Background: Coastal sediments in the northern Gulf of Mexico have a high potential of being contaminated by petroleum hydrocarbons, such as polycyclic aromatic hydrocarbons (PAHs), due to extensive petroleum exploration and transportation activities. In this study we evaluated the spatial distribution and contamination sources of PAHs, as well as the bioavailable fraction in the bulk PAH pool, in surface marsh and shelf sediments (top 5 cm) of the northern Gulf of Mexico. Results: PAH concentrations in this region ranged from 100 to 856 ng g⁻¹, with the highest concentrations in Mississippi River mouth sediments followed by marsh sediments and then the lowest concentrations in shelf sediments. The PAH concentrations correlated positively with atomic C/N ratios of sedimentary organic matter (OM), suggesting that terrestrial OM preferentially sorbs PAHs relative to marine OM. PAHs with 2 rings were more abundant than those with 5–6 rings in continental shelf sediments, while the opposite was found in marsh sediments. This distribution pattern suggests different contamination sources between shelf and marsh sediments. Based on diagnostic ratios of PAH isomers and principal component analysis, shelf sediment PAHs were petrogenic and those from marsh sediments were pyrogenic. The proportions of bioavailable PAHs in total PAHs were low, ranging from 0.02% to 0.06%, with higher fractions found in marsh than shelf sediments. Conclusion: PAH distribution and composition differences between marsh and shelf sediments were influenced by grain size, contamination sources, and the types of organic matter associated with PAHs. Concentrations of PAHs in the study area were below effects low-range, suggesting a low risk to organisms and limited transfer of PAHs into food web. From the source analysis, PAHs in shelf sediments mainly originated from direct petroleum contamination, while those in marsh sediments were from combustion of fossil fuels.

Future conditions of coastal Louisiana are highly uncertain due to the dynamic nature of deltas, climate change, tropical storms, and human reliance on natural resources and ecosystem services. Managing a system in which natural and socio-economic components are highly integrated is inherently difficult. Sediment diversions are a unique restoration tool that would reconnect the Mississippi River to its deltaic plain to build and sustain land. Diversions are innately adaptable as operations can be modified over time. An expert working group was formed to explore how various operational strategies may affect the complex interactions of coastal Louisiana’s ecological and social landscape and provide preliminary recommendations for further consideration and research. For example, initial operations should be gradually increased over 5 to 10 years to facilitate the development of a distributary channel network, reduce flood risk potential to communities, limit erosion of adjacent marshes and reduce stress to vegetation and fish and wildlife species. Diversions should operate over winter peaks to capture the highest sediment concentration, reduce vegetation loss while dormant, and reduce detrimental effects to fish and wildlife. Operations during the spring/summer should occur over shorter periods to capture the highest sediment load during the rising limb of the flood peak and minimize impacts to the ecosystem. Operational strategies should strive to build and sustain as much of the coastal landscape as possible while also balancing the ecosystem and community needs.

Healthy wetlands are among the most effective sinks for carbon on the planet, and thus contribute to mitigate climate change. However, in North Africa, coastal wetlands are under high pressure especially from urban sprawl and tourism development, due to the rapid population growth and migration. This paper analyzed the effects of land use/land cover changes on carbon stocks, over 20 years, in six North African coastal wetlands, and estimated the economic value of the carbon sequestered during the considered period. The methodology used combined remote sensing and modeling. The results showed that among the six studied sites, only two (Moulouya and Moulay Bouselham) showed an increase in stored carbon and therefore are potential carbon sinks. In turn, the other four showed a more or less significant loss of carbon, which will likely be released into the atmosphere. The underlying processes that drive changes in carbon dynamics are mainly urban expansion and land use conversion, which often occurs at the expense of the natural habitats surrounding the wetlands. Understanding these processes can provide valuable decision-making information for land use planning, wetlands conservation and carbon reduction policies.

Globally, mineral sediment supply to deltaic wetlands has generally decreased so these wetlands increasingly rely on accretion of organic matter to keep pace with relative sea‐level rise (RSLR). Because organic‐rich sediments tend to be more compressible than mineral‐dominated sediments, deltaic wetland strata are vulnerable to compaction and drowning. Using an unprecedented data set of almost 3,000 discrete bulk density and organic‐matter measurements, we examine organic‐rich facies from coastal Louisiana to quantify the thickness lost to compaction and investigate whether sediments are able to maintain sufficient volume for the associated wetlands to keep pace with RSLR. We find that organic content as well as overburden thickness and density (which together determine effective stress) strongly control sediment compaction. Most compaction occurs in the top 1–3 m and within the first 100–500 years after deposition. In settings with thick peat beds, successions up to 14 m thick have been compacted by up to ∼50%. We apply geotechnical modeling to examine the balance between elevation gained from accretion and elevation lost to compaction due to renewed sediment deposition over a 100‐year timescale. Wetlands overlying mineral‐dominated lithologies may support the weight of deposition and allow net elevation gain. Model results show that reintroduction of sediment to a representative Mississippi Delta wetland site will likely cause another ∼0.35–1.14 m of compaction but leave a net elevation gain of ∼0.01–1.75 m, depending on the sediment delivery rate and stiffness of underlying strata.