Ecosystem carbon dynamics along a productivity gradient following the BP Deepwater Horizon Oil Spill

Background/Question/Methods

Saltwater marsh carbon dynamics are affected by topography, geomorphology, hydro-edaphic conditions, vegetation composition, and other biotic and abiotic variables. Our primary objective is to examine ecosystem carbon dynamics in Spartina alterniflora (C₄) and Juncus roemerianus (C₃) dominated marshes along the Mississippi-Alabama coast following potential oil and dispersant exposure from the BP Deepwater Horizon spill. Three sites were chosen for this study based on potential oil/dispersant exposure level. Grand Bay had the highest potential exposure to oil/dispersants, Point Aux Pines experienced intermediate exposure potential and Dauphin Island had no potential exposure. At each site we measured NEE, R_eco, and GEE with static chamber techniques and spectral indices (NDVI, PRI and WI) using the UniSpec-SC Spectral Analysis System. LAI was measured using LI-COR LAI-2000 plant canopy analyzer. Previous studies have shown that fouling can result in smothering of plant and soil surfaces thereby altering plant gas-exchange and soil fluxes. This can also lead to tissue toxicity and reductions in transpiration and photosynthesis, reducing the overall productivity of the ecosystem. We hypothesized that GEE values would decrease with increasing potential oil exposure along the gradient due to smothering of leaves, reducing overall plant-atmosphere gas exchange. 

Results/Conclusions

Our results indicate that there is a significant difference in GEE between sites (p-value = 0.005) ; GEE increased with exposure level. Grand Bay (highest potential exposure to oil/dispersants) had the highest GEE (7.98 µ mol m^-2s^-1 ± 1.95 SE) which may be a function of LAI, which also increased with exposure level. Position of the marsh in the landscape, however may have a larger effect on carbon fluxes than exposure to oil and dispersants. Sites further inland that had less oil exposure are impacted by stresses associated with stagnant water and higher redox potential. We also found that Juncus roemerianus (C₃) had significantly higher GEE values than Spartina alterniflora (p-value = 0.0009). Juncus tends to reside at higher elevations avoiding some of the effects associated with lower redox potential. There has been little prior research on ecosystem carbon fluxes over the productivity gradient from west Mississippi to Alabama in saltwater marsh ecosystems, limiting our knowledge on carbon cycling preceding the BP oil spill. However, it should be stressed that additional research is needed to corroborate these short-term results and verify what long-term biological impacts the oil spill may have had on individual species and ecosystem processes.