Environmental warming is predicted to rise dramatically over the next century, yet few studies have investigated its effects in natural, multi-species systems. We present data collated over an 8-year period from a catchment of geothermally heated streams in Iceland, which acts as a natural experiment on the effects of warming across different organisational levels and spatiotemporal scales. Body sizes and population biomasses of individual species responded strongly to temperature, with some providing evidence to support temperature size rules. Macroinvertebrate and meiofaunal community composition also changed dramatically across the thermal gradient. Interactions within the warm streams in particular were characterised by food chains linking algae to snails to the apex predator, brown trout These chains were missing from the colder systems, where snails were replaced by much smaller herbivores and invertebrate omnivores were the top predators. Trout were also subsidised by terrestrial invertebrate prey, which could have an effect analogous to apparent competition within the aquatic prey assemblage. Top-down effects by snails on diatoms were stronger in the warmer streams, which could account for a shallowing of mass-abundance slopes across the community. This may indicate reduced energy transfer efficiency from resources to consumers in the warmer systems and/or a change in predator-prey mass ratios. All the ecosystem process rates investigated increased with temperature, but with differing thermal sensitivities, with important implications for overall ecosystem functioning (e.g. creating potential imbalances in elemental fluxes). Ecosystem respiration rose rapidly with temperature, leading to increased heterotrophy. There were also indications that food web stability may be lower in the warmer streams.

Excessive sediment pressure on aquatic habitats is of global concern. A unique dataset, comprising instantaneous measurements of deposited fine sediment in 230 agricultural streams across England and Wales, was analysed in relation to 20 potential explanatory catchment and channel variables. The most effective explanatory variable for the amount of deposited sediment was found to be stream power, calculated for bankfull flow and used to index the capacity of the stream to transport sediment. Both stream power and velocity category were highly significant (p ≪ 0.001), explaining some 57% variation in total fine sediment mass. Modelled sediment pressure, predominantly from agriculture, was marginally significant (p<0.05) and explained a further 1% variation. The relationship was slightly stronger for erosional zones, providing 62% explanation overall. In the case of the deposited surface drape, stream power was again found to be the most effective explanatory variable (p<0.001) but velocity category, baseflow index and modelled sediment pressure were all significant (p<0.01); each provided an additional 2% explanation to an overall 50%. It is suggested that, in general, the study sites were transport-limited and the majority of stream beds were saturated by fine sediment. For sites below saturation, the upper envelope of measured fine sediment mass increased with modelled sediment pressure. The practical implications of these findings are that (i) targets for fine sediment loads need to take into account the ability of streams to transport/retain fine sediment, and (ii) where agricultural mitigation measures are implemented to reduce delivery of sediment, river management to mobilise/remove fines may also be needed in order to effect an improvement in ecological status in cases where streams are already saturated with fines and unlikely to self-cleanse.

1. Detrimental impacts of excessive fine-grained sediment inputs to streams and rivers are well established. What is less well understood is the susceptibility of different elements of the freshwater biota to such perturbations and how such knowledge of their susceptibility could aid in identifying where excessive fine-grained sediment is impairing ecological condition. 2. Following the collection of biological and sediment data from 179 streams across England and Wales, representative of a range of river types over a gradient of fine sediment loading, objective statistical approaches were applied to establish relationships between the macroinvertebrate assemblage and fine-grained sediment inputs to river channels. 3. Having factored out that portion of the biological variation associated with natural environmental gradients, a model comprising mass of organic sediment in erosional areas of the stream bed [predominantly associated with the first axis of the partial canonical correspondence analysis (pCCA)], and mass of fine-grained sediment in the surface drape of depositional areas and % organic content in erosional areas (associated with the second axis of the pCCA) as explanatory variables best accounted for the residual variation in the macroinvertebrate assemblage. 4. The relative position of taxa along both axes of the pCCA, provided a ranking of taxa in relation to the two gradients of fine-grained sediment and provided the basis for a new empirically derived diagnostic index for fine-grained sediment stress in rivers. Two sub-indices were derived to capture the assemblage responses to both the gradient of organic sediment in erosional areas and the gradient of total fines in depositional areas. The two sub-indices were then combined to derive the new combined fine sediment index (CoFSI sp ). 5. The index was tested on an independent test data set (comprising 127 samples from 83 sites) and was found to provide a robust indication of benthic fine-grained sediment conditions (Spearman's rank correlations q = À0.519 to À0.703). The strength of correlation with the total fine-grained sediment gradient was always greater than that for other routinely used indices, confirming that CoFSI sp offered additional explanatory power when assessing this stressor of aquatic environments.

a b s t r a c tTwelve sites of tyre track pools and puddles situated in woodland, heath and pasture in Dorset UK were examined to determine their macroinvertebrate species richness and community changes over the course of one year.174 taxa were found with Diptera (59) and Coleoptera (48) contributing 61% of the total. The most frequently occurring and ubiquitous groups were nematoceran dipterans, Oligochaeta, Coleoptera, Crustacea and Lamellibranchiata.Species richness varied with season and on average was highest in March and November samples. On average only 26% (range 16-40%) of the combined total number of taxa found in spring (March) and autumn (November) samples from a site were also found there in each of these seasons individually, indicating a high species turnover through the year.The tyre track pools contributed to local aquatic biodiversity by adding 29 taxa to previously published taxa lists from aquatic habitats in the area. The relative richness of the tyre track pools is attributed to their successional variation in a heterogeneous landscape.Conservation value of 9 of the 12 sites was rated Very high to High and nine regionally notable or rare taxa were recorded. It is suggested that the important conservation status of the tyre track pools warrants greater recognition and further intensive study.

1. Improved water quality, through a reduction in diffuse pollution from agricultural sources, is an expected benefit of agri-environment schemes, but this has yet to be demonstrated in practice. Here, we evaluate the impact of Welsh agri-environment schemes on water quality and freshwater ecosystem condition through a combined monitoring and modelling framework. 2. To determine the influence of the agri-environment schemes on ecosystem condition, spatially independent catchments dominated by a single scheme (>40% of catchment) were compared to control catchments dominated (>70%) by agricultural land that was not part of any scheme. Biological indicators of water quality were monitored at the outfall of each catchment and a spatially explicit modelling framework of diffuse pollutant emissions applied to each of the 80 catchments. 3. Direct comparison (scheme/non-scheme) was unable to identify any significant effect of agrienvironment scheme participation. However, derived biological indicators that reflected organic pollution, eutrophication and pesticide run-off were strongly correlated with modelled concentrations of corresponding diffuse pollutants, thus providing a ground-truth for the models. Scenarios that assessed the correct counterfactuals (i.e. the influence of scheme entry on pollutant output) were developed for the whole of Wales. The models indicated an important effect of scheme entry on water quality, but this effect was not evenly distributed across the landscape. 4. Synthesis and applications. Our results indicate that agri-environment schemes can deliver improvements in water quality, through a reduction in diffuse pollution from agricultural sources. However, it is not easy to demonstrate scheme effectiveness; the combination of field survey and modelling used here provides a framework for addressing these difficulties. A spatially targeted approach for agri-environment scheme options to protect water resources from diffuse pollution is likely to be most effective at delivering water quality improvements.