High DON bioavailability in boreal streams during a spring flood

Abstract

Riverine inputs of nitrogen is an important factor that controls productivity of coastal marine waters. Dissolved organic nitrogen (DON) comprises most 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 about 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 composition during a spring flood in two streams in northern Sweden. Potential bioavailability was determined by employing bacterial regrowth bioassays with brackish medium and a coastal bacterial inoculum. In addition, concentrations of urea and amino acids and the proportion of D-isomers in total dissolved amino acids were analyzed because a high proportion of D-isomers is suggested to indicate old and refractory organic material.During the flood, potential DON bioavailability increased from 19-28% at baseflow to 55-45% during peak flow in the two streams, while DON concentration remained relatively constant. At the end of the flood, bioavailability returned 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 bacteria also utilized other DON compounds. Our results imply that a major portion of the annual export of labile nitrogen occurred during a few weeks of spring flood.

Assessing carbon dynamics in natural and perturbed boreal aquatic systems

“…The DOM in the lakes and reservoirs is also compositionally different from that of the leachates (C:N of 21.3 to 41.6 and d 13 C of À28.0‰ to À26.3‰; Table 5), likely consisting of a mixture of water-soluble materials either leached from the surficial soil layers, derived from in-lake OM production or obtained through reutilization/ bacterial reworking [Schiff et al, 1997;Stepanauskas et al, 2000;McCallister and del Giorgio, 2008]. Small bacteria with an effective diameter smaller than the porosity of the filters (0.45 mm) may also have contributed to the DOM pool.…”

“…More extensively leached litter, surface soils and leachates obtained later on in the season usually become depleted in N and are thus characterized by higher C:N ratios [Stepanauskas et al, 2000;Tremblay et al, 2005;Galimov, 2006;Berg and McClaugherty, 2008]. The DOM in the lakes and reservoirs is also compositionally different from that of the leachates (C:N of 21.3 to 41.6 and d 13 C of À28.0‰ to À26.3‰; Table 5), likely consisting of a mixture of water-soluble materials either leached from the surficial soil layers, derived from in-lake OM production or obtained through reutilization/ bacterial reworking [Schiff et al, 1997;Stepanauskas et al, 2000;McCallister and del Giorgio, 2008].…”

New insights on resource stoichiometry: assessing availability of carbon, nitrogen, and phosphorus to bacterioplankton

“…Bioavailable N was assessed using an alternative method described by Stepanauskas et al (2000) by counting the cells produced in growth bioassays with N-starved bacteria. For this test, two aliquots of 30 mL were used for bioassays and one of them was amended with N-NH 4 NO 3 to a final concentration of 0.405 mg N L −1 .…”

“…Such bioassays can increase our understanding of the direct controls on bacterial metabolism by bioavailable nutrient pools. Although growth bioassays have previously been applied to calculate bioavailability of single elements (Stepanauskas et al, 2000(Stepanauskas et al, , 2002Jansson et al, 2012), no efforts to date have quantified the bioavailability of more than two elements simultaneously so that the relative availability of multiple resources can be directly compared. In a recent review on bioavailability (Berggren et al, 2015), it was additionally suggested that nutrient bioavailability (as a fraction of the total pool) tends to increase from C to N and N to P in DOM-rich systems.…”

“…We first calibrated our method by detecting the response (leucine incorporation) of nutrient-starved bacteria to known added amounts of bioavailable resources. We then validated this bacterial response through comparison with common methods to detect bioavailability: lability incubations for DOC bioavailability (del Giorgio and Cole, 1998), cell production bioassays with N-starved bacteria for N bioavailability (Stepanauskas et al, 2000) and measuring P content in bacterial growth cultures harvested on filters . Specifically, by using this new bacterioplankton growth bioassay, our study addresses the following questions.…”