|
 Our RUSS data
set and Hennepin Park's water chemistry data from the summer of
1999 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 ultimately 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 the depth and
time patterns of dissolved oxygen, pH and turbidity. The nearby
West Upper Lake water column responded very 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).
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 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
impact 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 3x 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 BMPs, alone, may not be successful
in improving the water quality of these bays without concurrently
reducing internal 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.
|
