Differential associations of five riverine organism groups with multiple stressors.

The decline of river and stream biodiversity results from multiple simultaneous occuring stressors, yet few studies explore responses explore responses across various taxonomic groups at the same locations. In this study, we address this shortcoming by using a coherent data set to study the association of nine commonly occurring stressors (five chemical, one morphological and three hydraulic) with five taxonomic groups (bacteria, fungi, diatoms, macro-invertebrates and fish). According to studies on single taxonomic groups, we hypothesise that gradients of chemical stressors structure community composition of all taxonomic groups, while gradients of hydraulic and morphological stressors are mainly related to larger organisms such as benthic macro-invertebrates and fish. Organisms were sampled over two years at 20 sites in two catchments: a recently restored urban lowland catchment (Boye) and a moderately disturbed rural mountainous catchment (Kinzig). Dissimilarity matrices were computed for each taxonomic group within a catchment. Taxonomic dissimilarities between sites were linked to stressor dissimilarities using multivariable Generalized Linear Mixed Models. Stressor gradients were longer in the Boye, but did in contrast to the Kinzig not cover low stress intensities. Accordingly, responses of the taxonomic groups were stronger in the Kinzig catchment than in the recently restored Boye catchment. The discrepancy between catchments underlines that associations to stressors strongly depend on which part of the stressor gradient is covered in a catchment. All taxonomic groups were related to conductivity. Bacteria, fungi and macro-invertebrates change with dissolved oxygen, and bacteria and fungi with total nitrogen. Morphological and hydraulic stressors had minor correlations with bacteria, fungi and diatoms, while macro-invertebrates were strongly related to fine sediment and discharge, and fish to high flow peaks. The results partly support our hypotheses about the differential associations of the different taxonomic groups with the stressors.

How tolerances, competition and dispersal shape benthic invertebrate colonisation in restored urban streams.

Biotic communities often respond poorly to river restoration activities and the drivers of community recovery after restoration are not fully understood. According to the Asymmetric Response Concept (ARC), dispersal capacity, species tolerances to stressors, and biotic interactions are three key drivers influencing community recovery of restored streams. However, the ARC remains to be tested. Here we used a dataset on benthic invertebrate communities of eleven restored stream sections in a former open sewer system that were sampled yearly over a period of eleven years. We applied four indices that reflect tolerance against chloride and organic pollution, the community's dispersal capacity and strength of competition to the benthic invertebrate taxa lists of each year and site. Subsequently, we used generalised linear mixed models to analyse the change of these indices over time since restoration. Dispersal capacity was high directly after restoration but continuously decreased over time. The initial communities thus consisted of good dispersers and were later joined by more slowly dispersing taxa. The tolerance to organic pollution also decreased over time, reflecting continuous improvement of water quality and an associated increase of sensitive species. On the contrary, chloride tolerances did not change, which could indicate a stable chloride level throughout the sampling period. Lastly, competition within the communities, reflected by interspecific trait niche overlap, increased with time since restoration. We show that recovery follows a specific pattern that is comparable between sites. Benthic communities change from tolerant, fast dispersing generalists to more sensitive, slowly dispersing specialists exposed to stronger competition. Our results lay support to the ARC (increasing role of competition, decreasing role of dispersal) but also underline that certain tolerances may still shape communities a decade after restoration. Disentangling the drivers of macroinvertebrate colonisation can help managers to better understand recovery trajectories and to define more realistic restoration targets.

Effects of agriculture on river biota differ between crop types and organism groups.

While the general effects of agricultural land use on riverine biota are well documented, the differential effects of specific crop types on different riverine organism groups, remain largely unexplored. Here we used recently published land use data distinguishing between specific crop types and a Germany-wide dataset of 7748 sites on the ecological status of macroinvertebrates, macrophytes and diatoms and applied generalized linear mixed models to unravel the associations between land use types, crop types, and the ecological status. For all organism groups, associations of specific crop types with biota were stronger than those of urban land use. For macroinvertebrates and macrophytes, strong negative associations were found for pesticide intensive permanent crops, while intensively fertilized crops (maize, intensive cereals) affected diatoms most. These differential associations highlight the importance of distinguishing between crop types and organism groups and the urgency to buffer rivers against agricultural stressors at the catchment scales and to expand sustainably managed agriculture.

Stressors affecting the ecological status of temporary rivers in the Mediterranean region.

Temporary rivers are widespread in the Mediterranean region and impose a challenge for the implementation of the Water Framework Directive (WFD) and other environmental regulations. Surprisingly, an overarching analysis of their ecological status and the stressors affecting them is yet missing. We compiled data on the ecological status of 1504 temporary rivers in seven European Mediterranean region countries and related their ecological status (1) to publicly available data on pressures from the European WISE-WFD dataset, and (2) to seven more specific stressors modelled on a sub-catchment scale. More than 50 % of the temporary water bodies in the Mediterranean countries reached good or even high ecological status. In general, status classes derived from phytobenthos and macrophyte assessment were higher than those derived from the assessment of benthic invertebrates or fish. Of the more generally defined pressures reported to the WISE-WFD database, the most relevant for temporary rivers were 'diffuse agricultural' and 'point urban waste water'. Of the modelled more specific stressors, agricultural land use best explained overall ecological status, followed by total nitrogen load, and urban land use, while toxic substances, total phosphorus load and hydrological stressors were less relevant. However, stressors differed in relevance, with total nitrogen being most important for macrophytes, and agricultural land use for phytobenthos, benthic invertebrates and fish. For macrophytes, ecological quality increased with stressor intensity. The results underline the overarching effect of land use intensity for the ecological status of temporary water bodies. However, assessment results do not sufficiently reflect hydrological stress, most likely as the biological indicators used to evaluate these systems were designed for perennial water bodies and thus mainly target land use and nutrient impacts. We conclude that biomonitoring systems need to be updated or newly developed to better account for the specific situation of temporary water bodies.

