Patterns and processes of mercury release from leaves of two dominant salt marsh macrophytes, Phragmites australis and Spartina alterniflora

The release of mercury (Hg) from leaf tissue was compared between two dominant salt marsh macrophytes, Spartina alterniflora and Phragmites australis. Rates of Hg release were measured for individual leaves from late May to late July, along with concentrations of Hg in leaf tissue, rates of sodium (Na) release, and rates of transpiration. Leaves of S. alterniflora consistently released 2-3 times more Hg than leaves of P australis. Leaves of S. alterniflora also contained greater concentrations of Hg during these months. In contrast to P australis leaves, rates of Na release were high for S. alterniflora and were correlated with rate of Hg release. Transpiration rates averaged 2.2 times greater for Phragmites as compared to S. alterniflora, and were not correlated with the other variables at the leaf level for either species. Leaf Hg concentration was highly correlated with Hg release for both species, but the slope was significantly greater for S. alterniflora. Monthly differences were profound for all climate and physiological variables measured, with high measurements in May, and lower measurements in June and July. For both species, the highest Hg content was found in lower leaves in May, followed by upper leaves in May. Hg accumulation in leaf tissue and release from both species appear to be greatest in the spring, although differences between the species persist throughout these peak months of the growing season.