OSX-2007: Suspended Particulate Matter (SPM)

📌 1. Introduction to SPM

• SPM (Suspended Particulate Matter): Refers to fine particles suspended in the water column.

• Includes both inorganic particles (e.g., clay, silt, sand) and organic particles (e.g., phytoplankton, zooplankton fragments, faecal pellets, detritus).

• Often visualised via turbidity (water clarity/murkiness).

• Terminology:

  • SPM (Suspended Particulate Matter)

  • SSC (Suspended Sediment Concentration)

  • TSS (Total Suspended Solids)

  • MSS (Mineral Suspended Solids)

  • Organic SPM (via loss on ignition)

💡 2. Why Is SPM Important?

A. Physical and Geochemical Roles

• Sediment Transport: SPM is carried by tidal/wave currents and resettles elsewhere (important for harbour sedimentation).

• Biogeochemical Fluxes: Moves nutrients and organic/inorganic materials through systems.

• Pollutant Vector: Pollutants (e.g. heavy metals, microplastics) adhere to particle surfaces and are transported with SPM.

B. Light Attenuation & Primary Production

• Scattering of light by particles increases light path, making photons more likely to be absorbed.

• Decreases photosynthetically active radiation (PAR) → impacts phytoplankton, seagrass, kelp, and benthic algae productivity.

C. Food Source

• Organic fraction of SPM provides food for filter feeders (e.g. deep-sea sponges, corals, suspension feeders).

• Marine snow = visible aggregates (flocs) of organic/inorganic material.

🧪 3. SPM Types and Dynamics

A. Simple Suspensions

• Dominant in high-energy environments (e.g. near sandy seabeds).

• Higher flows = resuspension of coarser particles (sands, silts).

B. Flocs (Aggregates)

• Found in quiescent areas (low-energy), often during spring blooms.

• Composed of:

  • Mineral particles (clay, silt)

  • Organic material (algae, faeces)

  • Entrained water

• Turbulence:

  • Low: Allows growth of large flocs.

  • High: Breaks flocs apart into smaller particles.

• Controlled by Kolmogorov length scale: the smallest eddies in a turbulent flow constrain floc size.

🌊 4. Spatial and Temporal Variability in SPM

A. Vertical (Depth) Variation

• Higher SPM near the seabed due to gravity settling and turbulence from bed shear.

• Larger particles settle faster, but strong bottom shear can resuspend them.

B. Horizontal Variation

• Influenced by:

  • Tides: Strong tides = more SPM (especially in estuaries).

  • Wind/wave action: Storms = temporary SPM spikes.

  • Seabed type: Sandy vs. muddy vs. rocky.

  • Biological activity: Spring blooms increase organic floc formation.

C. Temporal Variation

• Tides (semi-diurnal, spring-neap cycles) influence regular SPM cycling.

• Storms cause episodic resuspension.

• Horizontal advection: Movement of sediment plumes across measuring stations causes periodic variation.

📊 5. Measuring SPM in the Field

A. Gravimetric Analysis (Gold Standard)

1. Collect samples with Niskin bottles (surface and near-bed).

2. Filter water through pre-weighed filter papers.

3. Dry and re-weigh filters to determine mass of SPM.

4. SPM concentration = mass of particles / volume of water.

• Loss on ignition can determine organic vs. mineral fractions (burning off organic material at ~500–550°C).

B. Optical & Indirect Methods

• Satellite Limitations: Only detects surface layers (top 10 cm – 2 m); requires cloud-free conditions.

📈 6. Modelling SPM Concentrations

A. Suspension-Settling Equilibrium

• Turbulence resuspends particles.

• Settling velocity (∝ particle diameter²) pulls them down.

• Result: Exponential SPM decay profile from seabed upward.

B. Rouse Profile:

• Predicts vertical SPM profile from particle size and turbulence.

• Fine particles → well-mixed in water column.

• Coarse particles → confined to near-bed zone.

C. Bed Shear Stress & Particle Mobilisation

• Tides and waves generate bed shear stress (τ₀).

• Different particles require different threshold τ₀ to be mobilised.

• Shields Parameter used to estimate this critical τ₀.

D. % Time Above Threshold

• Mapping the % of the year when τ₀ exceeds critical value shows areas of frequent sediment resuspension.

🧠 7. Twin-Peak Pattern in Time Series Data

• Observed in beam transmittance data.

• Caused by:

  • Advection of horizontal SPM gradient (semi-diurnal).

  • Vertical resuspension cycle (quarter-diurnal).

• Combined effect creates double peaks per tidal cycle.

Example: Observed by Alison Weeks in the 1990s, described via sketches and models of combined advection-resuspension cycles.

🛰 8. Satellite & Model Data

Satellite:

• High-resolution ocean colour imagery (e.g. MERIS).

• Shows spatial patterns of surface SPM (e.g. turbid estuaries, tidal mixing zones).

• Useful for large-scale patterns but limited to surface only.

Numerical Models:

• Can simulate:

  • Bed shear stress from tides/waves.

  • Advection & resuspension patterns.

• Example: Ratio of wave:tide bed shear reveals dominant process by region (e.g. tide-dominated Irish Sea vs. wave-dominated Dogger Bank).

🧾 9. Summary: Key Takeaways

• SPM affects light, productivity, sediment transport, pollution, and food webs.

• Controlled by physical (tides, waves, shear, turbulence) and biological (phytoplankton, flocculation) processes.

• Varies spatially and temporally — driven by dynamic interactions.

• Measurement techniques range from direct gravimetric sampling to indirect optical methods and satellite imagery.

• Floc formation and break-up are turbulence-dependent; turbulence both controls and is influenced by SPM.

• Tidal cycles and advective gradients generate distinctive twin-peak signals in SPM data.