Nitrogen and phosphorus, but not carbon, are quickly taken up in streams: assessing variability in nutrient uptake across six Lake Superior tributaries
An abundance of headwater streams along Lake Superior’s shores may comprise a major input of nutrients and account for nutrient sources that are currently unaccounted for in Lake Superior nutrient budgets. However, in-stream biogeochemical cycling can control how quickly nutrients are taken up or transformed, and consequently alter the timing and form of nutrients exported to the downstream lake. Our objective was to measure uptake of ammonium (NH4), soluble reactive phosphate (SRP), and dissolved organic carbon (DOC) in six streams to determine whether nutrients entering from terrestrial ecosystems may be modified or retained before being exported to Lake Superior. These streams represent regional land cover characteristics and exhibit a range in stream size (discharge: 3 to 756 L s-1). To measure nutrient uptake we released nutrients (NH4Cl and KH2PO4 or C12H22O11) and a conservative tracer (rhodamine WT) into the stream and quantified changes in concentrations along each stream reach. To account for variability among streams we measured canopy cover, periphyton biomass, nutrient concentrations, DOC quality, water temperature, discharge, and land cover, and analyzed whether these variables were correlated with nutrient uptake velocities.
We found that NH4 and SRP uptake velocities (Vf) varied across the tributaries (2.1 - 5.7 mm min-1 and 1.5 - 8.4 mm min-1, respectively). NH4 uptake was significant in all but one stream, where background NH4 concentrations were elevated at least 2.5x relative to other streams. Three tributaries exhibited significant uptake of SRP. Only one site exhibited significant uptake of DOC (Vf = 0.6 mm min-1). This site also had the lowest DOC concentrations and specific ultra-violet absorbance (SUVA254) values, indicating lower C aromaticity. Neither NH4 Vf nor DOC Vf correlated with measured environmental variables, while SRP Vf was correlated with [NH4], [DOC], and canopy cover. On average, NH4 traveled 141 to 1667 m, SRP traveled 370 to 455m, and DOC traveled 2500 m before being taken up, but nutrients must travel a distance of ~2000 to 13000 m from our study reaches before reaching Lake Superior. Our results suggest that N and P may be transformed or retained in the stream before being exported to Lake Superior, but DOC may not be. To better understand nutrient export to Lake Superior future research should focus on identifying pathways of nutrient transformation, and the location/duration of retention in streams.