Regulation of nutrient uptake in eutrophic lowland streams

We studied nutrient uptake in relation to water chemistry, stream hydrodynamics, and ecosystem metabolism in two eutrophic lowland streams located near Berlin, Germany. Ambient nutrient uptake rates ranged from 0.180 to 12.880 g NO3-N m^-2 d^-1, from 0.035 to 0.517 g NH4-N m^-2 d^-1, and from 0.017 to 0.750 g PO4-P m^-2 d^-1. Temporal and spatial variability in nutrient uptake rates within single streams were mainly controlled by concentrations of metabolic substrates (i.e., nutrients, dissolved organic carbon, and dissolved oxygen) and rates of ecosystem metabolism, highlighting the importance of assimilative nutrient uptake. According to stoichiometric accounts, dissimilative uptake of dissolved inorganic nitrogen was an important uptake mechanism. Thus, nutrient uptake was subject to controls similar to those reported from pristine study sites, indicating that basic patterns of nutrient retention are comparable in pristine and eutrophic streams. In contrast to pristine streams, eutrophic streams exhibited long nutrient uptake lengths (in the range of several kilometers), as elevated uptake rates could only partially compensate for high nutrient loads. Our results indicate that ecosystem nutrient uptake is unable to efficiently reduce nutrient exports from the investigated eutrophic lowland streams.