Sustained increase in suspended sediments near global river deltas over the past two decades.

River sediments play a critical role in sustaining deltaic wetlands. Therefore, concerns are raised about wetlands' fate due to the decline of river sediment supply to many deltas. However, the dynamics and drivers of suspended sediment near deltaic coasts are not comprehensively assessed, and its response to river sediment supply changes remains unclear. Here we examine patterns of coastal suspended sediment concentration (SSC) and river sediment plume area (RPA) for 349 deltas worldwide using satellite images from 2000 to 2020. We find a global increase in SSC and RPA, averaging +0.46% and +0.48% yr-1, respectively, with over 59.0% of deltas exhibiting an increase in both SSC and RPA. SSC and RPA increases are prevalent across all continents, except for Asia. The relationship between river sediment supply and coastal SSCs varies between deltas, with as much as 45.2% of the deltas showing opposing trends between river sediments and coastal SSCs. This is likely because of the impacts of tides, waves, salinity, and delta morphology. Our observed increase in SSCs near river delta paints a rare promising picture for wetland resilience against sea-level rise, yet whether this increase will persist remains uncertain.

Bioavailability of dissolved organic matter varies with anthropogenic landcover in the Upper Mississippi River Basin.

Anthropogenic conversion of forests and wetlands to agricultural and urban landcovers impacts dissolved organic matter (DOM) within streams draining these catchments. Research on how landcover conversion impacts DOM molecular level composition and bioavailability, however, is lacking. In the Upper Mississippi River Basin (UMRB), water from low-order streams and rivers draining one of three dominant landcovers (forest, agriculture, urban) was incubated for 28 days to determine bioavailable DOC (BDOC) concentrations and changes in DOM composition. The BDOC concentration averaged 0.49 ± 0.30 mg L-1 across all samples and was significantly higher in streams draining urban catchments (0.72 ± 0.34 mg L-1) compared to streams draining agricultural (0.28 ± 0.15 mg L-1) and forested (0.47 ± 0.17 mg L-1) catchments. Percent BDOC was significantly greater in urban (10% ± 4.4%) streams compared to forested streams (5.6% ± 3.2%), corresponding with greater relative abundances of aliphatic and N-containing aliphatic compounds in urban streams. Aliphatic compound relative abundance decreased across all landcovers during the bioincubation (average -4.1% ± 10%), whereas polyphenolics and condensed aromatics increased in relative abundance across all landcovers (average of +1.4% ± 5.9% and +1.8% ± 10%, respectively). Overall, the conversion of forested to urban landcover had a larger impact on stream DOM bioavailability in the UMRB compared to conversion to agricultural landcover. Future research examining the impacts of anthropogenic landcover conversion on stream DOM composition and bioavailability needs to be expanded to a range of spatial scales and to different ecotones, especially with continued landcover alterations.