Gardening by the psychomyiid caddisfly Tinodes waeneri: evidence from stable isotopes.

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.

16S rRNA assessment of the influence of shading on early-successional biofilms in experimental streams.

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.