Week 6 Sediment-biological interaction

Question 1

Biostabilisation vs. Biodestabilisation

Answer 1

Biostabilisation: Biological processes that make sediment harder to move. E.g. the secretion of extracellular polymeric substances (EPS) by microorganisms that bind sediment particles together

Biodestabilisation: Processes that make sediment easier to move. Occur through burrowing, grazing, or the movement of organisms that disturb the sediment structure

Question 2

How does Biology Influence Sediment Movement

Answer 2

Biology affects sediment dynamics by altering the physical properties of the sediment bed.

Examples:
- Microalgae and Biofilms: Increase sediment cohesion.

-Benthic Fauna: Bioturbation by worms or shellfish destabilizes the bed, making sediment easier to erode

Question 3

Biological Physical interactions

Answer 3

Biostabilisation and biodestabilistaion
Wave and Current Attenuation
Bed roughness and Baffling

• Context Dependency: The impact of these interactions is highly dependent on environmental factors like season, habitat type, and socio-economic influences.

Question 4

Biostabilisation and destabilisation effects

Answer 4

Affect sediment cohesion and erosion thresholds.

Question 5

Wave and Current Attenuation

Answer 5

Vegetation and biofilms can reduce the impact of hydrodynamic forces.

Question 6

Bed Roughness and Baffling

Answer 6

Created by biological structures, which modify flow and sediment transport patterns.

Question 7

Biofilms - impact on sediment stability

Answer 7

Three Stages of Biofilm Development:

1. Initial patches of organic matter on sediment grains. These have destabilizing effects, as they reduce sediment cohesion.
2. A more structured biofilm network develops, where stabilizing effects start to outweigh destabilizing ones, leading to increased sediment cohesion.
3. A mature biofilm mat forms, strongly attached to the bed. This stage significantly enhances sediment stability, making erosion less likely.

Question 8

Temporal and Spatial Variability in Biological Influence

Answer 8

Critical shear stress (𝜏𝑐
) is higher at night in lower intertidal zones and higher during the day in middle intertidal zones

Linked to phaeopigments (chlorophyll degradation products) and colloidal carbohydrate levels, affecting sediment cohesion.

Complexity: Multiple variables interact, making it unlikely that a single cause drives these patterns.

Question 9

Temporal and Spatial Variability – Tidal and Seasonal Patterns

Answer 9

Biomass fluctuations are driven by light availability, grazing, and tidal resuspension.

Seasonal Changes: Influenced by temperature and nutrient cycles, leading to variations in Chlorophyll a and sediment stability.

Question 10

Practical Applications of Biostabilisation

Answer 10

Nature-Based Solutions:

Bank Stabilisation: Vegetation roots increase soil cohesion, and above-ground vegetation reduces surface runoff and erosion.

Example: Root structures of plants like salt marsh grasses stabilize sediment, and this method is often combined with engineering solutions like geocells and anchors to reinforce banks.

Benefits: Combines erosion control with biodiversity conservation.

Question 11

Roots and Sediment Stability

Answer 11

Adapted Mohr-Coulomb Equation:
𝑆=𝐶𝑆+ 𝐶𝑅 + 𝜎𝑁 tan𝜙

​Cs: Shear resistance of unrooted soil.
CR : Additional shear resistance from roots.
𝜎𝑁 : Normal stress on the shear plane.
ϕ: Soil friction angle.

Implication: Sediment stability depends on both vegetation type and soil properties.

Question 12

Bioturbation

Answer 12

The transport of sediment by benthic organisms, affecting sediment matrices.

Reworking: Physical movement of sediment particles.

Ventilation: Water movement within sediment layers, altering oxygen and nutrient dynamics.

• Bioturbation is a key process in aquatic environments, affecting sediment stability and ecosystem function

Question 13

Methods of Quantifying Bioturbation

Answer 13

Direct Methods:

Collecting samples or trapping sediment disturbed by organisms.

Example: Modified Sediment Profile Imaging (SPI) cameras capture time-lapse images of sediment disturbance, tracking fluorescent particles mixed into the sediment.

Indirect Methods: Using tracers like luminiphores to observe particle movement.

Question 14

Bioturbation vs. Roughness Effects

Answer 14

Dairain et al., 2020, explored the effects of cockles (Cerastoderma edule) on sediment stability.

Findings:
- Increased roughness from cockle activity promotes erosion.
- The impact is density-dependent, but decreases when phytoplankton are abundant, as bioturbation activity lessens.

• Key Concept: Distinguishing between erosion caused by organism-induced roughness and other factors is critical for accurate predictions.

Question 15

Impact of Disarticulated Shells on Sediment Mobility

Answer 15

Thompson & Amos, 2002 demonstrated that shells from dead cockles can influence sediment erosion.

Mechanism: Shells act as mobile bedload, abrading the sediment surface.

Critical Shear Stress: Erosion is triggered when shear stress moves shells, not sediment properties alone.

Note: More rounded shells can saltate, adding to erosion through ballistic impacts.

Question 16

Bioroughness and Flow Modification

Answer 16

Biological elements like vegetation alter the near-bed flow structure

Effect: Reduces flow velocity, enhances sediment deposition, and protects the bed from erosion.

Examples:
- Seagrass beds reduce wave energy and modify boundary layer turbulence.
- Plant flexibility and density influence these protective effects.

Question 17

Vegetation and Flow Dynamics – Laboratory Observations (Seagrass Canopies)

Answer 17

Findings: Velocity reduction within the canopy, increased turbulence near the canopy height, and possible formation of skimming flow layers.

Hydroelasticity: Seagrass movements undulate (monami) this dissipates energy and limit vertical mixing.

Question 18

Vegetation Effects on Suspended Sediment Dynamics

Answer 18

Mechanisms:

1. Direct Sediment Capture: Sediment settles on blades of vegetation, especially during rainfall events.
2. Enhanced Sedimentation: Flow modifications increase the rate of sediment settling when upward turbulence is reduced.
3. Density Dependence: Denser vegetation traps more sediment, but this effect can vary with particle size.

Question 19

Habitat, Biotopes, and Marine Landscapes

Answer 19

Habitat: Physical environment, including sediment type, water conditions, and exposure levels.

Biotope: Combination of habitat and associated community species.

Marine Landscape: Assemblages of habitats consistently found together.

Question 20

Marine Protected Areas (MPAs) types

Answer 20

Conserving marine ecosystems while promoting sustainable resource use.

Marine Conservation Zones (MCZs): Protect specific species and habitats.

Special Areas of Conservation (SACs): Preserve habitats and species under EU directives.

Special Protection Areas (SPAs): Focus on bird species conservation.

Question 21

Vegetation and Flow – Influencing Factors and Flow Dynamics

Answer 21

Plant Morphology: Height, flexibility, and structure of the vegetation affect how water flows around and through the plants.

Stem Density: Higher density leads to greater resistance and more significant flow reduction.

Patchiness: The spatial distribution of vegetation influences flow patterns and energy dissipation.

Emergence/Submergence Ratio: Refers to whether the vegetation is fully submerged or partly emergent, which changes the flow impact.

Flow Velocity: The speed of the water affects how strongly the vegetation influences flow dynamics.