The varying role of water column nutrient uptake along river continua in contrasting landscapes

Abstract

Nutrient transformation processes such as assimilation, dissimilatory transformation, and sorption to sediments are prevalent in benthic zones of headwater streams, but may also occur in the water column. The river continuum concept (RCC) predicts that water column processes become increasingly important with increasing stream size. We predicted that water column nutrient uptake increases with stream size, mirroring carbon/energy dynamics predicted by the RCC. We measured water column uptake of ammonium (${\text{NH}}_{4}^{ + }$), nitrate (${\text{NO}}_{3}^{ - }$), and soluble reactive phosphorus (SRP) in 1st through 5th order stream and river reaches (discharge: 50–68,000 L s⁻¹) in three watersheds ranging from <1 to >70 % developed lands. We found that water column volumetric uptake (U _vol ) of ${\text{NH}}_{4}^{ + }$, ${\text{NO}}_{3}^{ - }$, and SRP did not significantly differ among watersheds and we did not find any longitudinal patterns for U _vol . Uptake velocity (v _f ) of ${\text{NH}}_{4}^{ + }$ increased with stream size, whereas ${\text{NO}}_{3}^{ - }$ and SRP v _f did not differ with stream size or among watersheds. Both U _vol and v _f were related to water column metabolism and material suspended in the water column, but specific relationships differed among solutes and uptake metrics. Median water column v _f across 15 sites was 4, 9, and 19 % of median whole-stream ${\text{NH}}_{4}^{ + }$, ${\text{NO}}_{3}^{ - }$, and SRP v _f based upon a previous meta-analysis. Thus, although we could not demonstrate a generalized longitudinal pattern in water column nutrient uptake, water column processes can be important. An improved mechanistic understanding of the controls on uptake and the ultimate fate of nutrients will facilitate effective management and restoration for mitigating downstream nutrient export.