PALEOLIMNOLOGICAL STUDY OF PHOSPHORUS-IMPAIRED LAKES IN THE CANNON RIVER WATERSHED

Document
Description
The Cannon River watershed in southeastern Minnesota contains 30 lakes that have phosphorus (P) levels exceeding state water-quality standards and are now on the EPA 303(d) list of impaired waters. A TMDL study completed by the MPCA (LimnoTech, 2016) identified P-load reductions needed to meet these standards, with the aim of developing restoration and protection strategies for subsequent implementation. Given the large number of lake-specific TMDLs in the watershed, uncertainties in modeled load reductions, and variability in lake characteristics, the question arises, what is the best allocation of implementation effort and what is the likelihood of success on a lake-by-lake basis? In some cases, small external load reductions could lead to rapid recovery, while in others, internal loading from legacy P in bottom sediments might overwhelm watershed restoration efforts for many years. Ideally, an accurate P mass balance would go a long way in addressing this question, but in reality internal loads are typically estimated by difference (model prediction vs. observed) and hence have substantial uncertainty. Another component of recovery that is seldom considered is the potential for internal loads to lag external load reductions. A decrease in internal loading ultimately depends on the gradual depletion or burial of P-rich surface sediments. Finally, there is natural variability among lakes in background trophic condition and the degree to which they have been altered by human perturbation. Some lakes may be naturally more productive than others or less sensitive to external P loads owing to differences in morphometry, hydrology, and watershed characteristics.
Date Issued
2019-07-15
Number of Pages
35
Decade
Publisher
Science Museum of Minnesota (St. Paul, Minnesota)
Rights Holder
Science Museum of Minnesota
Rights Management
Public Domain