Lecture 7

Question 1

What physical processes influence substance movement in ecosystems?

Answer 1

Advection, diffusion, dispersion, and resuspension.

Question 2

What is advection?

Answer 2

The transport of substances with the flow.

Question 3

What is diffusion?

Answer 3

The smoothing of concentration gradients.

Question 4

What is dispersion?

Answer 4

A combination of advection and diffusion.

Question 5

What is resuspension?

Answer 5

The reintroduction of sediment into the water column due to shear stress.

Question 6

What factors influence resuspension?

Answer 6

• 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.

Question 7

What does Stoke’s Law describe?

Answer 7

The settling velocity of small particles based on particle density, fluid density, particle radius, and fluid viscosity.

Question 8

What is the Reynolds number (Re)?

Answer 8

A value that determines whether flow conditions are laminar (low Re) or turbulent (high Re).

Question 9

What is the difference between settling and resuspension?

Answer 9

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.

Question 10

What does Stoke’s Law describe?

Answer 10

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.

Question 11

What assumptions are made in Stoke’s Law?

Answer 11

Assumes spherical particles, steady-state conditions, and laminar flow.

Question 12

How is the Reynolds number (Re) calculated, and what does it indicate?

Answer 12

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.

Question 13

What factors affect sedimentation rates?

Answer 13

Temperature (affecting viscosity and density), particle size and shape, and water movement.

Question 14

How does temperature affect settling rates?

Answer 14

Higher temperatures reduce viscosity and density, leading to faster settling rates.

Question 15

What is critical shear stress (τc)?

Answer 15

The minimum force needed to resuspend sediments. If shear stress (τ) exceeds τc, resuspension occurs.

Question 16

What is fetch, and why is it important?

Answer 16

Fetch is the distance over which wind transmits energy to water, influencing wave height and the extent of resuspension.

Question 17

How does sediment type affect resuspension?

Answer 17

Coarse sediments (e.g., sand) resuspend more easily than clay, which becomes more cohesive over time through consolidation.

Question 18

How is sedimentation modeled as a first-order kinetic process?

Answer 18

The rate of mass loss (dm/dt) is proportional to the settling rate (s) and suspended matter (m): dm/dt = -s * m.

Question 19

How does wind direction and speed influence resuspension?

Answer 19

Strong winds increase τ, while direction determines the affected shoreline area. Specific winds like Bora (from NE) can significantly alter fetch.

Question 20

What role does water depth play in resuspension?

Answer 20

In deeper water, energy dissipates before reaching the bottom, reducing resuspension compared to shallower areas.

Question 21

first order Michaelis-Menten-kinetics:

In a graph, y = growth grate and x = nutrients, where is K(n) ?

Answer 21