Mixing Events in Halsteds Bay, Lake Minnetonka
Our RUSS data and Hennepin Park's water chemistry data from last summer showed that sudden mixing events at Halsteds Bay could have dramatic effects on water quality. Severe thunderstorms with high winds and tornado warnings ripped through the Twin Cities area in August and September 1999, leading to sudden mixing and re-aeration events at the Halsteds Bay RUSS site in Lake Minnetonka. The event had profound water quality consequences by disrupting stable thermal stratification and introducing high nutrient/low oxygen bottom water into the upper sunlit layer of the bay resulting in an obnoxious algae bloom some days later. This, in turn, led to further changes in dissolved oxygen, pH and turbidity. The nearby West Upper Lake water column responded differently to the wind event and produced much smaller, if any, serious water quality effects.
In fact, there are economically important land and lake management implications that derive from these data in terms of how to most efficiently restore the degraded water quality in Halsteds Bay. Restorative management decisions involving millions of dollars of tax monies are currently being discussed for Halsteds Bay. Such storm-related events were suspected, but prior to our RUSS data, were not documented due to their transient nature and the danger of manual sampling during severe weather. In fact, several of these events were observed in August 1999 (see figures below) and again in August and September 2000.
In each case, temperature profiles didn't tell us the state of mixing of the bay since variations from surface to bottom were only about 1oC. However, dissolved oxygen (and to a lesser extent pH and EC25) clearly indicated that the lower half (~5 m) of the water column dramatically changed from extreme anoxia (no oxygen) to >75% saturation and then back again to anoxia over intervals of ~ 24 hours in mid and late August/early September. During the initial mixing, the influx of anoxic water to the "epilimnion" actually decreased the level of DO to ~5 mg/L, which could potentially harm fish communities. In addition, water samples collected on August 25, about 10 days after the first mixing event, showed that about 3200 kg (over 7000 lbs of P) was suddenly injected into the upper sunlit euphotic zone. This represented an areal phosphorus load of about triple the annual load estimated to enter Halsteds Bay from Six Mile Creek, its major tributary. The sudden input of P appears to have then caused an increase in algal growth, seen as a chlorophyll increase over the same manual monitoring interval (Barten and Vlach 1999, L. Minnetonka Annual Monitoring Report). Recall that 7000 lbs of P can potentially lead to over 3,500,000 lbs of algae !
These new data, that would likely not have been acquired without remote sensing, suggest that water quality in nutrient enriched lakes of intermediate depth (perhaps 5-10 meters) may often be controlled by weather events. The data also indicate that watershed Best Management Practices (BMPs), alone, may not be successful in improving the water quality of these bays without concurrently reducing internal, in-lake P-loading. Of course this does not diminish the importance of preventing increases in external P-loading from old and new developments over the long-term. However, it does offer important insights into the cost-effective management of water quality for 6 bays on the west and north end of the lake that have shown significant downward trends in water quality over the past 5 years.
Take a look at the DXT graphs below and see if you can find when or if the lakes mixed.