Exploring the ecosystem functional role of DOM in freshwaters - novel approaches and new insights from the NERC DOMAINE programme

Dissolved Organic Matter (DOM) is a key component of nutrient flux to freshwaters, yet its nature, origins and ultimate ecological impact in freshwater ecosystems are poorly understood. Results from our NERC DOMAINE programme designed to elucidate its role in freshwater ecosystems are presented here.

Initial laboratory-based bioassays demonstrated growth of phytoplankton, epilithon, bryophytes and higher plants in response to Dissolved Organic N (DON) compounds over NO₃^- by a range of biota under controlled conditions, but at higher dose rates than under ambient environmental conditions. We therefore set up a series of replicated stream microcosms (images 1-2), in an N-limited stream draining a peatland catchment in the headwaters of the Conwy, UK; a small recycling pump in each 6L jar simulated streamflow. The ¹⁵N-SIP experiment comprised a control, two inorganic nutrient control treatments (¹⁵KNO₃, ¹⁵NH₄SO₄) and 6 DON treatments: ¹⁵N-urea, ¹³C,¹⁵N-glutamate, ¹³C,¹⁵N-glycine, ¹⁵N-acetyl glucosamine, a commercial cell-free ¹³C,¹⁵N-amino acid (AA) mixture to mimic the likely profile in natural waters, and ¹⁵N-labelled sheep urine produced in the catchment. We added water, biofilm-coated pebbles (image 3) and bryophytes from the stream, to each jar and sampled them for analysis after 48 h incubation. The results obtained on freeze-dried tissues and extracts by EA-IRMS and GC-C-IRMS, demonstrate preferential uptake of different ¹⁵N-DON and ¹⁵NH₄⁺ over ¹⁵NO₃^- by both biofilm and bryophyte communities under ambient environmental temperatures.

In a parallel experiment we selected one substrate, ¹⁵N-labelled glutamate, a key substrate in cell metabolism, dosing at 15-min intervals for 8 days (192 h) into an adjacent tributary (images 4-6). Water, bryophytes and pebbles were sampled for analysis after 1, 2, 6, 12, 24, 48, 96 and 192 h. Results demonstrated direct uptake of glutamate by both bryophyte and biofilm as intact substrate (Fig. 1), and its rapid metabolism and incorporation into the biosynthesis of all measurable AAs within these biota (Fig. 2).

We conclude that DOM is a bioavailable nutrient resource supporting microbial metabolism and primary production in stream ecosystems, and as such should be considered and controlled in any efforts to restore nutrient enriched freshwater ecosystems.