A new review of suspended particulate matter (SPM) reveals that the seemingly ordinary cloudiness of rivers, estuaries and coastal waters may be one of the most powerful signals of how carbon, nutrients and contaminants move through the planet’s aquatic systems. Published in Nature Reviews Earth & Environment, the study synthesizes physical, sedimentological and biogeochemical evidence from across the land–ocean continuum. Its central message is striking: the concentration of particles in the water is closely linked to what those particles are made of, how large and dense they are, how quickly they settle and how they influence the movement of organic carbon.
SPM is not a single substance. It is a constantly changing mixture of mineral grains, organic fragments, microorganisms and chemical compounds that interact as they are carried downstream. Some particles originate in soils and eroding landscapes, while others are produced within rivers, wetlands, estuaries and coastal waters. Their properties can change rapidly as freshwater mixes with seawater, flow speeds shift, biological activity increases or decreases, and particles collide and bind together. These changing structures, known as flocs, determine whether material remains suspended, settles onto the bed or is transported farther toward the ocean.
The review identifies a robust relationship between SPM concentration and the physical and chemical character of suspended particles. When waters contain large quantities of particles, the material is generally dominated by small, mineral-rich grains. These particles remain suspended because turbulence and flow can counteract their tendency to sink. At lower SPM concentrations, however, organic matter becomes more prominent and particles are more likely to assemble into larger, fragile flocs. Although these aggregates may be physically larger, their porous structure can make their effective density lower than that of compact mineral grains, influencing how rapidly they settle.
This pattern is especially visible in turbidity maximum zones, which commonly form in estuaries where river flow meets tides and seawater. In these regions, strong mixing, tidal circulation and the convergence of opposing water movements trap enormous quantities of fine sediment. The result is a highly turbid environment in which flocs tend to be small and composed largely of minerals. As water moves away from the turbidity maximum and becomes clearer, particle size and organic matter content generally increase. This transition can transform the way material is transported, creating a switch between prolonged suspension and rapid deposition.
Organic matter plays a critical role in this process because it can act as a natural binding agent. Organic coatings produced by plants, microorganisms and decaying material help mineral particles adhere to one another. The resulting flocs can contain water, mineral surfaces and biological material in a complex, three-dimensional structure. Their formation is influenced by salinity, turbulence, oxygen conditions, microbial activity and the availability of sticky organic compounds. A change in any of these factors may alter floc size and stability, affecting whether carbon and associated contaminants remain in the water or become buried in sediment.
The relationship between SPM and carbon is equally important. The review reports that the ratio of particulate organic carbon to dissolved organic carbon tends to increase as SPM concentration rises. In other words, particle-rich waters generally carry a larger share of their organic carbon in suspended form, while clearer waters tend to contain proportionally more carbon dissolved in the water itself. This distinction matters because particulate and dissolved carbon follow different pathways. Suspended carbon can settle, be buried and potentially preserved in sediments, whereas dissolved carbon may be transported farther, transformed by microorganisms or exchanged with the atmosphere.
SPM also acts as a vehicle for nutrients and pollutants. Minerals and organic particles provide surfaces to which metals, phosphorus, hydrophobic chemicals and other contaminants can attach. When particles settle, these substances may be temporarily stored in riverbeds, floodplains, estuarine mud or coastal sediments. But changes in flow, salinity, acidity, oxygen availability or sediment disturbance can remobilize them. A particle that appears to be settling harmlessly may therefore be moving a contaminant into a temporary reservoir rather than removing it permanently from an ecosystem.
Human activity is now reshaping these natural particle pathways. Dams can trap sediment upstream, reducing the supply of mineral material reaching downstream wetlands, deltas and coastlines. Land-use change, including deforestation, intensive agriculture and urban development, can increase erosion and deliver additional particles to rivers. Channel engineering, dredging and shoreline modification can further alter turbulence, sediment storage and resuspension. At the same time, climate-driven changes in rainfall, floods, droughts, sea level and storm intensity may produce abrupt shifts in SPM concentration and composition, making long-term trends difficult to predict.
These changes have consequences far beyond water clarity. Reduced sediment delivery can leave wetlands and deltas unable to keep pace with sea-level rise, increasing their vulnerability to erosion and flooding. Excessive sediment input can smother habitats, reduce light penetration and disrupt aquatic productivity. Altered particle transport may change where contaminants accumulate and how easily they return to the water. It may also influence the global carbon cycle by modifying the balance between carbon that is exported in dissolved form, carbon that remains suspended and carbon that is buried in sediments. The same change in SPM can therefore create environmental risks in one location while producing different effects downstream.
The researchers argue that better monitoring is urgently needed to understand these connected processes. Traditional measurements often capture SPM concentration at isolated points, but concentration alone cannot reveal whether particles are mineral-rich or organic-rich, compact or flocculated, rapidly settling or remaining suspended. Future studies should combine in situ observations with remote sensing capable of tracking particle concentration, size and composition across entire river-to-ocean systems. Such data could improve models of sediment accretion and erosion, pollutant mobility and carbon export. By treating SPM not merely as a measure of muddy water but as a dynamic indicator of ecosystem processes, scientists may gain a clearer view of how natural systems are responding to human pressure and climate change.
Subject of Research: Suspended particulate matter and its role in sediment transport, carbon cycling, nutrient movement and contaminant mobility across rivers, estuaries, wetlands and coastal waters.
Article Title: Suspended particulate matter along the land–ocean continuum
Article References: Fettweis, M., Belliard, J.P., Silori, S. et al. “Suspended particulate matter along the land–ocean continuum.” Nature Reviews Earth & Environment (2026). https://doi.org/10.1038/s43017-026-00815-w
Image Credits: AI Generated
DOI: 10.1038/s43017-026-00815-w
Keywords: suspended particulate matter, SPM, flocs, sediment transport, land–ocean continuum, turbidity maximum, organic carbon, dissolved organic carbon, carbon cycle, wetlands, estuaries, contaminants, climate change, dams, remote sensing
