Water movements

Announcements• Canoe trip• Tuesday's lab: pick up

syllabus

Today:Last week's Q of the dayWater movementsBrainstorming independent

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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/l 8 mg/l

Surface gravity waves

Maximum wave length (m) influenced by maximum fetch (km)

Max. wave height = 0.332*(Fetchmax)0.5

Whitecaps occur at wind speed >4 - 5 m/s

Wave length

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

Currents (moving water)

• Mixing requires turbulent flow, not laminar flow

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

Convection (density) currents

Result:Onshore- offshore currents that exchange water between littoral and pelagic zones

a.k.a. The Thermal Siphon

Langmuir spirals

Wind direction

Langmuir spirals

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.

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

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)

Missing: Thermal Profile of Lake Kinneret over 4 days

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…

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

Currents created by seiches

The Dead Zone• Exposes benthic organisms

to oxygen-depleted waters

The Dead Zonethermocline