This version available http://nora.nerc.ac.uk/16072/ NERC has developed NORA to enable users to access research outputs wholly or partially funded by NERC. Copyright and other rights for material on this site are retained by the rights owners. Users should read the terms and conditions of use of this material at http://nora.nerc.ac.uk/policies.html#access NOTICE: this is the author's version of a work that was accepted for publication in Science of the Total Environment. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Science of the Total Environment [434, 201-212 (2012) . Decreasing ambient SRP concentration from 225 µg l -1 to 173 µg l -1 had no effect on periphyton biomass accrual rate or diatom assemblage. Phosphorus limitation became apparent at 83 µg SRP l -1 , at which point a 25 % reduction in periphyton biomass was observed. Diatom assemblage significantly changed when the SRP concentration was reduced to 30 µg l -1 . Such stringent phosphorus targets are costly and difficult to achieve for the River Thames, due to the high population density and intensive agriculture within the Thames basin. Reducing light levels by shading reduced the periphyton accrual rate by 50 %.Providing shading along the River Thames by planting riparian tree cover could be an effective measure to reduce the risk of excessive algal growth. If the ecology of the Thames is to reach the WFD's "good ecological status", then both SRP concentration reductions (probably to below 100 µg l -1 ) and increased shading will be required.

Sedentary species face a trade-off between the benefits of exploiting food close to their homes and the cost of defending it. In aquatic systems, it has been suggested that some sedentary grazers can increase the range of circumstances under which they are at an advantage over mobile grazers by enhancing food resources within their feeding territories through 'gardening'. We examined this for the retreat-building sedentary larvae of the caddis Tinodes waeneri, which are often dominant in the littoral of lakes. We hypothesised that T. waeneri gardens by fertilising its retreat (a fixed 'gallery' on which algae and other microorganisms grow), and that gardening would be more important in lower productivity lakes. We tested this by analysing the carbon and nitrogen stable isotope ratios of larvae, their galleries and the general background epilithon, collected from rocks in the littoral zones of six lakes spread across a natural nutrient gradient. We found evidence of nutrient recycling within the Tinodes gallery community in all lakes. Galleries were (15)N-depleted compared to the epilithon, suggesting that algae on galleries preferentially assimilated (14)N from larval excretions that were themselves (15)N-depleted relative to the larval food source. Mixing model results indicate that galleries formed an important larval carbon and nitrogen source, with mean gallery dietary contributions of over 40% in at least one sample period in all lakes studied. Gallery contributions were greater between April and October than in January and, contrary to our initial hypothesis, greater in the more productive lakes of those surveyed. Nevertheless, T. waeneri galleries do act as a fertilised garden. 'Gardening' appears to be widespread in this species, and may affect productivity and patterns of nitrogen retention within the stony littoral of lakes.

-The natural diversity of Bombus terrestris subspecies could be under threat from the commercialisation of bumblebees. Therefore, to determine whether commercially imported bumblebees are able to establish and spread, we carried out long-term observations of bumblebees in southern France. Our surveys occurred before, during, and after the importation (between 1989 and 1996) of thousands of colonies of the Sardinian subspecies B. t. sassaricus. Queens and males of B. t. sassaricus were observed foraging outside commercial greenhouses in 1991, 1993, and 1994 and feral workers were observed foraging on native vegetation nearly two years after the importation of B. t. sassaricus ceased. However, no B. t. sassaricus, or F1 hybrids were observed after 1998. We conclude that B. t. sassaricus remains inconspicuous in France and competition from the three native subspecies may have prevented it from becoming invasive. However, genetic interference through introgression cannot be ruled out.Bombus terrestris sassaricus / bumble bee trade / establishment / introduced subspecies / niche availability

Summary 1. Sedentary grazers can be numerous in fresh waters, despite the constraints on resource availability and the increased predation risk inherent in this lifestyle. The retreats of sedentary grazers have been assumed to provide protection to the resident (a ‘house’), but also may provide additional fertilised food for the grazer (i.e. a ‘garden’). If retreats function as a garden, then they should (i) contain a higher quality and/or quantity of food than the alternative food source. Furthermore, the proportion of retreat‐derived carbon and nitrogen assimilated by the resident should be (ii) related to overall resource availability (more when resources are limited) or (iii) perhaps also to larval density. Alternatively, if retreats provide a less risky food source, then (iv) assimilation of material from the retreat is likely to be greater under conditions in which the risk of emerging from the retreat is high. 2. We tested these four hypotheses for the common and widespread gallery‐building grazing caddisfly Tinodes waeneri. Resource availability, larval density and biomass, and exposure were measured for populations from six lakes of differing productivity in August, October and January. 3. Galleries always contained more algal food than the surrounding epilithon, suggesting that gardening is effective. Furthermore, gallery chlorophyll a content in August, and the disparity in food quality (assessed from the C : N ratio) between gallery and epilithon (quality higher in the former) in October were positively related to the proportion of larval biomass that was derived from the gallery. Larval density and wave exposure parameters were not related to larval assimilation of gallery material. 4. Galleries that are fertilised by the occupant provide more, and sometimes also better quality, food (in terms of the C : N ratio) than is otherwise available. Thus, the gallery plays a substantial role in larval nutrition, and this role is greater at key times of food shortage.

