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Algal Availability of Sediment Phosphorus in Drainage Water of the Black Creek Watershed¹

ABSTRACT

The water quality problems resulting from sediment P additions to rivers and lakes can only be assessed if the biological availability of sediment-borne P is known. A study was conducted to evaluate the availability to algae of sediment P in drainage water samples collected from an agricultural watershed in northeastern Indiana. Sediments suspended in drainage water were separated, resuspended in a nutrient medium, inoculated with Selanastrum capricornutum, and incubated under illumination for 2 weeks. The sediment-treated media were analyzed for algal numbers and P assimilated by algal cells by sequential extraction with NH₄F, NaOH, and HCl initially and after 2 weeks of incubation. Phosphorus assimilated by algal cells was calculated as the decrease in each sediment P fraction observed during the incubation period.

The proportions of sediment P available for algal uptake in drainage water samples collected in March and June averaged 20.7% of the total sediment P and 30.1% of the sediment inorganic P(P_i). the three sediment P_i fractions investigated, the NH₄F-extractable fraction contributed the largest proportion (42.2%) of the sediment P_i utilized by algae. The NaOH- and HCl-extractable fractions of sediment P_i contributed 36.3% and 21.6%, respectively, of the P_i utilized by algae. A significant proportion (59.8%) of the NH₄F-extractable P present in sediment prior to incubation was utilized by algal cells during the 2-week incubation period. Of the NaOH- and HCl-extractable P_i present initially, 26.8% and 12.8%, respectively, were utilized by algae during the 2-week incubation period. Although all of the soluble P_i initially present in the sediment-medium mixtures was rapidly utilized by algae, sediment P contributed a significantly larger quantity of P than soluble P for algal growth during incubation.

Key Words: S. capricornutum • inorganic P • eutrophication • agricultural runoff • suspended sediment

¹ A contribution of the Indiana Agric. Exp. Stn., Purdue Univ., West Lafayette, IN 47907. Supported in part by Grant no. G6005103 from the USEPA under the Great Lakes National Program. Journal Paper no. 7646.

² Research Assistant, Professor, and Professor, respectively, Agron. Dep., Purdue Univ.

Received for publication July 2, 1979.

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