Foliar DMSO:DMSP ratio and metal content as indicators of stress in Spartina alterniflora

We evaluated 2 potential indicators of stress, viz. the ratio of dimethylsulfoxide to dimethylsulfoniopropionate (DMSO:DMSP) and foliar metals, in Spartina alterniflora collected from areas affected by 4 different disturbances (sudden marsh dieback, horse grazing, increased snail densities, wrack de position) across 20 marshes in Georgia, USA. The DMSO:DMSP ratio was a stronger and more consistent indicator of stress than either DMSP or DMSO concentrations alone, with significantly higher ratios occurring in leaves and stems collected from affected compared to healthy areas in all 4 disturbance types. Foliar metal concentrations also differed in affected compared to healthy areas. Of 20 metals evaluated, concentrations of 19 were increased in leaves collected from edge and affected areas. Multidimensional scaling using the entire suite of metals showed separation between plants from affected and healthy areas, but no difference among disturbance types. In contrast, chlorophyll a concentrations were not significantly different between affected and healthy areas, and did not correlate with variation in either of the 2 indicators. These results suggest that the DMSO:DMSP ratio and foliar metal suite are sensitive indicators of sublethal stress in Spartina, capable of identifying stress before there are visible signs such as chlorophyll loss. The fact that both indicators were consistent across a variety of disturbance types suggests that they may be primarily responsive to general oxidative stress and thus, broadly useful tools for evaluating the health of salt marsh habitat in the field.