High DON bioavailability in boreal streams during a spring flood

Riverine inputs of nitrogen is an important factor that controls productivi ty of coastal marine waters. Dissolved organic nitrogen (DON) comprises mos t of the N in boreal rivers. During spring floods, DON flux may exceed the baseflow flux by several orders of magnitude. However, little is known abou t the biological availability of spring flood DON and, thus, its potential effect on coastal productivity. We have investigated the dynamics of DON bioavailability and chemical compo sition during a spring flood in two streams in northern Sweden. Potential b ioavailability was determined by employing bacterial regrowth bioassays wit h brackish medium and a coastal bacterial inoculum. In addition, concentrat ions of urea and amino acids and the proportion of D-isomers in fetal disso lved amino acids were analyzed because a high proportion of D-isomers is su ggested to indicate old and refractory organic material. During the flood, potential DON bioavailability increased from 19-28% at ba seflow to 55-45% during peak how in the two streams, while DON concentratio n remained relatively constant. At the end of the hood, bioavailability ret urned to the baseflow values. Potential DON bioavailability was positively correlated with the concentration of dissolved combined amino acids and the proportion of L-enantiomers of amino acids. However only 5-18% of DON was identified as urea and free and combined amino acids, suggesting that bacte ria also utilized other DON compounds. Our results imply that a major porti on of the annual export of labile nitrogen occurred during a few weeks of s pring flood.