Potential trace metal co-limitation controls on N2 fixation and NO3- uptake in lakes with varying trophic status

The response of N-2 fixation and NO3 uptake to environmental conditions and nutrient enrichment experiments in three western U.S. lake systems was studied (eutrophic Clear Lake; mesotrophic Walker Lake; oligotrophic Lake Tahoe). We tested the effect of additions of bioactive trace metals molybdenum as Mo(V) and iron (Fe) as well as phosphate (P), N-2 fixation, NO3-, carbon (C) fixation, chlorophyll a (Chla), and bacterial cell counts under both natural conditions and in mesocosm experiments. We found distinct background N-2 fixation and NO3- uptake rates: highest at Clear Lake (N-2 fixation: 44.7 +/- 1.8 nmol N L-1 h(-1)), intermediate at Walker Lake (N-2 fixation: 1.7 +/- 1.1 nmol N L-1 h(-1); NO3- uptake: 113 +/- 37 nmol N L-1 h(-1)), and lowest at Lake Tahoe (N-2 fixation: 0.1 +/- 0.07 nmol N L-1 h(-1); NO3- uptake: 37.2 +/- 10.0 nmol N L-1 h(-1)). N-2 fixation was stimulated above control values with the addition of Fe and Pin Clear Lake (up to 50 and 63%, respectively); with Mo(V), Fe, and P in Walker Lake (up to 121, 990, and 85%, respectively); and with Mo(V) and P in Lake Tahoe (up to 475 and 21%, respectively). NO3- uptake showed the highest stimulation in Lake Tahoe during September 2010, with the addition of P and Mo(V) (similar to 84% for both). High responses to Mo(V) additions were also observed at some sites for C fixation (Lake Tahoe: 141%), Chla (Walker Lake: 54% and Clear Lake: 102%), and bacterial cell counts (Lake Tahoe: 61%). Overall our results suggest that co-limitation of nutrients is probably a common feature in lakes, and that some trace metals may play a crucial role in limiting N-2 fixation and NO3- uptake activity, though primarily in non-eutrophic lakes.