Many aspects of Lake Superior's nitrogen cycle are poorly described in spite of the fact that the lake's nitrate concentration has risen dramatically this past century. One important, yet under-described parameter is the concentration of ammonium. Here, we present data to resolve spatial and temporal variation along with vertical profiles of ammonium concentration in Lake Superior. Lake-wide average concentrations were low (0.21 μM, n = 166) with considerable spatial and temporal variation. During the onset of summer, the western margin of the lake had higher average concentrations than open and eastern parts. Surface layer (<10 m) ammonium concentrations showed an increase from January to October. Relatively higher ammonium concentration in the near bottom waters at a number of sites during August indicated efflux from sediment to be an important process. Subsurface maxima near the thermocline were observed in late August and persisted until September-October suggesting that ammonium might be controlled by food web processes during warm, stratified conditions. The higher potential for ammonium uptake compared to external inputs suggested rapid turnover of ammonium in the lake.
|Original language||English (US)|
|Number of pages||11|
|Journal||Journal of Great Lakes Research|
|State||Published - 2007|
Bibliographical noteFunding Information:
We thank Mike Mellesmoen and Ann McNally for assisting in sample collection. Constructive suggestions about sampling and study design were pro- vided by R. Sherrell, G. Bullerjahn, and M. McKay. We are grateful to the captain and crew of the R/V Blue Heron, and R/V Limnos for their assistance with sample collection. We also thank Environment Canada for support during Limnos cruises. This research was funded by an NSF-Chemical Oceanography grant no. 0352291.
- Lake Superior