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 m22 d21, from 0.035 to 0.517 g NH4-N m22 d21, and from 0.017 to 0.750 g PO4-P m22 d21. 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. The intrinsic ability of stream ecosystems to store or even eliminate inorganic nutrients is termed nutrient retention. Nutrient retention is generally analyzed by applying the nutrient spiraling concept, which combines the evaluation of the antagonistic processes of nutrient cycling and transport in running waters (Webster and Patten 1979; Newbold et al. 1981; Stream Solute Workshop 1990). In pristine headwater streams, nutrient retention has been demonstrated to effectively reduce nutrient loads (e.g., Peterson et al. 2001). These results give rise to the assumption that nutrient retention could potentially counteract water quality problems originating from point and diffuse anthropogenic sources. However, excessive nutrient loads due to wastewater discharge can cause low load-specific nutrient retention efficiencies (Haggard et al. 2001; Martı́ et al. 2004). Under pristine conditions, channel morphology, hydrologic interaction between surface and hyporheic water, and the geologic origin and grain size of streambed sediments (Munn and Meyer 1990; Valett et al. 1996; Gücker and Boëchat 2004), as well as biological productivity and ambient nutrient concentration (Martı́ and Sabater 1996; Dodds et al. 2002; Hall and Tank 2003), appear to be important determinants of nutrient retention. However, the functioning of nutrient retention is especially interesting in eutrophic streams, whose nutrient exports affect downstream rivers, lakes, and estuaries. Nevertheless, little is known about the rates, mechanisms, and controls of nutrient uptake in eutrophic streams. In this study, we examined nutrient uptake in two eutrophic lowland streams. To assess ammonium, nitrate, and phosphate uptake rates and lengths in relation to a variety of potential controls, such as water chemistry, stream hydrodynamics, and ecosystem metabolism, we conducted seasonal short-term nutrient addition experiments in four eutrophic stream reaches. We aimed to test the following four hypotheses: 1) Nutrient uptake rates in eutrophic lowland streams