Factors controlling concentration, export, and decomposition of dissolved organic nutrients in the Everglades of Florida

Abstract

Water draining from the Everglades marshes of southern Florida contains high concentrations of dissolved organic C (DOC), N (DON), and in some locations, P (DOP). These dissolved organic nutrients carry over 90% of the N and organic C, and about 25% of the P exported downstream in the Everglades. Our objectives were to describe the most important aspects of the origin and fate of dis- solved organic matter (DOM) in the Everglades, and to describe the processes controlling its concen- tration and export. Concentrations of dissolved organic nutrients are influenced by local plant produc- tion, decomposition, and sorption equilibrium with peat. The drained peat soils of the Everglades Agricultural Area and the more productive marshes of the northern Everglades produce some of the highest concentrations of DOC and DON in the Everglades watershed. In portions of the marshes of the northern Everglades, P enrichment was correlated with higher local DOC and DON concentrations and greater production of soluble plant matter. Microbial degradation of Everglades DOM was very slow; less than 10% of the DOC was lost after 6 months of incubation in the laboratory and supplements of inorganic nutrients failed to speed the decomposition. Exposure to solar radiation increased the subse- quent decay rate of the remaining DOC (25% in 6 mo.). Solar radiation alone mineralized 20.5% of the DOC, 7% of the DON, and degraded about 50% of the humic substances over 21 days in sterile pore- water samples and thus degraded DOM faster than microbial degradation. The humic substances ap- peared to inhibit biodegradation of the other fractions of the DOC since hydrophilic organic acids de- composed faster when isolated from the humic substances. The fate of DOC and DON is closely linked as indicated by a generally narrow range of C/N ratios. In contrast, high concentrations of DOP were associated with P enrichment (at least in pore water). The DOC was composed of about 50% humic substances, 33% hydrophilic acids, and 15% hydrophilic neutral substances, typical of DOC from other environments, despite the fact that it originates from a neutral to slightly alkaline peatland. Despite high exports of DON (3.9 g m2 y1 from one area), the marshes of the northern Everglades are a sink for DON on a landscape scale. The agricultural fields of the Everglades Agricultural Area, however, ex- ported net quantities of DON. High concentrations of DOC desorbed from the agricultural soils when water with no DOC was added. Sorption experiments indicated that high concentrations of dissolved organic matter flowing into the marshes from the Everglades Agricultural Area could suppress the fur- ther desorption of additional soluble organic matter through physicochemical mechanisms. While bio- logical factors, plant production and microbial decomposition are important in producing potentially soluble organic nutrients, physicochemical sorption equilibria, hydrology, and degradation by solar ra- diation are also likely to control the export of this material on the landscape scale.