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

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 inputs from Lake Okeechobee as well as agricultural and urban runoff from the surrounding watershed. Water samples were collected bimonthly for 1 year beginning December 2014 at three stations along the river. Treatments included 28-day dark incubations with and without prior photo-irradiation. 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 for selective samples. 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% (−12–61% 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. Effects of light exposure prior to incubation did not have a measurable effect on %BDON but did affect bacterial biomass and DON composition. 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.