Study Examines How Multiple Plant Species Affect Marsh Response to Oiling

Mesocosm experiment at Dauphin Island Sea Lab tested the effects of Avicennia germinans and Spartina alterniflora on marsh responses to oiling. Photo credit: R. Hughes

Mesocosm experiment at Dauphin Island Sea Lab tested the effects of Avicennia germinans and Spartina alterniflora on marsh responses to oiling. Photo credit: R. Hughes

Scientists conducted a year-long mesocosm experiment to assess if the expansion of tropical black mangroves (Avicennia germinans) into Gulf of Mexico saltmarshes dominated by temperate cordgrass (Spartina alterniflora) affected marsh response to oiling. Compared to mesocosms that contained a single plant species, mesocosms containing both plant species resulted in a reduced magnitude in the oil’s impact on mangrove seedling survival (21% vs 12% mortality) and belowground biomass production (71% vs 19% reduction). There was not an observed reduction in the magnitude of oil’s impact on cordgrass. Genetic diversity appeared to have little influence on marsh plants’ response to oiling. Findings suggest that mangrove colonization did not increase, and may have decreased, overall marsh habitat’s susceptibility to oiling and that mixed assemblies of mangroves and cordgrass mediated oil impacts on mangroves. The researchers hypothesized that the observed reductions in oiling effects may facilitate the expansion of black mangroves into marsh habitats.

The researchers published their findings in Ecosphere: Effects of oil exposure, plant species composition, and plant genotypic diversity on salt marsh and mangrove assemblages.

Coastal salt marshes and mangroves are intertidal habitats that provide food, shelter, and nursery areas for marine life. Alterations, such as to distribution and abundance, from climate change to these habitat-providing plants can have cascading effects on ecological functions and humans who depend on their ecosystem services. Previous studies suggest that greater species or functional diversity could enhance an ecosystem’s resilience to environmental disturbances. The black mangroves’ expansion into salt marshes, therefore, provides an opportunity to examine the responses of climate-mediated transition zones to disturbance, which can inform the trajectory of change and conservation and restoration practices.

Using Deepwater Horizon oiling as the disturbance, this study team sought to improve our understanding of how a mixed-species transition zone responded to an environmental stressor. The researchers used Louisiana sweet crude oil to test the interactive effects of emulsified weathered oil exposure, plant species identity/composition, and plant genotypic diversity on mangrove and marsh grass survival and production.

“We used weathered oil vs fresh oil because that was most relevant to the impacts in marsh systems, and because there were published and reproducible protocols for that type of manipulation,” explained study author A. Randall Hughes. Because heavy oiling can be an extreme stress that causes complete plant mortality and likely overwhelms any positive effects of plant species or genotypic diversity, they used a one-time high exposure to oil levels that were within those observed during Deepwater Horizon and then used a sustained moderate-to-low level exposure that previous experiments showed led to partial stem mortality in marsh plants.

The experiment setting was an important component, too, as Hughes explained, “The large, outdoor mesocosms at Dauphin Island Sea Lab allowed us to maintain realistic environmental conditions while experimentally manipulating oil exposure in a controlled manner that is generally impossible at field sites.”

Results indicated that oil negatively affected both mangroves and cordgrass regardless of mixed or single species present, with cordgrass being more negatively affected than mangroves. Cordgrass showed negative effects two months post-oiling, but the absence of negative effects on aboveground traits (density, stem height, leaf growth) at the experiment’s end suggests resilience to oil exposure. Mangroves did not show negative effects until 6 – 12 months post-oiling and only showed modest negative responses, suggesting a greater resistance to oil than cordgrass.

The authors noted that the aboveground recovery of cordgrass was likely the result of a resource transfer from belowground tissues since there was reduced belowground biomass at the experiment’s end. Thus, although there were no visible signs of oiling damage to cordgrass one year following exposure, these plants likely had reduced ability to withstand future stresses or disturbances. Further, the reduction in belowground biomass suggests that key ecosystem functions such as erosion prevention and carbon storage were likely compromised.

The authors explained that although the levels of genotypic diversity tested (one genotype vs. three genotypes) were relatively low, it reflects realistic diversity for Gulf of Mexico marshes. They also noted that the multi-species benefit for black mangroves and the lack of benefit for cordgrass raises the possibility that disturbances such as oil may allow mangroves to establish and dominate in sites where it would otherwise be at a competitive disadvantage.

“The expansion of black mangroves into salt marshes in the northern Gulf of Mexico may change the susceptibility of these habitats to disturbances such as oil spills,” said Hughes. “Our study demonstrates that mangroves will not increase the susceptibility of marsh habitats to oiling, though they may still impact other services. Many questions remain about what their presence means for the services that marshes provide to us, such as fish enhancement, coastal protection, and disturbance resilience.”

Data are publicly available through the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at doi:10.7266/n7k93629 and doi:10.7266/N7ZG6QS6.

The study’s authors are A.R. Hughes, J. Cebrian, K. Heck, J. Goff, T.C. Hanley, W. Scheffel, and R.A. Zerebecki.

By Nilde Maggie Dannreuther and Stephanie Ellis. Contact maggied@ngi.msstate.edu with questions or comments.

************

This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Alabama Center for Ecological Resilience (ACER).

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.

© Copyright 2010-2020 Gulf of Mexico Research Initiative (GoMRI) – All Rights Reserved. Redistribution is encouraged with acknowledgement to the Gulf of Mexico Research Initiative (GoMRI). Please credit images and/or videos as done in each article. Questions? Contact web-content editor Nilde “Maggie” Dannreuther, Northern Gulf Institute, Mississippi State University (maggied@ngi.msstate.edu).