The four Yahara River chain of lakes – Mendota, Monona, Waubesa, and Kegonsa (Fig. left, Table) – have experienced undesirable blue-green algal blooms fueled by excessive phosphorus (P) inputs for years. Analyzing over 30 years of highly variable P loading and in-lake data (Fig. right), NTL-LTER scientists developed P load reduction targets needed for improving water quality in the lakes. These reduction targets were based on lower P loads measured during a 2-year drought when water quality in all four lakes was substantially better.
An important part of NTL’s work was quantifying the load reductions that would occur to the downstream Yahara lakes from instituting management practices in upstream lake drainage basins. Because the downstream lakes are shallower and have much shorter water residence times than Mendota (especially Waubesa and Kegonsa), the upstream lake outlet’s river load is the principal source of P to each downstream lake (Table). Thus, decreases in P inputs to a lake would translate to lower P concentrations in the lake’s surface waters and then less P leaving the lake’s outlet.
These analyses by NTL scientists are the cornerstone to a new lake clean-up effort forged as a public-private partnership to install agricultural and urban best management practices necessary to achieve the P load reductions needed. Much of the pollution reduction practices will be installed in Mendota’s drainage basin because of its much greater land area (with its 60% agricultural land use) than the direct drainage basins of the other three Yahara lakes combined (Table), although management practices also need to be put in place in the drainage basins of the lower lakes to achieve their target P load reductions. NTL-LTER will continue to track the water quality responses in all four Yahara lakes as a way of measuring the project’s success.
|Direct drainage area (km2)||564||105||113||141|
|Lake area (km2)||40.0||13.3||8.4||13.0|
|Maximum depth (m)||25.3||22.6||11.3||9.8|
|Mean depth (m)||12.7||8.3||4.7||5.1|
|Water residence time (yr)||4.4||0.79||0.23||0.33|
|Percent upstream lake outlet P load to lake relative to all surface water inputs||N/A||60%||83%||76%|
|Proportion of P input load that leaves the lake outlet||0.265||0.586||0.935||0.779|