Learning from Lake of the Woods

Transcription

1 Learning from Lake of the Woods Internal loading and resuspension in shallow lakes Julie Blackburn, Minnesota Area Manager

2

3 Lake of the Woods Aug In 2008, Lake of the Woods was placed on the federal 303 (d) list as impaired for aquatic recreation due to excess nutrients.

4 Lake of the Woods Basin 27,000 square miles >30,000 Waterbodies 9 Linked HSPF models Bigger than West Virginia!

5 Lake of the Woods 5 bays within & upstream of US borders Largest: Big Traverse Mean depth 27 feet 31 total tributaries Main Tributary: Rainy River drains 80% of Basin

6 Lake of the Woods Rainy River primary water and P source ~75% reduction in P concentrations since s. 5 southern lake segments P concentration doubles over summer Well-mixed Aphanizomeon blooms (summer, fall)

7 Observed TP Concentrations Day Rolling Mean TP Concentration 50 TP Concentration (μg/l) Ordinal Day Big Traverse Little Traverse Muskeg

8 Chlorophyll-a Concentration (µg/l) Observed Chlorophyll-a Concentration Mean Chlorophyll-a Concentration by Month, Late summer/falls blooms Aphanizomenon which can translocate P from lake sediments via akinetes and form surface scums 0 June July August September 4-Mile Big Traverse Muskeg Little Traverse Water Quality Standard (9 µg/l)

9 Large and Small Shallow Lake Processes Phosphorus (P) key nutrient determining algal blooms Physical processes: Sedimentation: physical settling of particles Resuspension: physical mixing (winds, rough fish) Peak summer temperatures & dissolved oxygen concentrations at sediment/water boundaries influence lake sediment P release: Lower rates with aerobic sediments Higher rates with anaerobic sediments (up to ~35 times aerobic rates) Both contribute dissolved P to lake waters contributing to algal blooms In spite of LoW sediments with high iron content

10 Understanding all P loading sources External sources Tributaries Local runoff directly to lake Point sources Septic systems Atmospheric sources Internal sources Sediment phosphorus release Resuspension of particleattached phosphorus P P P P P P

11 Shallow Lake Processes Wind mixing, resuspension, temporary loss of dissolved oxygen along sediment/water interface, aerobic and anaerobic P release rates as to temperature. WIND P TSS Sediment P Runoff P Sediment P release

12 Internal Loading: Past Work Bill James (2015) Temperature and dissolved oxygen mediated sediment P release Sediment P Release Rate vs. Water Temperature Sediment P Release Rate (mg/m 2 /d) Water Temperature (⁰C) 9 tons/day from Big Traverse potentially ~53 tons/year from Big Traverse Aerobic release rate Anaerobic release rate

13 Internal Loading: Past Work Science Museum of Minnesota s buoy monitoring Observed Bottom Dissolved Oxygen Concentration (mg/l) Dissolved oxygen profiles to 50cm above lake bottom Large oxygen depletion rates during calm periods No anoxia observed (summer, winter) in southern basins Bottom Dissolved Oxyen Concentration, /21/2015 6/26/2015 7/1/2015 7/6/2015 7/11/2015 7/16/2015 7/21/2015 7/26/2015 7/31/2015 MPCA Muskeg Big Traverse

14 Lake of the Woods Continuous Buoy Temperature and DO Profile Measurements Observed Bottom Dissolved Oxygen Concentration (mg/l) Bottom Dissolved Oxyen Concentration, 2015 Sustained depletion rates of ~ mg/l/d Anaerobic conditions take min days to develop (assuming 8 mg/l starting DO concentration) 0 6/21/2015 6/26/2015 7/1/2015 7/6/2015 7/11/2015 7/16/2015 7/21/2015 7/26/2015 7/31/2015 MPCA Muskeg Big Traverse

15 Lake of the Woods BATHTUB Model Review Consecutive Calm Days (Kenora) 35 Frequency of Consecutive Calm Days (Max. wind speed <7mph, <3.1m/s), Frequency Consecutive Calm Days

16 July Wind Rose for Jackson, MN Annual Wind Rose for Flag Island (LOW), MN From: Iowa State University, Department of Agronomy

17 Long-Term Temperatures

18 TP Mass Balance Approach Δ TTTT = Σ iiiiiiiiiiii Σ ooooooffffffff + UUUUUUUUUUUUUUUUUUUUUU RRRRRRRRRRRRRRRR Unexplained residual internal loading, or more accurately Net TP release, resuspension, and settling We know that both TP release and sedimentation/settling are happening all the time, but TP release dominates during the growing season due to warmer temperatures, biological activity, and wind mixing Monthly net estimates are therefore minimums for TP release into the water column

19 Monthly TP Budget 200,000 Entire Lake Monthly TP Budget Net P release 150, ,000 50,000 Monthly TP Flux (kg) - (50,000) (100,000) (150,000) (200,000) (250,000) (300,000) Net sedimentation May Jun Jul Aug Sep Oct Total Surface Inflow (kg) Total Outflow Net Inflow Unexplained Residual Observed Change in In-Lake TP

20 Shallow Lake Restoration: Lake Shaokotan, Lincoln County, MN 996 acres max depth 10 feet. Early 1990 s massive algal blooms and degradation noted. Restoration actions since 1993: 58% P Load external source reduction 3 feedlot upgrades 4 wetlands rehabilitated Shoreline septic upgrades Apparent reduction of sediment P loss Clear water phase with diversified aquatic plant community

21 Shallow Lake Shaokotan, Lincoln County, MN.

22 Summary of Key Points Peak summer conditions indicate net P release from sediments Shallow lake sediments can contribute substantially to annual P loads Not just during anaerobic conditions Resuspension, temperature, and wind are more important than originally thought Calm days set up the algal bloom conditions; windy days pump more phosphorus into the water column Smaller lake example (Lake Shaokotan) with 58% P load reduction went from turbid to clear lake Lake of the Woods: internal loading is key; control of watershed P loads will assist flushing of lake sediment P loads which are declining over time.

23 QUESTIONS?

24 Existing P Loads Determine loads entering lake from all sources External loading from: Tributaries Local runoff directly to lake Point sources Septic systems Atmospheric sources Internal loading: Sediment phosphorus release Resuspension of particle-attached phosphorus

25 Lake of the Woods Lake Segments n from HSPF our Mile abaskong uskeg ig Traverse ittle Traverse total tributaries luding lakesheds) n Tributary Rainy r drains 80% of in

26 Lake of the Woods Basin 27,000 square miles >30,000 Waterbodies Bigger than West Virginia!

27 Fish species vary relative to lake trophic status (Carlson s TSI) Based on work of Dennis Schupp & paper by Schupp & Wilson 1993

28 DEEP - Dimic Epilimnion Metalimnion Hypolimnion p P D.O. Temp

29 HSPF/Bathtub Integration Advantage to HSPF approach Continuous simulated Q and concentration time series Time series for ungauged tributaries are available by applying land use loading and runoff from gauged tributaries HSPF is very accurate over long periods (10 years for TMDL) Without HSPF: More difficult to estimate loading for ungauged tributaries BATHTUB - P model reflected resuspension for estimating lake responses.