Seasonal bioavailability of dissolved organic carbon and nitrogen from pristine and polluted freshwater wetlands

We examined the chemical composition and bioavailability of dissolved organic carbon (DOC) and nitrogen (DON) from two pristine and two polluted cedar bog wetlands across three seasons. Pristine and polluted wetlands differed in DOC and DON concentrations, chemical characteristics, and bioavailability. DOC and DON concentrations were higher in the polluted than in the pristine wetlands. In contrast, a higher percentage of the dissolved organic matter (DOM) was more aromatic in the pristine (54% 6 19) than in the polluted (27% 6 4) wetlands. A higher percentage of DOC was bioavailable in the pristine (22% 6 9) than the polluted (12% 6 4) wetlands. A similar percentage of DON was consumed in both the pristine (33% 6 25) and polluted (28% 6 25) wetlands. Seasonally, the bioavailability of DOC and DON varied and differed between the pristine and polluted wetlands. The availability of phosphate appeared to affect the amount of DOC incorporated into bacterial biomass, whereas inorganic nutrient availability did not affect the assimilation of DON. Bioavailable DOC primarily fueled bacterial respiration, whereas DON supported bacterial growth. Overall, our results demonstrate that anthropogenic activities and season affect the quantity and quality of wetland DOM exported to rivers and that different factors control the utilization and fate of DOC and DON within the bacterial community. Carbon and nitrogen are important components of riverine dissolved organic matter (DOM). Dissolved organic carbon (DOC) is often the largest organic carbon pool in rivers (Schlesinger and Melack 1981). Likewise, dissolved organic nitrogen (DON) can comprise up to 90% of the total dissolved nitrogen (TDN) concentration and export from some rivers (e.g., Seitzinger and Sanders 1997). Recent studies have shown that DOM is metabolically important in rivers and estuaries; it supplies energy (carbon) and nutrients (nitrogen) to bacteria and some algae and potentially contributes to coastal eutrophication and hypoxia (e.g., Seitzinger and Sanders 1997; Stepanauskas et al. 1999; Glibert et al. 2001; Wiegner and Seitzinger 2001). Riverine DOM is composed of organic matter from terrestrial, atmospheric, and autochthonous sources. The bioavailability of DOM from these sources differs from one another and varies seasonally (e.g., Wiegner and Seitzinger 1 Corresponding author ([email protected]). Present Address: University of Hawaii at Hilo, Marine Science Department, 200 West Kawili Street, Hilo, Hawaii 96720.