Groundwater controls ecological zonation of salt marsh macrophytes

• Published in: Ecology (Durham) Vol. 96; no. 3; pp. 840 - 849
• Main Authors: Wilson, Alicia M, Tyler Evans, Willard Moore, Charles A. Schutte, Samantha B. Joye, Andrea H. Hughes, Joseph L. Anderson
• Format: Journal Article
• Language: English
• Published: United States Ecological Society of America 01.03.2015
• Subjects: aquifers; drainage; ecological zones; flooded conditions; freshwater; Georgia; Groundwater; groundwater flow; highlands; hydrologic factors; Juncus; macrophytes; Magnoliopsida - growth & development; Salicornia europaea; saline water; Salinity; salt marshes; Salt-Tolerant Plants - growth & development; sand; soil; Soil - chemistry; South Carolina; Spartina alterniflora; stratigraphy; streams; tides; Water Movements; water table; Wetlands; Southeastern United States
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/13-2183.1

Ecological zonation of salt marsh macrophytes is strongly influenced by hydrologic factors, but these factors are poorly understood. We examined groundwater flow patterns through surficial sediments in two salt marshes in the southeastern United States to quantify hydrologic differences between distinct ecological zones. Both sites included tall‐ or medium‐form Spartina alterniflora near the creek bank; short‐form Spartina alterniflora in the mid‐marsh; salt flats and Salicornia virginica in the high marsh; and Juncus roemarianus in brackish‐to‐fresh areas adjacent to uplands. Both sites had relatively small, sandy uplands and similar stratigraphy consisting of marsh muds overlying a deeper sand layer. We found significant hydrologic differences between the four ecological zones. In the zones colonized by S. alterniflora, the vertical flow direction oscillated with semi‐diurnal tides. Net flow (14‐day average) through the tall S. alterniflora zones was downward, whereas the short S. alterniflora zones included significant periods of net upward groundwater flow. An examination of tidal efficiency at these sites suggested that the net flow patterns rather than tidal damping controlled the width of the tall S. alterniflora zone. In contrast to the S. alterniflora zones, hypersaline zones populated by S. virginica were characterized by sustained periods (days) of continuous upward flow of saline water during neap tides. The fresher zone populated by J. roemarianus showed physical flow patterns that were similar to the hypersaline zones, but the upwelling porewaters were fresh rather than saline. These flow patterns were influenced by the hydrogeologic framework of the marshes, particularly differences in hydraulic head between the upland water table and the tidal creeks. We observed increases in hydraulic head of