The state of Minnesota recently released their report, “Swimmable, fishable, fixable? What we’ve learned so far about Minnesota waters.” Minnesota is halfway through a watershed-by-watershed assessment, which, among other things, has decoupled lakes from their watersheds when looking at improvement strategies.
This report outlines strategies for fixing polluted lakes and streams and recognizes that “nutrient-impaired” lakes (that is lakes with algae blooms) will require: “treating sediments to control phosphorus recycling, supporting fish and plant communities, and protecting the lakeshore”– no mention of watershed practices.
The report does outline the need for watershed management as an overall strategy. And this strategy is appropriate and effective for streams, but not so much for lakes.
Nutrient reductions in watersheds using best management practices (BMPs) are insufficient and impaired lakes are unresponsive.
So how do we fix these lakes and how long will it take?
If we rely on watershed management, lake improvements will take lifetimes. The Yahara watershed upstream from Lake Mendota, WI has been actively implementing watershed BMPs since the 1970s with this result – “long term trends in water quality are not significant,” meaning no change.
Most nutrient-impaired lakes stay impaired because of sediment phosphorus cycling. When we preform the analysis, restoring an impaired lake requires a reduction of watershed phosphorus of 80 to 90% or a reduction of sediment phosphorus by the same amount. Either one will accomplish the objective.
Watershed phosphorus reductions are slow – decades or more. As in the Yahara case, underlying land use alterations are hardwired and BMPs, in the best cases, may reduce phosphorus by 25%, if they are strategically and comprehensively deployed, appropriately designed and maintained and their phosphorus reduction targets are validated by field measurements. Even then, a 25% reduction is insufficient.
Many agencies and organizations urge landowners to install rain gardens or buffer strips or shoreland plantings, etc., implying that by doing so, our lakes will become clean. However, these pleas too seldom include the math. We should do what we can to keep runoff water away from our lakes. But we need to do the math to tell the whole story. We cannot undo the hardwired land alterations that have been made unless we re-engineer whole watersheds – something that has yet to be discussed.
Sediment phosphorus reductions are (or can be) fast – days. Alum applications are the most common way to treat phosphorus-enriched sediments. Alum application technologies have been in use since the 1970s with a long track record of safety and efficacy. These treatments will reduce sediment phosphorus by 80 to 90% and will last about 10 to 15 years.
Why not address phosphorous impairment in both the watershed and in individual lakes? This is a management philosophy question more than a management strategy question. We could argue that reducing watershed phosphorous as the ultimate source of the problem is the first priority. But here’s the rub – even if we could reduce watershed phosphorus sufficiently (80 – 90%), the sediment phosphorus would still require treatment. Sediment treatments are also much more cost-effective than BMPs — less than 10% of the cost, short-term and long-term.
We should implement both BMPs to slowly minimize watershed phosphorus and sediment treatments to quickly fix nutrient-impaired lakes. But to have cleaner lakes, treating the sediments is required. It can be done sooner or later.