earthen hatchery pond sediments may provide additional nutrients to the water column and act as a sink for nutrients and excess fertilizer. In 2008 and again in 2013 soil nutrient concentrations and phosphorus (P) adsorption were examined in five earthen ponds at Minor Clark Fish Hatchery, Kentucky. These ponds are annually filled with oligotrophic water from an upstream reservoir, fertilized, and drained during normal hatchery operations. Over time, pond nitrogen (N) and P additions have not kept up with soil losses. Soil extractable P concentrations decreased about an order of magnitude between 2008 and 2013. Soil %N decreased by nearly 50% over the same time period. Other soil nutrients (organic carbon, calcium, and manganese) remained the same. Soil collected in 2008 desorbed P under normal fish rearing water quality conditions. Using higher isotherm P-concentrations on 2013 soils, more P was adsorbed as incubation concentrations increased, with greatest adsorption occurring at the highest loading (1500 μg P L-1). Phosphorus adsorption was correlated with concentrations of extractable aluminum and % clay, but not with extractable iron. Soils collected in 2013 incubated with unspiked reservoir water still desorbed P to the water, but at a rate an order of magnitude lower than 2008 soils. The ability of soils to release P to the water under normal conditions has decreased over time, as pond soils exhausted their extractable and labile P supplies. Regular annual flooding and flushing with low ionic strength water has resulted in P depletion in Minor Clark Fish Hatchery pond soils. Given the relatively low contribution of sediments to pond P budgets during fish rearing, lining ponds could be a worthwhile management strategy.

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