The Bode catchment (Germany) shows strong land use gradients from forested parts of the National Park (23% of total land cover) to agricultural (70%) and urbanised areas (7%). It is part of the Terrestrial Environmental Observatories of the German Helmholtz association. We performed a biogeochemical analysis of the entire river network. Surface water was sampled at 21 headwaters and at ten downstream sites, before (in early spring) and during the growing season (in late summer). Many parameters showed lower concentrations in headwaters than in downstream reaches, among them nutrients (ammonium, nitrate and phosphorus), dissolved copper and seston dry mass. Nitrate and phosphorus concentrations were positively related to the proportion of agricultural area within the catchment. Punctual anthropogenic loads affected some parameters such as chloride and arsenic. Chlorophyll a concentration and total phosphorus in surface waters were positively related. The concentration of dissolved organic carbon (DOC) was higher in summer than in spring, whereas the molecular size of DOC was lower in summer. The specific UV absorption at 254 nm, indicating the content of humic substances, was higher in headwaters than in downstream reaches and was positively related to the proportion of forest within the catchment. CO₂ oversaturation of the water was higher downstream compared with headwaters and was higher in summer than in spring. It was correlated negatively with oxygen saturation and positively with DOC concentration but negatively with DOC quality (molecular size and humic content). A principle component analysis clearly separated the effects of site (44%) and season (15%), demonstrating the strong effect of land use on biogeochemical parameters.

As benthic biofilms mediate essential functions in stream ecosystems (e.g., carbon flux, storage of nutrients and other substances), the element-specific regulation of the biofilm composition is of great interest. We tested whether (1) the elemental composition of biofilms is related to that of the water column and (2) there are different accumulation patterns from the dissolved phase (adsorption) and the particulate phase (incorporation of suspended matter). We analysed biomass parameters, nutrients and metals in biofilms and surface waters at 28 sites within a stream network (Bode catchment, Germany). Algal biomass in biofilms was dominated by diatoms. The P/C ratio in biofilms was positively related to total phosphorus of surface water (and to the proportion of agricultural area in the catchment) indicating phosphorus limitation of biofilms, whereas the N/C ratio was not related to nitrate levels of surface water, and neither the P/C nor the N/C ratio to the concentration of dissolved organic carbon (DOC) of surface water. Biofilms were enriched in metals compared to their concentrations in water. The metals in biofilms were positively related to the concentration of dissolved metals in surface water for iron and strontium (but not for manganese, copper, zinc, arsenic or lead) and to the concentrations of particle-associated metals of surface waters for strontium and lead. Manganese and arsenic were the metals with a negative effect on the biomasses of biofilm diatoms and cyanobacteria. Overall, we observed element-specific accumulation patterns in biofilms with selected elements being related to the water column while others were probably subject to biofilm-internal processes.

Citation: Marcus, H., J. K. Wey, H. Norf, and M. Weitere. 2013. Disturbance alters the response of consumer communities towards warming: A mesocosm study with biofilm-dwelling ciliates. Ecosphere 5(1):10. http://dx.Abstract. Environmental warming can have negative effects on the carrying capacity of communities because metabolic rates increase at the expense of biomass. Here, we tested the hypothesis that such warming effects are reversed in communities experiencing disturbance, as temperature-driven growth processes gain relevance and can compensate for negative disturbance effects. Model communities of seminatural, biofilm-dwelling ciliates were cultivated in mesocosms (river bypass systems) under two temperature regimes (ambient temperature and increased by 48C). The interactive effects between these different temperatures and seven disturbance intensities were tested in a nested design. Disturbance generally reduced total ciliate abundances, whereas only small effects on the prevalence of functional diversity were detected. Temperature effects differed between different disturbance intensities and seasons: Whereas warming reduced the carrying capacity of undisturbed communities irrespective of the season, pronounced positive warming effects were detected under disturbance in winter and, to a lesser extent, in spring. Neither significant temperature nor disturbance effects were recorded in summer, probably because ciliate growth rates were not temperature limited due to high summer background temperatures. Our results show that disturbance can markedly alter warming effects on temperature limited communities. Since natural communities commonly face disturbance, it should therefore be considered in models of future environmental warming responses.