Understanding particle behavior in the aquatic environment is fundamental to unraveling key processes in biogeochemistry, sediment transport, and ecosystem dynamics. Since 1995, Sequoia’s LISST and Hyper instruments have become important tools for quantifying particle size distributions (PSDs), concentrations and optical properties in situ. How do particle dynamics differ across diverse aquatic environments—and what can Sequoia instruments reveal about these differences?
Why Particle Size Matters
Particle size influences everything from light attenuation and nutrient cycling to the sinking rates and deposition of organic and inorganic matter and sediment. Disaggregated fine particles tend to remain suspended longer, affecting turbidity and light penetration. Larger aggregates (e.g., flocs, marine snow) play a crucial role in sedimentation, coastal sediment transport, tidal flat dynamics and vertical carbon flux.
Rivers: A One-way Conveyor Belt for Sediment
In rivers, sediments are continuously eroded, transported and deposited, eventually discharging in a delta or coastal plume. A significant feature of sediment transport in rivers is its temporal variability. This variability is guided not only by watershed-scale vents like precipitation, but also small-scale processes such as streaks, sweeps, ejections, vortices, and kolks. LISST data (LISST-SL2) have shown that:
- These events necessitate temporal averaging, sometimes for as long as ~10 minutes to get a representative value of the suspended sediment concentration.
- Sediment flux estimates based only on the product of mean concentration and mean velocity may underestimate the true transport rate when (coarse) sediment is transported in bursts.
Tidal Flats: Daily Cycles of Erosion and Deposition
Tidal flats are among the most productive habitats on the planet. Influenced by sediment dynamics and characteristics, the daily primary productivity on a mid-latitude mudflat can be as high as in a tropical rainforest (~3 g C/m2/day). LISST data has revealed that
- Flocculation is responsible for producing large deposition rates near secondary channels on intertidal mudflats, and less so on the flats themselves.
- Filter feeders alter floc properties and may enhance flocculation by excretion of exopolymer particles, potentially increasing the mud flat stability and erosion resistance
Fjords: A Dynamic Sediment Conveyor
Fjords, particularly those influenced by glacial melt, are hotspots of fine sediment input. They can be optically complex and a challenge to study using remote sensing. Sills at the fjord entrance often restrict the exchange of water and particles with the open ocean. This creates vastly different environments between the surface waters and the more stagnant bottom waters. LISST measurements in these systems have revealed:
- Remote Sensing atmospheric correction algorithms have performed better during bloom than non-bloom scenarios.
- How labile iron affects floc sizes, shapes and densities, yielding low settling velocities and extended sediment plumes: Understanding flocculation mechanisms when examining fluxes of meltwater transported iron in polar regions becomes important.
- High concentrations of fine silt and clay near glacial termini, with consequences for the optical properties.
- Strong vertical gradients in particle size due to stratification and flocculation.
- Seasonal variability driven by meltwater discharge and freshwater plumes.
Continental Shelves: A Patchwork of Processes
On continental shelves, LISST data often show:
- Mixed particle populations, impacted by resuspended sediments, advected sediment, phytoplankton, and detritus.
- Tidal and storm-driven variability in PSDs, especially in shallow, energetic zones.
- Localized flocculation near river mouths or estuarine fronts.
Deployments from ships, on moorings or on autonomous vehicles allow for high-resolution temporal monitoring, revealing episodic events like storm-induced resuspension or phytoplankton blooms.
Open Ocean: The Realm of Biogenic Particles
Particle dynamics in the open ocean are largely unaffected by particles derived from land – almost all particles are produced locally. In oligotrophic gyres and deep ocean basins, LISST instruments detect:
- Diatom blooms monitored for months using LISST on USV.
- Vertical diatom distribution
- Low particle concentrations, dominated by biogenic material such as coccolithophores or diatom frustules.
- Narrow PSDs, often centered around small phytoplankton or microzooplankton.
- Vertical flux signatures, where larger aggregates appear in deeper layers, indicating sinking marine snow.
Here, LISST data are often paired with sediment traps or optical backscatter sensors to quantify carbon export and validate biogeochemical models.
Comparative Insights
| Environment | Dominant Particle Types | Key LISST Observations |
|---|---|---|
| Rivers | Sand and silt eroded from land | Turbulence impacts manual bottle sampling |
| Tidal Flats | Cohesive sediments | Flocculation processes, sediment stability |
| Fjords | Glacial silt, clay, flocs | Atmospheric correction, flocculation importance, high turbidity, sharp vertical PSD gradients |
| Continental Shelves | Mixed sediments, plankton | Tidal variability, episodic resuspension, river inputs |
| Open Ocean | Biogenic particles, aggregates | Low concentrations, vertical flux patterns |
Toward a Global Understanding
In addition to ships and benthic landers, LISST instruments are deployable on gliders, moorings, profiling floats, and wire-walkers. PSD comparison across environments enhances our understanding of the local or environment-specific processes. At the same time, all datasets add to global knowledge and our collective understanding of particle dynamics. We try to keep tab on it – check back frequently for updates to our website article library!