Transport and utilization of dissolved organic matter in a lotic wetland ecosystem

Although the importance of hydrology to wetland ecosystem productivity is recognized, few detailed analyses exist on shallow subsurface water flows ({dollar}<{dollar}1 m depth) through wetland ecosystems. Subsurface water movements and biotic interactions with shallow subsurface hydrology were evaluated in a lotic wetland located in Hale County, AL. Sediment characteristics varied across the wetland with a distinctive demarcation in sediments between the upper (0 to 30-cm) and lower (30 to 80-cm) depths. This demarcation was created by increased organic matter deposition in the upper sediments which resulted in significantly faster hydraulic conductivity in the upper compared to lower (0.5 to 1100 and {dollar}1times10sp{lcub}-3{rcub}{dollar} to 55 cm d{dollar}sp{lcub}-1{rcub},{dollar} respectively) sediments.; Water table elevation and subsurface recharge zones were spatially and temporally variable at the wetland and were directly correlated to rainfall. During periods of low average monthly rainfall (May through November), wetland subsurface water was recharged from the hyporheic region of the main inflow stream. With increased monthly rainfall (December though August), subsurface water recharge shifted to recharge from an upgradient area dominated by alder, Alnus serrulata. Net photosynthesis and photosynthetic efficiency by the dominant emergent macrophyte, Juncus effusus, was seasonally variable and was not correlated to sediment saturation. Transpirative water loss by J. effusus was seasonally variable and affected by sediment saturation; diel measurements of transpirative water loss found significantly greater water loss during low light and dark periods.; Nutrient and dissolved organic carbon (DOC) concentrations were compared between subsurface waters of unvegetated sediments and sediments among stands of J. effusus. Total dissolved iron and inorganic nitrogen concentrations were greater in samples collected from the unvegetated compared to the vegetated zone. Soluble reactive phosphorus was greater in interstitial water of vegetated samples collected in the winter compared to higher concentrations in sediments of the unvegetated zone in spring. DOC concentrations of interstitial waters were greater in sediments of the unvegetated zone in the winter and spring compared to those from the vegetated zone. Although DOC concentrations in hydrosoils collected from both zones increased from winter to spring, bacterial productivity per mg DOC in spring decreased compared to winter.