Perceived multiple stressor effects depend on sample size and stressor gradient length.

Multiple stressors are continuously deteriorating surface waters worldwide, posing many challenges for their conservation and restoration. Combined effect types of multiple stressors range from single-stressor dominance to complex interactions. Identifying prevalent combined effect types is critical for environmental management, as it helps to prioritise key stressors for mitigation. However, it remains unclear whether observed single and combined stressor effects reflect true ecological processes unbiased by sample size and length of stressor gradients. Therefore, we examined the role of sample size and stressor gradient lengths in 158 paired-stressor response cases with over 120,000 samples from rivers, lakes, transitional and marine ecosystems around the world. For each case, we split the overall stressor gradient into two partial gradients (lower and upper) and investigated associated changes in single and combined stressor effects. Sample size influenced the identified combined effect types, and stressor interactions were less likely for cases with fewer samples. After splitting gradients, 40 % of cases showed a change in combined effect type, 30 % no change, and 31 % showed a loss in stressor effects. These findings suggest that identified combined effect types may often be statistical artefacts rather than representing ecological processes. In 58 % of cases, we observed changes in stressor effect directions after the gradient split, suggesting unimodal stressor effects. In general, such non-linear responses were more pronounced for organisms at higher trophic levels. We conclude that observed multiple stressor effects are not solely determined by ecological processes, but also strongly depend on sampling design. Observed effects are likely to change when sample size and/or gradient length are modified. Our study highlights the need for improved monitoring programmes with sufficient sample size and stressor gradient coverage. Our findings emphasize the importance of adaptive management, as stress reduction measures or further ecosystem degradation may change multiple stressor-effect relationships, which will then require associated changes in management strategies.

Environmental ranges discriminating between macrophytes groups in European rivers.

Riverine macrophytes form distinct species groups. Their occurrence is determined by environmental gradients, e.g. in terms of physico-chemistry and hydromorphology. However, the ranges of environmental variables discriminating between species groups ("discriminatory ranges") have rarely been quantified and mainly been based on expert judgement, thus limiting options for predicting and assessing ecosystem characteristics. We used a pan-European dataset of riverine macrophyte surveys obtained from 22 countries including data on total phosphorus, nitrate, alkalinity, flow velocity, depth, width and substrate type. Four macrophyte species groups were identified by cluster analysis based on species' co-occurrences. These comprised Group 1) mosses, such as Amblystegium fluviatile and Fontinalis antipyretica, Group 2) shorter and pioneer species such as Callitriche spp., Group 3) emergent and floating species such as Sagittaria sagittifolia and Lemna spp., and Group 4) eutraphent species such as Myriophyllum spicatum and Stuckenia pectinata. With Random Forest models, the ranges of environmental variables discriminating between these groups were estimated as follows: 100-150 µg L-1 total phosphorus, 0.5-20 mg L-1 nitrate, 1-2 meq L-1 alkalinity, 0.05-0.70 m s-1 flow velocity, 0.3-1.0 m depth and 20-80 m width. Mosses were strongly related to coarse substrate, while vascular plants were related to finer sediment. The four macrophyte groups and the discriminatory ranges of environmental variables fit well with those described in literature, but have now for the first time been quantitatively approximated with a large dataset, suggesting generalizable patterns applicable at regional and local scales.

The interplay of nutrients, dissolved inorganic carbon and algae in determining macrophyte occurrences in rivers.

Nitrogen and phosphorous concentrations are widely considered to drive macrophyte assemblages in rivers. However, Dissolved Inorganic Carbon (DIC) - available for plants as CO2 and HCO3- - is also of major relevance. Based on literature, we present a conceptual model on the interaction between algae, macrophytes, DIC, pH, light, N, P and the surface water and sedimental compartment. Analysing two separate datasets (i) on river physico-chemistry and chlorophyll-a, and (ii) on river physico-chemistry and macrophytes we quantify three connections within this concept: (1) the correlation of chlorophyll-a versus pH, (2) the correlation of TP versus chlorophyll-a and (3) the occurrence of HCO3-users and CO2-only-users among macrophytes along the DIC gradient. Chlorophyll-a correlated positively with pH (R-squared = 77%, p < .001) due to increased carbon dioxide uptake of phytoplankton. Surface water TP did not linearly correlate with chlorophyll-a concentrations. Obligate and optionally submerged macrophyte species that utilise HCO3- were separated from CO2-only-users by HCO3- concentrations, with an area under the curve (AUC) of 68% and 70% (both p < .001) between groups. Obligate and optionally submerged macrophyte assemblages only composed of HCO3-users and those exclusively composed of CO2-only-users showed an even stronger separation based on the HCO3- concentration, with both an AUC of 82% and 78% (both p < .001). Our results underline that DIC can greatly affect riverine macrophytes. However, absolute concentrations of HCO3- are less relevant, while the connection to pH is more important, reflecting CO2 concentrations. River monitoring and management should consider the interaction between nutrients DIC, surface water and sedimental compartment as important factors affecting macrophyte occurrence, rather than solely focussing on surface water nutrients.