Lecture 7 Flashcards
What physical processes influence substance movement in ecosystems?
Advection, diffusion, dispersion, and resuspension.
What is advection?
The transport of substances with the flow.
What is diffusion?
The smoothing of concentration gradients.
What is dispersion?
A combination of advection and diffusion.
What is resuspension?
The reintroduction of sediment into the water column due to shear stress.
What factors influence resuspension?
- Shear Stress: The force exerted by moving water that can lift particles.
- Sediment Type: Fine-grained sediments like clay resuspend more easily than coarse sand.
What does Stoke’s Law describe?
The settling velocity of small particles based on
- particle density + radius
- fluid density + viscosity
What is the Reynolds number (Re)?
A value that determines whether flow conditions are laminar (low Re) or turbulent (high Re).
What is the difference between settling and resuspension?
Settling refers to the downward movement of particles (e.g., phytoplankton leaving the euphotic zone), while resuspension refers to the upward movement of benthic sediments into the water column.
What does Stoke’s Law describe?
It describes the settling velocity (v) of particles, influenced by
- buoyancy (Fb),
- friction (Ff), and
- gravity (Fg).
- Formula: v = (g * (ρp - ρf) * d²) / (18μ),
- where ρp and ρf are particle and fluid densities, d is diameter, and μ is viscosity.
What assumptions are made in Stoke’s Law?
- spherical particles
- steady-state conditions
- laminar flow
How is the Reynolds number (Re) calculated, and what does it indicate?
Re = (d * ρf * v) / μ. It indicates the ratio of inertial to viscous forces, with Re < 1 indicating laminar flow and Re > 1000 indicating turbulence.
What factors affect sedimentation rates?
- Temperature (affecting viscosity and density)
- particle size and shape
- water movement
How does temperature affect settling rates?
- Higher temperatures reduce viscosity and density
- leading to faster settling rates
imagine something flowing through honey vs heated honey
What is critical shear stress (τc)?
The minimum force needed to resuspend sediments. If shear stress (τ) exceeds τc, resuspension occurs.
What is fetch, and why is it important?
Fetch is the distance over which wind transmits energy to water, influencing wave height and the extent of resuspension.
How does sediment type affect resuspension?
Coarse sediments (e.g., sand) resuspend more easily than clay, which becomes more cohesive over time through consolidation.
What is a first-order kinetic process?
rate of change proprtional to concentration
How is sedimentation modeled as a first-order kinetic process?
How does wind direction and speed influence resuspension?
Strong winds increase τ, while direction determines the affected shoreline area. Specific winds like Bora (from NE) can significantly alter fetch.
What role does water depth play in resuspension?
In deeper water, energy dissipates before reaching the bottom, reducing resuspension compared to shallower areas.
first order Michaelis-Menten-kinetics:
In a graph, y = growth grate and x = nutrients, where is K(n) ?
- K(n) represents the substrate (nutrient) concentration at which the system achieves half of its maximum growth rate.
- It is a key parameter for understanding nutrient uptake efficiency.
- A low K(n) indicates that algae can achieve high growth rates even at low nutrient concentrations, meaning they are highly efficient at nutrient uptake.
- A high K(n) means that algae require higher nutrient concentrations to reach similar growth rates, indicating lower efficiency.