Methane oxidation produces biomass that is a potential source of particulate carbon for consumers, and is in addition to photosynthetic production. We assessed methanotrophy and photosynthetic production under differing conditions of light and methane concentration. We measured methane oxidation and photosynthesis in gravel sediments from adjacent shaded and unshaded stretches of 15 chalk rivers in southern England, and also in 30 artificial channels in which we manipulated light and methane experimentally. The capacity for methane oxidation was 78% higher in the shade than unshaded areas, indicating a denser, or more active, methanotrophic assemblage on shaded riverbeds, and the difference was most pronounced when methane concentration was high. Across the 15 rivers, methanotrophic production ranged from 16 to 650 nmol C cm 22 d 21 and net photosynthetic production from 256 to 35,750 nmol C cm 22 d 21. The relative importance of methanotrophy to their total production (i.e., photosynthetic and methanotrophic) increased with methane concentration and ranged from 0.1-2.4% and 0.2-13% in unshaded and shaded areas, respectively. Over an annual cycle in one river, the response of the methanotrophs in the shade to a high summer methane concentration was five times greater than in the open; in winter, there was no effect of shading on methane oxidation. The response of methanotrophy to shading and methane concentration in the artificial channels resembled that found in the rivers. Methanotrophy makes a non-negligible (here up to 13%) contribution to particulate carbon production in these streams, is disproportionately greater in the shade, and constitutes a distinct carbon pathway available for their food webs.

Elevated nutrient levels can lead to excessive biofilm growth, but reducing nutrient pollution is often challenging. There is therefore interest in developing control measures for biofilm growth in nutrient-rich rivers that could act as complement to direct reductions in nutrient load. Shading of rivers is one option that can mitigate blooms, but few studies have experimentally examined the differences in biofilm communities grown under shaded and unshaded conditions. We investigated the assembly and diversity of biofilm communities using in situ mesocosms within the River Thames (UK). Biofilm composition was surveyed by 454 sequencing of 16S amplicons (∼400 bp length covering regions V6/V7). The results confirm the importance of sunlight for biofilm community assembly; a resource that was utilized by a relatively small number of dominant taxa, leading to significantly less diversity than in shaded communities. These differences between unshaded and shaded treatments were either because of differences in resource utilization or loss of diatom-structures as habitats for bacteria. We observed more co-occurrence patterns and network interactions in the shaded communities. This lends further support to the proposal that increased river shading can help mitigate the effects from macronutrient pollution in rivers.

Stream ecosystems are supported by both green (i.e. based on grazing) and brown (i.e. detritus) food webs, whereas methane‐derived carbon is not considered generally to be important; here, we add circumstantial evidence for this potential third way. Grazing cased‐caddis (Trichoptera) larvae in the family Glossosomatidae can be very abundant in springs and headwaters and frequently have much lower stable carbon isotope ratios (i.e. they are depleted in the heavier 13C stable isotope) than the biofilm (epilithon) on the upper surfaces of the stones on which they live, and which is their presumed diet. Evidence for similar isotopic depletion in other lotic invertebrates is currently limited, however; even for glossosomatids it has been observed so far only in some streams draining the southern English cretaceous chalk and in a few headwaters in northern California. If this phenomenon proves to be more widespread, among streams or taxa, it could imply a more general underpinning of stream food webs by isotopically light carbon derived from methane and accessed via consumers feeding on methanotrophic bacteria. Here, we sampled 58 stream sites to examine whether caddis larvae are also 13C‐depleted in streams draining other geologies. We focused mainly on carboniferous limestone and sandstone, as well as on further chalk streams representative of most of the British chalk aquifer: together, these new sites covered an area of almost 90,000 km2, around three times greater than that surveyed previously. At all 58 sites methane gas was supersaturated relative to the atmospheric equilibrium, and at 49 of them larvae were conspicuously 13C‐depleted (from −17.5‰ to −3.6‰) relative to the bulk epilithon (components of which we know can oxidise methane). Although still most pronounced on chalk, this phenomenon was geographically and geologically much more widespread than shown previously and suggests methane‐derived carbon could indeed play a prominent role in stream food webs (i.e. the third way).

Summary Sedentary herbivores may improve the food resources available to them by ‘gardening’, and most obviously by fertilising primary producers with excreted nutrients such as nitrogen. In five English lakes, spanning a gradient of nutrient availability, we predicted that fertilisation of the larval retreat by the littoral, gallery‐building caddisfly Tinodes waeneri would result in: (a) a distinct algal assemblage from that in the background epilithon, and that (b) the difference would be greatest in the least productive lakes (where the importance of the nutrient subsidy from larvae should be greatest). Classes of algae present in samples of galleries and epilithon were investigated using chlorophyll and carotenoid pigment analysis and diatoms were identified. Galleries were characterised by a greater content of pigments indicative of diatoms, including fucoxanthin, than the background epilithon (which contained a higher proportion of chlorophyte algae). Redundancy analysis (RDA) of diatom counts indicated a clear separation between gallery and epilithic assemblages in all lakes, supporting hypothesis (a). Furthermore, in agreement with hypothesis (b), the assemblage of diatoms on galleries was most similar to that of the epilithon in the more productive lakes, with the greatest divergence in Windermere and Coniston, mainly due to a much greater relative proportion of Gomphonema in the epilithon than in the galleries, suggesting that fertilisation had a greater impact where background nutrient concentrations were low. Lakes in the RDA triplot were arranged in order of productivity along axis 1, with gallery assemblages in each case located towards the more productive end relative to the epilithon. This sedentary, retreat‐building grazer, probably in common with many other taxa with such traits in aquatic ecosystems, improves its own food resources by gardening. Larvae can modify algal assemblages in their feeding patches by weeding and fertilisation. They create new surfaces for growth of the biofilm via continually adding to the front of their silken galleries and harvesting the biofilm (dominated by diatoms) that has developed on older parts of their galleries to the rear. It is also feasible that the physical structure of the gallery (separately from the effects of nutrients) plays some role in increasing the availability of algae within the territory. This behaviour is likely to extend the range of conditions under which such species can persist, raise the carrying capacity of their habitat, and has possible wider ecosystem consequences for productivity and nutrient cycling.