Elucidating the effects of diversion-relevant salinity and hydrologic regimes on biogeochemical responses and growth of foundational plant species (Spartina patens, Spartina alterniflora, Typha latifolia) in a mesocosm setting

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Ashleigh Lambiotte and Jonathan M. Willis, Biological Sciences, Nicholls State University, Thibodaux, LA

Background/Question/Methods
Southeastern Louisiana is facing extreme rates of coastal wetland loss. The wetland systems occurring in this region provide critical ecosystem services for the state and nation. While a myriad of stressors contribute to this loss, one major aspect is the disconnection of annual floodwaters from the Mississippi River to surrounding wetlands. This isolation from river sediment input, in conjunction with the natural process of subsidence, results in reduced elevations and increased flooding stress. River sediment diversions are large-scale restoration techniques intended to rehabilitate Louisiana’s degraded wetlands by reconnecting the river and surrounding wetlands via controlled, periodic flooding that deliver sediment to subsiding wetlands, stimulating growth and building land. Utilizing such large-scale restoration techniques requires an appropriate knowledge base of vegetation growth and soil biogeochemical responses to altered hydrologic conditions. In this research, sods of three foundational marsh species (Spartina patens, Spartina alterniflora, Typha latifolia) are being exposed to inundation depths (20 and 40 cm), durations (8 and 16 weeks), and salinities (0 and 5 psu) consistent with likely sediment diversion scenarios in a mesocosm setting. Various indicators of vegetation growth and soil physicochemical status are being determined to enhance understanding of likely impacts to overall marsh health and sustainability.
Results/Conclusions
Preliminary results indicate that both surface and deep soil redox potential became highly reduced within two weeks of the 20 cm flooding depth being initiated. In contrast, soil redox potential measurements for control mesocosms displayed only moderate reduction in deep measurements and minimal reduction in surface measurements. The increase in flooding stress intensity is particularly apparent for Spartina patens, which demonstrates a trend towards decreased live cover with increasing flooding depth. As expected, Spartina alterniflora appears to be much less impacted by flooding, with reduced live cover only notable at the greatest depth. Interestingly, differences in Typha latifolia live cover are not readily discernible among inundation depths. Thus far, preliminary data suggests differential growth responses of these important species at the inundation depths employed in this study, which would have implications for possible impacts of diversion operation. However, overall integrated growth responses and assessment of recovery subsequent to inundation duration treatments will be necessary to more fully estimate effects.