Isotopic Tracking of Organic Matter in Estuarine Systems

Estuarine systems lie at the interface of rivers and the coastal ocean, where organic matter from terrestrial, marine and anthropogenic sources mixes, transforms and is buried. Tracking stable carbon and nitrogen isotopes in particulate and dissolved organic pools has emerged as a powerful means of distinguishing these sources, quantifying fluxes and understanding biogeochemical processing under varying hydrodynamic regimes. Isotopic signatures reveal contributions from C3 versus C4 vegetation, marine phytoplankton and sewage or fertiliser inputs, while shifts in δ15N highlight nutrient cycling and anthropogenic influence. By combining isotope ratios with elemental stoichiometry, sedimentological observations and mixing models, researchers can resolve spatial and temporal patterns of organic matter origin, transport and degradation. Such insights advance our understanding of carbon sequestration in marginal sediments, the fate of river-borne pollutants and the resilience of estuarine habitats to urbanisation and climate-driven hydrological change.

Research from Nature Portfolio

One recent investigation applied stable carbon and nitrogen isotope analysis to sediments in urban stormwater retention tanks, revealing that stormwater runoff delivers mixed sources of organic carbon—dominated by C3 land plants, wood and petroleum residues—and that organic nitrogen largely originates from synthetic fertilisers. Extreme rainfall events amplified nutrient and organic matter loads, with torrential storms causing dramatic spikes in nitrate, phosphate and organic carbon concentrations. Another foundational study in the Yellow River Estuary combined measurements of total organic carbon, total inorganic carbon and their isotopic compositions in surface sediments. It demonstrated a north–south gradient in both TOC and TIC, attributed to hydrodynamic energy and resuspension, and showed that most organic carbon derives from marine production except in sheltered bays where terrigenous inputs prevail. This work also provided evidence that resuspension-induced decomposition can transfer organic carbon to inorganic forms in the water column, influencing carbon burial efficiency.

Isotopic Tracking of Organic Matter in Estuarine Systems publication trend

The graph below shows the total number of articles in isotopic tracking of organic matter in estuarine systems across all publications each year (not limited to Nature Index journals).

Technical terms

Stable isotope ratio (δ13C, δ15N): The per mil deviation of the heavy to light isotope ratio in a sample relative to an international standard, used to distinguish organic matter sources and processing.

Particulate organic matter (POM): Organic particles suspended in water, comprising detritus, plankton and terrestrial debris, distinct from dissolved organic compounds.

Dissolved organic matter (DOM): Organic molecules in the water column small enough to pass through filters, influencing nutrient cycling and light attenuation.

C/N ratio: The molar ratio of carbon to nitrogen in organic material, indicative of source type (e.g., terrestrial plants vs. algae).

Bayesian mixing model: A statistical framework that uses isotope data to estimate proportional contributions of multiple sources to a mixture.

Autochthonous organic matter: Organic matter produced within the aquatic system, such as by phytoplankton or benthic microalgae.

Allochthonous organic matter: Organic matter derived from external sources, typically terrestrial soils, plants or anthropogenic inputs.

References

1. Sources of contamination in sediments of retention tanks and the influence of precipitation type on the size of pollution load. Scientific Reports (2023).
2. Organic and inorganic carbon and their stable isotopes in surface sediments of the Yellow River Estuary. Scientific Reports (2018).
3. Distribution and sources of particulate organic matter from the anthropogenically disturbed Iyidere River to the Black Sea coast. Frontiers in Environmental Science (2023).
4. Biogeochemical markers across a pollution gradient in a Patagonian estuary: A multidimensional approach of fatty acids and stable isotopes. Marine Pollution Bulletin (2018).
5. Natural and anthropogenic signatures on sedimentary organic matters across varying intertidal habitats in the Korean waters. Environment International (2019).