Organic Carbon Composition and Oxygen Metabolism across a Gradient of Seasonally Inundated Limesink and Riparian Wetlands in the Southeast Coastal Plain,USA

A set of three relatively pristine seasonally inundated limesink wetlands and one riparian wetland was studied over a 4–6 month long inundation period in 2001. Patterns in organic matter properties and oxygen consumption in the water column followed a previously documented ecological gradient based on soil composition, vegetation type, and canopy cover. The full canopy, cypress-gum swamp had the highest mean concentrations of dissolved organic carbon (DOC; 26.2 mg/l) and dissolved lignin (sum 6; 299 μg/l) with lower concentrations observed in the partial canopy, cypress savanna (22.0 mg/l DOC; 252 μg/l sum 6) and the open marsh savanna (20.6 mg/l DOC; 135 μg/l sum 6), respectively. During the inundation period, DOC increased in concentration, dissolved lignin decreased, and δ¹³C shifted to more positive values which collectively indicate a large reduction in the percentage of aromatic carbon during the inundation period. All wetlands had very high concentrations of organic matter, yet microbial oxygen consumption was almost always stimulated by the addition of glucose rather than inorganic nutrients. Stimulation by glucose suggests that there were very small pools of highly bioavailable forms of DOC in the wetlands. A larger pool of moderately bioavailable organic matter had the capacity to sustain microbial oxygen consumption rates under dark conditions for at least 15 d. During the inundation period, the cypress-gum swamp had the lowest average rates of whole water oxygen consumption (1.0 μM/h) with increasing rates observed in the cypress savanna (1.3 μM/h), marsh savanna (1.6 μM/h), and riparian wetland (1.9 μM/h), respectively. The lignin compositional fingerprint varied across the gradient of limesink wetlands, and was useful for identifying different sources of vascular plant-derived DOM. Vascular plant production, algal production, microbial respiration, and UV degradation are all important drivers of DOM cycling, and the consistencies observed in this initial assessment of seasonally inundated limesink wetlands suggest they vary in predictable ways across the ecological gradient.