Algal nutrients: sources and patterns of flow in the Occoquan Watershed

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Virginia Tech

The movement of algal nutrients was studied in the Occoquan Watershed. During the period April through September 1973, weekly water samples collected by the Occoquan Monitoring Laboratory were split for chemical analysis and algal growth potential (AGP) assay.

Variation in AGP observed between stations indicated that the major source of algal nutrients was sewage treatment plants in the Bull Run portion of the Occoquan Watershed. Chemical analyses, likewise, indicated that the Bull Run waters were highest in phosphate and nitrate concentration.

Both phosphate and nitrate concentrations were found to correlate highly with AGP for green and blue green test species. For the green alga Selenaetrum capricornutum, the chemical nutrient most highly correlated with AGP was nitrate, whereas for the bluegreen alga Anabaena flosaquae, total phosphate gave the highest correlation coefficient. A graphic analysis of AGP versus nitrate-nitrogen concentration indicated that Selenastrum yield does not increase with nitrate concentration when orthophosphate concentration is less than 0.010 mg-P/liter. At concentrations greater than this, however, nitrate appears to control the Selenastrum AGP. A similar analysis of Anabaena AGP versus nitrate-nitrogen indicated that nitrate concentration does not affect Anabaena AGP unless the total phosphate concentration is greater than 0.150 mg-P/liter. At total phosphate concentration greater than 0.250 mg-P/liter a definite stimulation of Anabaena yield due to nitrate concentration was observed.

The weekly changes in chemical nutrient flux were examined through spring and summer seasons. The nitrate and phosphate loadings of Bull Run, which receives sewage treatment effluent, were found to be high and relatively constant throughout spring and summer. The agricultural region of Occoquan Creek, however, showed high nitrate loadings only during periods of stormflow. At baseflow, Occoquan Creek nutrient loadings were found to be insignificant. A decrease in nutrient content of the Occoquan Creek tributary was observed during summer due to the influence of a small recreational lake in Occoquan Creek upstream from the reservoir.

Finally, analysis of material and energy transfer between water strata in the stratified reservoir indicated that hypolimnetic nutrients do not enter the epilimnion at a rate sufficient to cause algal bloom conditions. Storm events were observed to cause mixing of the reservoir and so provided the nutrient concentration necessary for algal blooms.

It is suggested that removal of phosphates from all sewage treatment effluent in the Occoquan Watershed will reduce the frequency of blue green algal blooms and allow more effective control with copper sulfate.