Aquatic macrophytes can contribute to the retention of organic contaminants in streams, whereas knowledge on the dynamics and the interaction of the determining processes is very limited. The objective of the present study was thus to assess how aquatic macrophytes influence the distribution and the fate of organic contaminants in small vegetated streams. In a first study that was performed in vegetated stream mesocosms, the peak reductions of five compounds were significantly higher in four vegetated stream mesocosms compared to a stream mesocosm without vegetation. Compound specific sorption to macrophytes was determined, the mass retention in the vegetated streams, however, did not explain the relationship between the mitigation of contaminant peaks and macrophyte coverage. A subsequent mesocosm study revealed that the mitigation of peak concentrations in the stream mesocosms was governed by two fundamentally different processes: dispersion and sorption. Again, the reductions of the peak concentrations of three different compounds were in the same order of magnitude in a sparsely and a densely vegetated stream mesocosm, respectively, but higher compared to an unvegetated stream mesocosm. The mitigation of the peak reduction in the sparsely vegetated stream mesocosm was found to be fostered by longitudinal dispersion as a result of the spatial distribution of the macrophytes in the aqueous phase. The peak reduction attributable to longitudinal dispersion was, however, reduced in the densely vegetated stream mesocosm, which was compensated by compound-specific but time-limited and reversible sorption to macrophytes. The observations on the reversibility of sorption processes were subsequently confirmed by laboratory experiments. The experiments revealed that sorption to macrophytes lead to compound specific elimination from the aqueous phase during the presence of transient contaminant peaks in streams. After all, these sorption processes were found to be fully reversible, which results in the release of the primarily adsorbed compounds, once the concentrations in the aqueous phase starts to decrease. Nevertheless, the results of the present thesis demonstrate that the processes governing the mitigation of contaminant loads in streams are fundamentally different to those already described for non-flowing systems. In addition, the present thesis provides knowledge on how the interaction of macrophyte-induced processes in streams contributes to mitigate loads of organic contaminants and the related risk for aquatic environments.
The estimation of the potential risk of pesticide entries into streams - and therefore the potential risk for the ecosystems - is an important requirement for the planning of risk mitigation strategies. Especially on the landscape level the required event triggered sampling methods are conjuncted with considerable efforts with regard to input data, time and personnel. To circumvent these problems simulation models form a reasonable alternative. The aims of this work were (A) the development of a simulation tool for the estimation of pesticide entries into surface waters on the landscape level, and (B) the application of the simulator for an exposure- and risk-assessment as well as the assessment of negative effects of pesticides on aquatic communities. Section 1 - Exposure-, Risk- and Effects In sections 1.1 and 1.2 the simulation model was applied to a multitude of small and medium sized streams in an agricultural impacted study area around the city of Braunschweig, Germany. Section 1.3 gives an overview of the simulators field of application and the general system structure. Section 1.1 - Scenario based simulation of runoff-related pesticide entries into small streams on a landscape level (English publication, p. 27): In this paper we present a simulation tool for the simulation of pesticide entry from arable land into adjacent streams. We used the ratio of exposure to toxicity (REXTOX) model proposed by the OECD which was extended to calculate pesticide concentrations in adjacent streams. We simulated the pesticide entry on the landscape level at 737 sites in small streams situated in the central lowland of Germany. The most significant model parameters were the width of the no-application-zone and the degree of plant-interception. The simulation was carried out using eight different environmental scenarios, covering variation of the width of the no-application-zone, climate and seasonal scenarios. The highest in-stream concentrations were predicted at a scenario using no (0 m) buffer zone in conjunction with increased precipitation. According to the predicted concentrations, the risk for the aquatic communities was estimated based on standard toxicity tests and the application of a safety factor. Section 1.2 - Linking land use variables and invertebrate taxon richness in small and medium-sized agricultural streams on a landscape level (English publication, p. 50): In this study the average numbers of invertebrate species across an arable landscape in central Germany (surveys from 15 years in 90 streams at 202 sites) were assessed for their correlation with environmental factors such as stream width, land use (arable land, forest, pasture, settlement), soil type and agricultural derived stressors. The stress originating from arable land was estimated by the factor "risk of runoff", which was derived from a runoff-model (rainfall induced surface runoff). Multivariate analysis explained 39.9% of the variance in species number, revealing stream width as the most important factor (25.3%) followed by risk of runoff (9.7%). Section 1.3 - Informationssystem zur ökotoxikologischen Bewertung der Gewässergüte in Bezug auf Pflanzenschutzmitteleinträge aus der Landwirtschaft - Systemaufbau und Anwendungsmöglichkeiten (German publication, p. 61): Section 1.3 contains a short overview of the simulation tool, the field of application and some examples of use, covering the effects of the width of the buffer zone as well as the creation of risk maps on the landscape level. Section 2 - The simulation tool An important aspect for the employment of a simulation model in the context of risk assessment is the applicability in practice: the accessibility of the needed input data, the conversion of the mathematical model into a software application that can be run on any current personnel computer and also an appropriate end-user documentation of the system. Section 1.4 - Informationssystem zur ökotoxikologischen Bewertung der Gewässergüte in Bezug auf Pflanzenschutzmitteleinträge aus der Landwirtschaft - Simulationsmodell und Systemaufbau (German report, p. 67): In this section a general overview of the simulation model as well as the schematic system structure given. Section 1.5 - Benutzerhandbuch (German report, p. 71): The user manual contains details concerning the installation of the system, generation of the required input data and the general use of the system. Moreover it presents some application examples (what-if analyses). Section 1.6 - Technical documentation (German report, p. 104): The technical documentation describes internal structures and processes of the simulation system. Section 1.6 provides information regarding the required structure of input/output tables.