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Purposes of this study were to determine the relative importance of organic matter and mineral matter to marsh vertical accretion and to determine if insufficient vertical accretion was a factor in land loss in a Terrebonne Basin (Louisiana, USA) marsh. '"CS dating indicated that vertical accretion (0.98 cm yr-l) was extremely rapid relative to other marshes, but insufficient to counter submergence (1.38 cm yr-'). Mineral and organic matter accumulation were similar to that in other marshes. Variation in vertical accretion was accounted for by variation in organic matter accumulation rather than mineral matter accumulation: thus, inadequate vertlcal accretion resulted from inadequate organic matter accumulation. Inadequate organic matter accurnulat~on (593 g m-2 yr-') was attributed to inadequale plant production resulting primarily from flooding stresses; inadequate mineral matter accumulation (1629 g m-' yr-') was attributed to inadequate mineral matter availability. These data suggest that this marsh 1s threatened by a positive feedback loop of plant flooding stress and inadequate vertical accretion. Inadequate plant growth limits vertical accretion, which further increases flooding and decreases plant proditction. Thus plant production, which has previously been considered only in the context of trophic dynamics, also partly determines the degree of submergence some coastal and estuarine marshes will tolerate.

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.