THE DISTRIBUTION OF AMMONIUM AND PHOSPHATE IN THE POCOTALIGO SWAMP, SOUTH CAROLINA: EVIDENCE FOR NET EXPORT AND NITROGEN LIMITATION

Depth profiles are reported for ammonium (NH4+) and soluble reactive phosphorus (SRP) in sediment pore water and concentrations in surface water along the length of the Pocotaligo Swamp in South Carolina. The distributions were examined in relation to the concentrations of NH4+ and SRP in the effluent from a wastewater treatment plant (WTP) on the swamp. The swamp discharges marginally more NH4+ than it receives in the combined inputs above the WTP and its effluent, which suggests that nonpoint sources of NH4+ are important. There appears to be no possibility of movement of NH4+ into the sediment because the NH4+ gradient is strongly in the direction of diffusion out of the sediment into the water column. Unlike NH4+, the concentration of SRP in effluent was significantly elevated above what we observed anywhere in the river or in pore water. The discharge of SRP into the swamp was an order of magnitude greater than upstream inputs, and downstream outputs were even greater, suggesting that both point and nonpoint sources of phosphorus are very important, resulting in nitrogen-limitation. With the exception of the 50 cm depth at an upstream site, nitrogen-to-phosphorus (N:P) ratios were less than 8:1, suggesting that the Pocotaligo Swamp is nitrogen-limited.