Martin T. Pusch

Effects of wastewater treatment plant discharge on ecosystem structure and function of lowland streams

Abstract Secondary and tertiary wastewater treatment is common in developed countries, but little is known about the responses of lotic ecosystems to contemporary wastewater treatment plant (WWTP) discharge. We examined the effects of WWTP discharge on various ecosystem components and functions of 2 morphologically and chemically impacted lowland streams near Berlin, Germany. We sampled one reach upstream and one reach downstream of a WWTP in each stream during each of 5 sampling campaigns. Discharge of treated wastewater resulted in increased concentrations of total organic C, total N, and total P in the sediments and in elevated macrophyte and benthic invertebrate biomasses. However, adverse effects of the WWTPs on the benthic invertebrate communities were small compared to effects reported in previous studies. This difference was a result of the higher purification efficiency of modern WWTPs, but also of significant structural degradation and eutrophication of the streams that already had impoverished the invertebrate community upstream of the WWTPs. Whole-stream community respiration (CR24) and gross primary production (GPP) were both enhanced by WWTP discharge. WWTP discharge generally caused diminished NH4- and PO4-uptake efficiencies, but did not necessarily lead to diminished NO3-uptake efficiencies of streams. Increases in areal NO3-uptake rates caused by the discharge of a large municipal WWTP were high enough to result in increased load-specific NO3-uptake efficiencies. Our study shows that the effects of present-day WWTPs on stream ecosystem functioning clearly differ from the former impacts of poorly treated wastewater. Present-day WWTP discharges mainly cause eutrophication and subsequent side effects and low nutrient-retention efficiencies relative to the high nutrient concentrations and loads of impacted streams. Our results highlight the need for efficient tertiary treatment of wastewater and for the refinement of agricultural practices to reduce diffuse nutrient loadings. We found evidence that even efficiently treated wastewater can have extensive effects on stream ecosystem structure and function. Therefore, adequate dilution rates always should be considered when routing treated wastewater through lotic networks. Our findings on the response of key ecosystem variables to present-day WWTP loading underline the importance of scientifically based stream management.

Agriculture has changed the amount and composition of dissolved organic matter in Central European headwater streams

Dissolved organic matter (DOM) is an important part of the global carbon cycle and significantly influences aquatic ecosystem functions. Recent studies suggest that its amount and composition in freshwaters may be altered by agricultural land use, whereby the influence of preceding in-stream production and processing is not clear. To assess the land use effect on DOM amount and composition for the export from terrestrial to freshwater systems at the land-water interface, we sampled headwater streams draining agricultural and near-pristine catchments (forested and wetland) in the North German plains. To account for spatial and seasonal variation, we conducted a screening of DOM amount (53 sites) and composition (42 sites), and conducted bi-weekly samplings to investigate seasonal variation at eight sites over one year. Concentrations of dissolved organic carbon (DOC) were significantly higher for agricultural and wetland catchments than for forested catchments. Moreover, DOC loads exhibited higher seasonal variation for agricultural and wetland catchments than for forested catchments, which was due to higher variation in discharge. Parallel Factor Analysis revealed that the composition of DOM in agricultural catchments was significantly different from the other studied catchment types, and was characterized by low redox state and high structural complexity. Moreover, a gradient from protein- to humic-like fluorescence significantly separated forested from agricultural and wetland catchments. The contribution of humic-like DOM was strongly and positively related to DOC concentration, suggesting a mechanistic coupling of both. The effects of land use on patterns of DOC concentration and DOM composition were consistent across seasons, implying that land use strongly regulates DOM export. Overall, this study clearly shows the seasonally independent importance of agricultural land use for the amount and composition of DOM fluxes from the terrestrial zone to surface waters. These altered fluxes may affect ecosystem metabolism and health of agricultural headwaters and downstream situated aquatic ecosystems.

Nutrient enrichment homogenizes lake benthic assemblages at local and regional scales

The compositional heterogeneity of biotic assemblages among sites, or beta-diversity, regulates the relationship between local and regional species diversity across scales. Recent work has suggested that increased harshness of environmental conditions tends to reduce beta-diversity by decreasing the importance of stochastic processes in structuring assemblages. We investigated the effect of nutrient enrichment on the compositional heterogeneity of lake benthic invertebrate assemblages in Ireland at both local (within-lake) and regional (among-lake) scales. At local scales, we found that the compositional heterogeneity of benthic assemblages was related inversely to the extent of nutrient enrichment (as indicated by measurements of water column total phosphorus, total nitrogen, and chlorophyll a), after effects of lake morphology (i.e., surface area, connectivity, and depth of sampling) and alkalinity were accounted for. At regional scales, we found that nutrient-rich lakes had significantly more homogenous benthic assemblages than nutrient-poor lakes, over and above the effect of alkalinity and across a similar range of lake morphologies. These findings have profound implications for global aquatic biodiversity, as the homogenization of benthic assemblages at both local and regional scales may have important and unpredictable effects on whole aquatic ecosystems, with potentially considerable ecological and evolutionary consequences.

