Water as an EnvironmentWater as an Environment

LightLightWater MovementsWater Movements

Part 3Part 3

Light and Primary Production•The depth of primary production in lakes is dependent on light penetration• k is used to determine the compensation depth (1% of subsurface light)

Compensation depth

Payne, 1986

•Phytoplankton circulates vertically through the mixed layer.

•The thickness of the euphotic zone (Zeu) in relation to the thickness of the mixing layer (Zmix)has a large effect on primary production. •If zeu:zmix is low, phytoplankton will be spending much of their time below the compensation depth.Irradiance and mixing depth change with the seasons, which helps to explain seasonal patterns of primary production.

Light and Predation

DMV: Diel (daily) vertical migration

Vertical Mixing and Nutrient Cycles

Nutrient concentrations (N and P) increase after spring thaw due to tributary input and isothermal mixing (whole lake in contact with sediments)

Nutrient concentrations in epilimnion decline over summer because

Tributary inputs decline (less external loading) Surface waters are not in contact with sediments

(less internal loading) Nutrient concentrations in epilimnion increase

after fall turnover

Phytoplankton growth cycles (Typical temperate-zone lake)

Phytoplankton Growth Cycle are the product of

Seasonal Temperature Cycles Light Nutrients

JanJan JuneJune Dec DecNut

rient

s in

Epi

limni

on

Nut

rient

s in

Epi

limni

on

Ice-outIce-out TurnoverTurnover

JanJan JuneJune Dec Dec

Ligh

tLi

ght

Ice-outIce-outTurnoverTurnover

Light available to organisms in the lake changes over the seasons Low light under snow/ice cover Increased light as snow melts and ice thins. Very low light during spring isothermal period (high turbidity, deep

mixing) Light in epilimnion increases after stratification (longer daylength,

increased clarity) Light levels decline at fall turnover

Light

JanJan JuneJune Dec Dec

Tem

pera

ture

Tem

pera

ture

Ice-outIce-outTurnoverTurnover

Temperature of the epilimnion follows a regular seasonal pattern

Temperature

Tem

pera

ture

Tem

pera

ture Ice-outIce-out

TurnoverTurnover

Combine all three factors

Ligh

tLi

ght

JanJan JuneJune Dec Dec

Nut

rient

sN

utrie

nts

For phytoplankton growth in the epilimnion

1. During spring mixing, conditions are poor – very low light

2. Following spring stratificaton conditions are excellent. (high light, high nutrients, cool temperature)

3. Nutrient depletion in epilimnion, High temperature causes high sinking rates. OK conditions

4. Higher nutrients as hypolimnion begins to mix with epilimnion. Good conditions

5. Poor conditions due to low light

11 22 33 44 55

Annual Phytoplankton Growth Cycle

JanJan JuneJune Dec DecPh

yto

pla

nkto

n G

row

thP

hyt

opl

ank

ton

Gro

wth

Ice-outIce-outTurnoverTurnover

11

22

5533

44

Water MovementsWater Movements

Gravity WavesGravity Waves SeichesSeiches Internal wavesInternal waves Laminar flow vs. turbulent flowLaminar flow vs. turbulent flow

Gravity (Surface) wavesGravity (Surface) waves Characterized by wavelength (L), height (h), and period Water molecules mainly move up and down, not sideways Wave amplitude (height) attenuates with depth Maximum wave height in a lake is proportional to fetch

H = 0.105 (x)1

/

2

SeichesSeiches Strong, sustained winds cause water to “slosh” to the far end of the lake When wind stops, the water sloshes back and forth several times. Seiche may have one or more nodes Seiche period can be calculated by T= 2L/(n(gh)1/2)

L=lake length, n=# of nodes, g=gravity, h=depth of lake

Lake Erie seiche period is about 14 hours.

Internal wavesInternal waves A surface seiche can cause internal waves in a stratified lake between the epilimnion and hypolimnion. Internal waves typicallly have many nodes and continue for long after the surface seiche ends. Internal waves cause limited mixing between the l

Nutrients from the hypolimnion can be mixed into the epilimnionIncrease in epilmnion phytoplankton productivity

http://www.youtube.com/watch?v=oljinlD2yho&feature=fvsr

Turbulent vs Laminar Flow At low speeds and small scales, water flow is laminar,

adjacent layers of water do not mix with each other At higher speeds and larger scales, laminar flow breaks

down and flow becomes turbulent (chaotic). Laminar and turbulent characteristics are important for

nutrient dynamics of phytoplankton and feeding of zooplankton

http://www.youtube.com/watch?v=p08_KlTKP50&feature=related