EFFECT OF MAT MOVEMENT ON SOIL-SALINITY FLUCTUATIONS IN SOME COASTAL LOUISIANA MARSHES

We studied patterns of interstitial soil salinities in relation to adjacent surface-water salinity fluctuations in two floating marshes (in which the upper 50 cm of substrate move vertically in response to water level fluctuations) and one firmly rooted marsh occurring in close proximity to each other in a low-salinity (0.5-5) region of coastal Louisiana. Soil salinities were measured 1 and 50 m inland from the adjacent canal at 5-25 cm (mat) depth in 5-cm intervals, and at 50, 75 and 100 cm depth (sludge). Under the fresh (salinity 1-2 g) conditions prevailing during most of the 3-year study, mean salinities in the firmly anchored marsh were considerably higher than adjacent surface waters and also higher than in the two floating marshes. The highest single-point concentrations during the study were also measured in the firmly rooted marsh. In the floating marshes soil salinities were relatively fresh, higher than but tracking open-water salinity fluctuations closely throughout the study. At all three sites salinities increased with depth as well as distance from the adjacent canal except at the freely floating site, where the deeper salinities at the edge of the creek and 50 m inland were not significantly different. Soil salinities were more homogeneously distributed in the floating marshes than in the firmly anchored marsh. Vertical and lateral salinity gradients in the firmly anchored marsh were steeper and showed greater seasonal variability. From the observed patterns of soil salinity fluctuations in the two marsh types it is possible to infer greater below-ground exchange of water between marsh and adjacent surface water in the floating marsh type than in the rooted marsh type. The differing response of the two marsh types to forcing from the same external source of salt, with resultant differing soil salt distributions, may allow plants with varying degrees of salt tolerance to coexist in the same general area.