Marsh mat flotation in the Louisiana delta plain

(1) Vertical mat movement in relation to surface-water fluctuations was measured for 1 year at three marshes differing in dominant emergent vegetation and location in the Mississippi River delta plain of coastal Louisiana, U.S.A. (2) The freshwater marsh, dominated by Panicum hemitomon, floated directly with ambient water levels, provided they were high enough to float the mat. Water levels varied by c. 70 cm and mat movement by 55 cm. An intermediate-salinity marsh closer to the Gulf of Mexico and dominated by Sagittaria falcata moved 35 cm vertically during the study period, and water levels moved 70 cm. A brackish marsh, dominated by Spartina patens, moved only 3 cm in response to c. 40 cm of vertical water movement. (3) The freshwater marsh floated throughout the year, provided ambient water levels were high enough, and mat movement followed water-level movement directly (r2 = 0.97). In contrast, the intermediate marsh exhibited seasonal buoyancy. In the summer and early autumn, this marsh responded to water-level oscillations directly (r2 = 0.94). It was the least buoyant in late winter (r2 = 0.29). (4) Mat movement reflected differences between marshes in substrate bulk density, mineral density and degree of decomposition. The shallow substrate of the two fresher marshes, which could respond directly to water levels, contained almost no mineral sediment (9% by dry weight at the fresh site and 17% at the intermediate), whereas the brackish marsh, which showed only small fluctuations relative to water movement, contained almost 50% mineral sediment by weight. The fresh marsh had the most fibric substrate in the upper 30 cm; the organic substrate material became progressively more decomposed with proximity to the Gulf of Mexico and with depth.