Water as an Environment Light Water Movements Part 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 (Z eu ) in relation to the thickness of the mixing layer (Z mix )has a large effect on primary production. If z eu :z mix 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

JanJune Dec Nutrients in Epilimnion Ice-outTurnover

JanJune Dec Light Ice-out Turnover   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

JanJune Dec Temperature Ice-out Turnover   Temperature of the epilimnion follows a regular seasonal pattern Temperature

Temperature Ice-outTurnover Combine all three factors Light JanJune Dec Nutrients   For phytoplankton growth in the epilimnion 1. 1.During spring mixing, conditions are poor – very low light 2. 2.Following spring stratificaton conditions are excellent. (high light, high nutrients, cool temperature) 3. 3.Nutrient depletion in epilimnion, High temperature causes high sinking rates. OK conditions 4. 4.Higher nutrients as hypolimnion begins to mix with epilimnion. Good conditions 5. 5.Poor conditions due to low light 12345

Annual Phytoplankton Growth Cycle JanJune Dec Phytoplankton Growth Ice-outTurnover

Water Movements  Gravity Waves  Seiches  Internal waves  Laminar flow vs. turbulent flow

Gravity (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 = (x) 1/2

Seiches 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 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 epilimnion Increase in epilmnion phytoplankton productivity tch?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 8_KlTKP50&feature=related