The metabolism of organic matter in the hyporheic zone of a mountain stream, and its spatial distribution

Community respiration in hyporheic sediments (HCR) was studied in a characteristic riffle-pool-sequence of a mountain stream. HCR activity at the riffle site strongly exceeded that at the corresponding pool site with a mean ratio of 5.3. The vertical distribution of HCR activity was homogeneous in the pool, while there was a distinct maximum in the uppermost layer in the riffle. Similarly, the spatial distribution of certain fractions of particulate organic matter (POM), and their turnover, was largely determined by stream morphology. Mean annual HCR per unit area of stream bed was estimated as 1.71 g O2 m−2 d−1. Hence, HCR contributes significantly to total heterotrophic activity in streams, thus enhancing the relative importance of heterotrophic processes in running waters containing hyporheic zones.

Horizontal and vertical movements of unionid mussels in a lowland river

Abstract Freshwater mussels are important constituents of freshwater ecosystems, yet much of their basic biology remains to be examined. The behavior of 3 species of unionid mussels (Unio tumidus, Unio pictorum, and Anodonta anatina) was examined in the lowland River Spree (Germany). Mussels were marked individually, and their positions on the sediment surface and depth below the sediment surface were recorded weekly between May and October 2004. The average rate of horizontal movement was 11 ± 15 cm/wk (mean ± 1 SD). The direction of the movements seemed erratic; however, a significant net shoreward displacement of ~17 cm, possibly caused by rising water levels, was observed during the study. A surprisingly high percentage of the mussels (74 ± 7%) was burrowed entirely in the sediment to depths as great as 20 cm during the summer. Smaller mussels and individuals not infested by the zebra mussel, Dreissena polymorpha, burrowed deeper in the sediments than larger or infested mussels. Burrowing reduced infestation densities in a laboratory experiment. Significantly more U. tumidus individuals were found on the sediment surface during the reproductive period in early summer than in late summer, suggesting that reproductive activity may influence burrowing. Burrowing was significantly related to current velocity (discharge), day length, and water temperature (multiple linear regression, R2 = 0.74, p < 0.001), but current velocity appeared to be the dominant factor driving vertical movements (R2 = 0.53, p < 0.01). We propose that movement behaviors are important adaptations of unionid mussel populations to the flow and food conditions in rivers. Movement behavior also may help unionids escape predators and control infestation by D. polymorpha.

Ecological status assessment of European lakes: a comparison of metrics for phytoplankton, macrophytes, benthic invertebrates and fish

Data on phytoplankton, macrophytes, benthic invertebrates and fish from more than 2000 lakes in 22 European countries were used to develop and test metrics for assessing the ecological status of European lakes as required by the Water Framework Directive. The strongest and most sensitive of the 11 metrics responding to eutrophication pressure were phytoplankton chlorophyll a, a taxonomic composition trophic index and a functional traits index, the macrophyte intercalibration taxonomic composition metric and a Nordic lake fish index. Intermediate response was found for a cyanobacterial bloom intensity index (Cyano), the Ellenberg macrophyte index and a multimetric index for benthic invertebrates. The latter also responded to hydromorphological pressure. The metrics provide information on primary and secondary impacts of eutrophication in the pelagic and the littoral zone of lakes. Several of these metrics were used as common metrics in the intercalibration of national assessment systems or have been incorporated directly into the national systems. New biological metrics have been developed to assess hydromorphological pressures, based on aquatic macrophyte responses to water level fluctuations, and on macroinvertebrate responses to morphological modifications of lake shorelines. These metrics thus enable the quantification of biological impacts of hydromorphological pressures in lakes.

Bacterial abundance and production in river sediments as related to the biochemical composition of particulate organic matter (POM)

The major proportion of heterotrophic activity in running waters islocalized on the solid surfaces of sediments in the benthic and hyporheic zoneand is dominated by microorganisms. However, this assertion is based on thestudies of small streams, and little is known about the microbial metabolism oforganic matter in river ecosystems. We therefore explored the relationshipsbetween bacterial abundance and production and the gradients of organic matterquality and quantity in sediments of a sixth-order lowland river (Spree,Germany). We found vertical gradients of detrital variables (particulateorganicmatter (POM), particulate organic carbon (POC), nitrogen (PN), and protein) andof bacterial variables (abundance, production, turnover time, and proportion ofbacterial carbon in total POC) in two different sediment types. These gradientswere steeper in stratified sediments than in the shifting sediments. Detritalvariables correlated strongly with bacterial abundance and production. The bestcorrelation was found for detrital variables indicating substrate quantity andquality (rS = 0.90 for PN with abundance). Although bacterialbiomasscomprised only 0.7% of the POC (1.9% of PN, 3.4% of the protein) in sediments,the turnover of sedimentary organic carbon was fast (median = 62d), especially in the shifting sediments. Our findings demonstratethat sediment dynamics significantly foster organic carbon metabolism in riversystems. Thus, these sediments, which are typical for lowland rivers, stronglyinfluence the metabolism of the whole ecosystem.

