Eutrophication of Tenkiller Reservoir, Oklahoma, from nonpoint agricultural runoff

Abstract

Tenkiller Ferry Reservoir, a large (51.6 km2) US midcontinent reservoir in Oklahoma, switched from oligomesotrophic prior to 1975 to eutrophic by 1986, evidenced by changes in phytoplankton taxa, chlorophyll (Chl), total phosphorus (TP), transparency, and areal hypolimnetic oxygen deficit. External TP loading increased 2.5-fold between 1974 and 2004, mainly as nonpoint loading from disposal of an annual (2001-2004) average 406,818 metric tons (mt) of untreated poultry litter on watershed pastures, which added an annual (2001-2006) average 4120 mt of TP. Phosphorus runoff from litter, estimated as 5%of applied, was 63%of external loading to the Illinois River from 2001- 2004, 71 % after the 2004 waste water treatment plant upgrade. The 9 % load decrease from the upgrade did not affect Chl. Sediment TP release, seldom determined for reservoirs, accounted for 16 % of annual external plus internal TP load. Trophic state graded from riverine to lacustrine zone. In wet summers with low residence times, the lacustrine zone was eutrophic; in dry summers it was mesotrophic. Transition and riverine zones were always eutrophic or hypereutrophic. Trophic state assessment ideally requires multiple sampling years in all reservoir zones. A similar Oklahoma reference reservoir, Broken Bow, with modest watershed poultry activities and low inflow TP concentrations, was oligo-mesotrophic. Rehabilitation of Tenkiller Reservoir requires large reductions of TP loading by ceasing watershed litter disposal, changes in watershed management practices, and application of in-reservoir procedures.