Phosphorus mass-balance studies of Dingle Marsh, Idaho indicated that the marsh was an annual net sink for total suspended sol ids and all forms of phosphorus under consideration: total phosphorus, particulate phosphorus, total dissolved phosphorus, dissolved organic phosphorus, and orthophosphate phosphorus. During some months, however, more phosphorus was exported from the marsh than entered. Phosphorus mass-balance dynamics were compared between night and day. Total suspended solids and particulate phosphorus dynamics were controlled by sedimentation of particulate material, a physical process, and were unaffected by light conditions. Dissolved organic and ortho-phosphorus were affected by light conditions indicating, at least indirectly, that biological processes were affecting dissolved phosphorus dynamics. These phosphorus fractions were occasionally released in large amounts by the marsh at night. An in situ enclosure experiment was performed to determine major sites of phosphorus uptake/release. A twentyfour-hour rate of particulate phosphorus decrease in the enclosures could be predicted (r2=0.89) by knowing the initial concentration of total suspended solids. On a twentyfour-hour basis, periphyton, detritus, and live bulrushes tended to remove more dissolved organic phosphorus from the water than they added to it while plankton and sediments added dissolved organic phosphorus to the water. Orthophosphates were removed from the water by plankton, detritus, and periphyton with plankton dominating the changes. Live bulrushes or sediments within the bulrush system tended to be a source for orthophosphates. The open water ecosystem was a greater net source for dissolved organic phosphorus than the bulrush system. The open water system was a greater net sink, however, for orthophosphates, than was the bulrush community.