Multifunctionality of floodplain landscapes: relating management options to ecosystem services

The concept of green infrastructure has been recently taken up by the European Commission for ensuring the provision of ecosystem services (ESS). It aims at the supply of multiple ESS in a given landscape, however, the effects of a full suite of management options on multiple ESS and landscape multifunctionality have rarely been assessed. In this paper we use European floodplain landscapes as example to develop an expert based qualitative conceptual model for the assessment of impacts of landscape scale interventions on multifunctionality. European floodplain landscapes are particularly useful for such approach as they originally provided a high variety and quantity of ESS that has declined due to the strong human impact these landscapes have experienced. We provide an overview of the effects of floodplain management options on landscape multifunctionality by assessing the effects of 38 floodplain management interventions on 21 relevant ESS, as well as on overall ESS supply. We found that restoration and rehabilitation consistently increased the multifunctionality of the landscape by enhancing supply of provisioning, regulation/maintenance, and cultural services. In contrast, conventional technical regulation measures and interventions related to extraction, infrastructure and intensive land use cause decrease in multifunctionality and negative effects for the supply of all three aspects of ESS. The overview of the effects of interventions shall provide guidance for decision makers at multiple governance levels. The presented conceptual model could be effectively applied for other landscapes that have potential for a supply of a high diversity of ESS.

Filtration and Respiration Rates of Two Unionid Species and Their Impact on the Water Quality of a Lowland River

As a consequence of their filter-feeding activity, mussels produce a zone around them that is depleted in suspended particles. As the inhalant and exhalant openings of freshwater mussels are generally situated adjacent to each other, this effect may deteriorate the efficiency of filter feeding in standing waters (O’Riordan 1993; Lenihan et al. 1996). In running waters, however, water is permanently mixed by turbulent flow, so that no vertical food gradients may be produced. Therefore, streams and rivers are favourable habitats for freshwater mussels, which may there dominate the total biomass of benthic invertebrates. In streams and small rivers in the uplands of Europe, the great populations of unionid mussels that once lived there are mostly extinguished. However, in the lowlands there are still plankton-rich lake outlets or larger rivers, where this rich nutritional resource is exploited by large populations of unionid mussels of the genera Unio, Anodonta and Pseudanodonta (Unionidae) (Kasprzak 1986). In these type of streams, a second aspect favours mussel colonization, which is the absence of catastrophic spates, which may affect mussel populations in the uplands through the mobilization of bed sediments, and physical abrasion or cracking of the shells.

Regulation and Seasonal Dynamics of Extracellular Enzyme Activities in the Sediments of a Large Lowland River

We tested whether seasonal changes in the sources oforganic substances for microbial metabolism were reflected changes in the activities of five extracellular enzymes in the eighth order lowland River Elbe, Germany. Leucine aminopeptidase showed the highest activities in the water column and the sediments, followed by phosphatase > β-glucosidase > α-glucosidase > exo-1,4-β-glucanase. Individual enzymes exhibited characteristic seasonal dynamics, as indicated by their relative contribution to cumulative enzyme activity. Leucine aminopeptidase was significantly more active in spring and summer. In contrast, the carbohydrate-degrading enzymes peaked in autumn, and β-glucosidase activity peaked once again in winter. Thus, in sediments, the ratio of leucine aminopeptidase/β-glucosidase reached significant higher medians in spring and summer (5-cm depth: ratio 7.7; 20-cm depth: ratio 10.1) than in autumn and winter (5-cm depth: ratio 3.7, 20-cm depth: ratio 6.3). Therelative activity of phosphatase in the sediments was seasonally related to both the biomass of planktonic algae as well as to the high content of total particulate phosphorus in autumn and winter. Due to temporal shifts in organic matter supply and changes in the storage capacity of sediments, the seasonal peaks of enzyme activities in sediments exhibited a time lag of 2–3 months compared to that in the water column, along with a significant extension of peak width. Hence, our data show that the seasonal pattern of extracellular enzyme activities provides a sensitive approach to infer seasonal or temporary availability of organic matter in rivers from autochthonous and allochthonous sources. From the dynamics of individual enzyme activities, a consistent synoptic pattern of heterotrophic functioning in the studied river ecosystem could be derived. Our data support the revised riverine productivity model predicting that the metabolism of organic matter in high-order rivers is mainly fuelled by autochthonous production occurring in these reaches and riparian inputs.