Role of Internal Phosphorus Loading in Two Shallow, Productive Lakes in Alberta, Canada

The contribution of internal loading from the sediments to the total phosphorus (TP) budget and to the [TP] in the surface water was investigated from May to November in Nakamun and Halfmoon lakes, Alberta. During the summer, Nakamun Lake was intermittently stratified and Halfmoon Lake was weakly, thermally stratified. During the stratified periods, water overlying the sediments was anoxic, TP levels increased in the deep water, and sediments were the main source of TP, contributing 1468 and 147 kg of TP to Nakamun and Halfmoon lakes, respectively. During these same periods, internal loading to Nakamun and Halfmoon lakes (12.7 and 15.6 mg∙m−2∙d−1, respectively) were slightly higher than the rates predicted from laboratory experiments on sediment cores (9.7 and 7.2 mg∙m−2∙d−1, respectively). Quantitative estimates of vertical water exchange in Nakamun Lake and in-lake TP budgets for both lakes indicated that phosphorus released from the sediments was transported to the surface water during mixing. After eight of the nine mixing events that immediatley followed stratified periods, [TP] increased 3–43% and 31–52% in the surf ace water of Nakamun and Halfmoon lakes, respectively.