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Title: Molecular composition and bioavailability of dissolved organic nitrogen in a lake flow-influenced river in south Florida, USA

item Pisani, Oliva
item BOYER, JOSEPH - University Of Plymouth
item PODGORSKI, DAVID - Florida State University
item THOMAS, CASSONDRA - South Florida Water Management District
item COLEY, TERESA - South Florida Water Management District
item JAFFE, RUDOLF - Florida International University

Submitted to: Aquatic Sciences - Research Across Boundaries
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2017
Publication Date: 5/13/2017
Citation: Pisani, O., Boyer, J., Podgorski, D., Thomas, C., Coley, T., Jaffe, R. 2017. Molecular composition and bioavailability of dissolved organic nitrogen in a lake flow-influenced river in south Florida, USA. Aquatic Sciences - Research Across Boundaries. 79(4):891-908.

Interpretive Summary: The Caloosahatchee River and Estuary (CRE) is an important component of the aquatic landscape of South Florida. Its hydrology and ecology have been impacted by a combination of anthropogenic activities and nitrogen inputs from these activities to the CRE are of significant concern to the environmental health of the system. Since most of the nitrogen in this system is in the form of dissolved organic nitrogen (DON), a complex mixture of molecularly uncharacterized compounds with varying degrees of bioavailability, the environmental dynamics and reactivity of this biogeochemical component need to be assessed. Here we studied the composition and reactivity of DON along the Caloosahatchee River over both spatial and temporal scales. DON composition was categorized as total hydrolysable amino acids (THAA), urea, and an uncharacterized pool. The bioavailable DON (BDON) was determined by difference after 28 days and reported as % loss from the DON pool (%BDON). Inorganic N, bacterial numbers, and leucine aminopeptidase activity (LAPA) were also measured. Ambient TDN composition showed a high degree of variability on seasonal scales but was consistent in being dominated by DON (ca. 80%), where THAA and urea represented only a minor fraction of the DON. The variability in %BDON suggests variations in DON composition (and thus quality) combined with variations in bacterial community structure. Elevated values of BDON were observed during high flow from Lake Okeechobee. In general terms however, while BDON was observed to be highly variable seasonally, it was fairly consistent along the Caloosahatchee River transect. Because of the large component of relatively recalcitrant DON, reductions in TN loads as a means to reduce estuarine eutrophication and associated harmful algal blooms would not necessarily guarantee a reduction in estuarine eutrophication and the associated ecological impacts.

Technical Abstract: Dissolved organic nitrogen (DON) represents a large percentage of the total nitrogen in rivers and estuaries, and can contribute to coastal eutrophication and hypoxia. This study reports on the composition and bioavailability of DON along the Caloosahatchee River (Florida), a heavily managed system receiving point source input from Lake Okeechobee as well as agricultural and urban runoff from the surrounding watershed. Samples were collected bimonthly for one year beginning December 2014 at three stations along the river. Treatments included pre-photo-irradiation prior to 28-day dark incubation as well as non-irradiated samples. Concentrations of DON, ammonium, nitrate-nitrite, total hydrolyzable amino acids (THAA), and urea, as well as bacterial numbers, leucine aminopeptidase activity, and fluorescent optical properties were measured. Ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to characterize the molecular composition of DON before and after incubation. The total dissolved N pool was dominated by DON (61-99%), with low inorganic N (1-39%), and small amounts of THAA-N (0.1-23%) and urea-N (0.6-3.2%). The mean percentage of biologically available DON (BDON) for the study was 15% (0-66% range) with highest values occurring when water inputs from Lake Okeechobee were the most dominant freshwater source. FT-ICR MS analysis revealed the presence of a wide range of N-containing formulas and the generation of aliphatic and peptide-like structures likely due to microbial alteration of the carbon skeleton of DON compounds. These findings may help predict nutrient loading effects to the Caloosahatchee River estuary and may aid in understanding wetland potential as a treatment technology for removing N in this and other freshwater systems sensitive to N loading.