1. Water movements Announcements Canoe trip Tuesday's lab: pick up syllabus Today: Last week's Q of the day Water movements Brainstorming independent project ideas

2. Question of the day In the summer, in this Northfield-area lake, why does the oxygen profile indicate low oxygen below the thermocline? 0 mg/l8 mg/l

3. Surface gravity waves Maximum wave length (m) influenced by maximum fetch (km) Max. wave height = 0.332*(Fetch max ) 0.5 Whitecaps occur at wind speed >4 - 5 m/s Wave length

4. Surface gravity waves In deep water, water 'parcels' oscillate elliptically & do not travel horizontally with wave In shallow water, some horizontal movement when ellipses contact sediments If orbital velocity is great enough, (shallow water) sediments are re-suspended Scan of pg 182, fig 12-3

5. Currents (moving water) Mixing requires turbulent flow, not laminar flow

6. Eckman spiraling currents Caused by the wind Resemble a spiral staircase of horizontal currents that ultimate move the opposite direction of the wind Coriolis effect - trying to go straight on a turning planet

7. Convection (density) currents Result: Onshore- offshore currents that exchange water between littoral and pelagic zones a.k.a. The Thermal Siphon

8. Langmuir spirals Wind direction

9. Langmuir spirals

10. Surface seiche (standing surface wave) Epilimnetic water piles up on one end of the lake due to sustained strong winds When wind ceases, water sloshes back (think: bathtub) Usually, but not always, visible as minor changes in water height. In the Great Lakes, water levels can fluctuate meters. Short-lived.

11. Lake Erie storm surges (from Ohio Sea Grant's Homepage)

12. Internal circulations Things are more turbulent below the surface than we might have thought… Lake Kinneret in Isreal as an example (thanks to folks at the Centre for Water Research, at the University of Western Australia)

14. Missing: Thermal Profile of Lake Kinneret over 4 days

15. Thermocline seiches Caused by wind stress forcing epilimnetic water towards one end of the lake The extra water tilts the thermocline When the wind lessens, the thermocline tilts back and forth Regular displacement in Lake Kinneret due to daily sea breezes Missing: movies of thermocline waves…

16. Results of thermocline tilting Horizontal currents Entrainment of epilimnetic waters or hypolimnetic waters Transfers nutrient-rich waters, heat, etc across the thermocline Erodes weak thermoclines

17. Currents created by seiches

18. The Dead Zone Exposes benthic organisms to oxygen-depleted waters The Dead Zone thermocline