D. von Schiller

Contraction, fragmentation and expansion dynamics determine nutrient availability in a Mediterranean forest stream

Temporary streams are a dominant surface water type in the Mediterranean region. As a consequence of their hydrologic regime, these ecosystems contract and fragment as they dry, and expand after rewetting. Global change leads to a rapid increase in the extent of temporary streams, and more and more permanent streams are turning temporary. Consequently, there is an urgent need to better understand the effects of flow intermittency on the biogeochemistry and ecology of stream ecosystems. Our aim was to investigate how stream nutrient availability varied in relation to ecosystem contraction, fragmentation and expansion due to hydrologic drying and rewetting. We quantified the temporal and spatial changes in dissolved nitrogen (N) and phosphorus (P) concentrations along a reach of a temporary Mediterranean forest stream during an entire contraction–fragmentation–expansion hydrologic cycle. We observed marked temporal changes in N and P concentrations, in the proportion of organic and inorganic forms as well as in stoichiometric ratios, reflecting shifts in the relative importance of in-stream nutrient processing and external nutrient sources. In addition, the spatial heterogeneity of N and P concentrations and their ratios increased substantially with ecosystem fragmentation, reflecting the high relevance of in-stream processes when advective transport was lost. Overall, changes were more pronounced for N than for P. This study emphasizes the significance of flow intermittency in regulating stream nutrient availability and its implications for temporary stream management. Moreover, our results point to potential biogeochemical responses of these ecosystems in more temperate regions under future water scarcity scenarios.

Attenuation of pharmaceuticals and their transformation products in a wastewater treatment plant and its receiving river ecosystem

Pharmaceuticals are designed to improve human and animal health, but may also be a threat to freshwater ecosystems, particularly after receiving urban or wastewater treatment plant (WWTP) effluents. Knowledge on the fate and attenuation of pharmaceuticals in engineered and natural ecosystems is rather fragmented, and comparable methods are needed to facilitate the comprehension of those processes amongst systems. In this study the dynamics of 8 pharmaceuticals (acetaminophen, sulfapyridine, sulfamethoxazole, carbamazepine, venlafaxine, ibuprofen, diclofenac, diazepam) and 11 of their transformation products were investigated in a WWTP and the associated receiving river ecosystem. During 3 days, concentrations of these compounds were quantified at the influents, effluents, and wastage of the WWTP, and at different distances downstream the effluent at the river. Attenuation (net balance between removal and release from and to the water column) was estimated in both engineered and natural systems using a comparable model-based approach by considering different uncertainty sources (e.g. chemical analysis, sampling, and flow measurements). Results showed that pharmaceuticals load reduction was higher in the WWTP, but attenuation efficiencies (as half-life times) were higher in the river. In particular, the load of only 5 out of the 19 pharmaceuticals was reduced by more than 90% at the WWTP, while the rest were only partially or non-attenuated (or released) and discharged into the receiving river. At the river, only the load of ibuprofen was reduced by more than 50% (out of the 6 parent compounds present in the river), while partial and non-attenuation (or release) was observed for some of their transformation products. Linkages in the routing of some pharmaceuticals (venlafaxine, carbamazepine, ibuprofen and diclofenac) and their corresponding transformation products were also identified at both WWTP and river. Finally, the followed procedure showed that dynamic attenuation in the coupled WWTP-river system could be successfully predicted with simple first order attenuation kinetics for most modeled compounds.

Microbial carbon processing along a river discontinuum

The hydrological continuum in rivers can be altered by the presence of small dams that modify the water residence time (WRT) and prevailing habitat, turning lotic river sections into lentic ones and influencing downstream reaches. The structure and activity of the microbial community occurring in the benthic and planktonic compartments can be modified by these small dams. We studied the microbial community processing of organic C along a sequence of 4 lentic–lotic sections in a medium-size Mediterranean river during base flow (spring) and low flow (summer). We hypothesized that longitudinal anomalies in WRT would influence the relative contribution of benthic vs planktonic compartments and their relevance in C processing along the river network, particularly during low flows. The biomass of free-living and particle-associated bacterioplankton was higher in the lentic sections, which had longer WRT, resulting in higher organic C processing (enzymatic activities and respiration). Microbial aggregates occurred in the lentic sections especially during the low-flow period and resulted in hotspots of organic C processing. The lotic reaches received a significant contribution of C in the form of bacterio- and phytoplankton. The small dams subsidized the lotic sections downstream and increased their respiration activity. Our results reveal the influence of small dams on organic C processing along the river network. Accounting for their effect, together with that of large dams, may be essential for accurate estimations of organic-matter transformation in river networks.

Author Correction: A global analysis of terrestrial plant litter dynamics in non-perennial waterways

In the version of this Article originally published, the affiliation for M. I. Arce was incorrect; it should have been: 5Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany. This has now been corrected in the online versions of the Article.

Ecosystem Responses to Emerging Contaminants: Fate and Effects of Pharmaceuticals in a Mediterranean River

There is concern about the environmental effects of pharmaceuticals, since these substances have strong biological impacts and are found in an increasing number of sites, especially downstream from wastewater treatment plants (WWTP). Most information existing on the effects of pharmaceutical products is based on simple laboratory assays with single compounds, whereas pharmaceuticals in the environment typically appear in complex mixtures that include secondary metabolites as well as other pollutants. Therefore, real-world situations may contribute to the understanding of the fate and effects of pharmaceuticals in freshwaters. Here we report the effects of pharmaceuticals in the river Segre (Pyrenees, Iberian Peninsula) in a river segment affected by the effluent of a WWTP. The removal efficiencies of pharmaceuticals and their metabolites in both the WWTP and the river were analyzed by comparing the inflow and outflow concentrations at the WWTP and along the studied river segment, and their transformations and interactions were modeled. The WWTP had a higher removal efficiency (45%) than the river segment (20%), but the latter was also important. In general, the compounds most efficiently removed in the WWTP were also those more efficiently removed in the river. The removal efficiency in the river was higher during the day than during the night, suggesting that attenuation was driven by either photodegradation or biological transformation by primary producers. The effects of pharmaceuticals were analyzed across different scales, from those on biofilms to functional impairment of the river ecosystem. Laboratory toxicity tests showed that stream biofilms at the most polluted site developed community tolerance to anti-inflammatory drugs. Biofilms in the field also showed altered metabolic profiles and reduced algal diversity. WWTP effluents were able to alter the balance between autotrophic and heterotrophic processes: while ecosystem respiration was subsidized, gross primary production showed some stress effects.