Bending the rules: exploitation of allochthonous resources by a top-predator modifies size-abundance scaling in stream food webs.

Body mass-abundance (M-N) allometries provide a key measure of community structure, and deviations from scaling predictions could reveal how cross-ecosystem subsidies alter food webs. For 31 streams across the UK, we tested the hypothesis that linear log-log M-N scaling is shallower than that predicted by allometric scaling theory when top predators have access to allochthonous prey. These streams all contained a common and widespread top predator (brown trout) that regularly feeds on terrestrial prey and, as hypothesised, deviations from predicted scaling increased with its dominance of the fish assemblage. Our study identifies a key beneficiary of cross-ecosystem subsidies at the top of stream food webs and elucidates how these inputs can reshape the size-structure of these 'open' systems.

Persistence in the longitudinal distribution of lotic insects in a changing climate: a tale of two rivers.

The longitudinal distribution of many taxa in rivers is influenced by temperature. Here we took advantage of two older datasets on net-spinning caddisflies (Hydropsychidae) from contrasting European rivers to assess changes in species occurrence and relative abundance along the river by resampling the same sites, postulating that an increase in river temperature over the intervening period should have resulted in cool-adapted species retreating into the headwaters and warm adapted species expanding upstream. Distributional changes in the Welsh Usk were slight between 1968/69 and 2010, one rare species appearing at a single headwater site and one warm-adapted species disappearing from the main river. Distributional changes in the French Loire, between 1989-93 and 2005, were similarly modest, with no consistent movement of species up- or downstream. We estimate that the decadal rate of increase in the mean summer daily maximum in the Usk was only 0.1°C at one 'summer cool' headwater site, while a neighbouring 'summer warm' tributary increased by 0.16°C per decade, and the main river by 0.22°C. The Loire is warmer than the Usk and the mean decadal rates of increase, over the period 1989-2005, at three sites along the lower reaches were 0.39, 0.48 and 0.77°C. Increases in stream and river temperature, therefore, were spatially variable and were not associated with consistent upstream movement of species in either of these (very different) rivers. We conclude that either the temperature increases have hitherto been insufficient to affect species distribution or, more speculatively, that it may not be possible for river organisms (that do not respond only to temperature) to move upstream because of a developing spatial mismatch between key habitat characteristics, some of them changing with the climate but others not.

Long-term amelioration of acidity accelerates decomposition in headwater streams.

The secondary production of culturally acidified streams is low, with a few species of generalist detritivores dominating invertebrate assemblages, while decomposition processes are impaired. In a series of lowland headwater streams in southern England, we measured the rate of cellulolytic decomposition and compared it with values measured three decades ago, when anthropogenic acidification was at its peak. We hypothesized that, if acidity has indeed ameliorated, the rate of decomposition will have accelerated, thus potentially supporting greater secondary production and the longer food chains that have been observed in some well-studied recovering freshwater systems. We used cellulose Shirley test cloth as a standardized bioassay to measure the rate of cellulolytic decomposition, via loss in tensile strength, for 31 streams in the Ashdown Forest over 7 days in summer 2011 and 49 days in winter 2012. We compared this with data from an otherwise identical study conducted in 1978 and 1979. In a secondary study, we determined whether decomposition followed a linear or logarithmic decay and, as Shirley cloth is no longer available, we tested an alternative in the form of readily available calico. Overall mean pH had increased markedly over the 32 years between the studies (from 6.0 to 6.7). In both the previous and contemporary studies, the relationship between decomposition and pH was strongest in winter, when pH reaches a seasonal minimum. As in the late 1970s, there was no relationship in 2011/2012 between pH and decay rate in summer. As postulated, decomposition in winter was significantly faster in 2011/2012 than in 1978/1979, with an average increase in decay rate of 18.1%. Recovery from acidification, due to decreased acidifying emissions and deposition, has led to an increase in the rate of cellulolytic decomposition. This response in a critical ecosystem process offers a potential explanation of one aspect of the limited biological recovery that has been observed so far, an increase in larger bodied predators including fish, which in turn leads to an increase in the length of food chains.

Grazing and detritivory in 20 stream food webs across a broad pH gradient.

Acidity is a major driving variable in the ecology of fresh waters, and we sought to quantify macroecological patterns in stream food webs across a wide pH gradient. We postulated that a few generalist herbivore-detritivores would dominate the invertebrate assemblage at low pH, with more specialists grazers at high pH. We also expected a switch towards algae in the diet of all primary consumers as the pH increased. For 20 stream food webs across the British Isles, spanning pH 5.0-8.4 (the acid sites being at least partially culturally acidified), we characterised basal resources and primary consumers, using both gut contents analysis and stable isotopes to study resource use by the latter. We found considerable species turnover across the pH gradient, with generalist herbivore-detritivores dominating the primary consumer assemblage at low pH and maintaining grazing. These were joined or replaced at higher pH by a suite of specialist grazers, while many taxa that persisted across the pH gradient broadened the range of algae consumed as acidity declined and increased their ingestion of biofilm, whose nutritional quality was higher than that of coarse detritus. There was thus an increased overall reliance on algae at higher pH, both by generalist herbivore-detritivores and due to the presence of specialist grazers, although detritus was important even in non-acidic streams. Both the ability of acid-tolerant, herbivore-detritivores to exploit both autochthonous and allochthonous food and the low nutritional value of basal resources might render chemically recovering systems resistant to invasion by the specialist grazers and help explain the sluggish ecological recovery of fresh waters whose water chemistry has ameliorated.

River bed carbon and nitrogen cycling: state of play and some new directions.

The significance of freshwaters as key players in the global budget of both carbon dioxide and methane has recently been highlighted. In particular, rivers clearly do not act simply as inert conduits merely piping carbon from catchment to coast, but, on the whole, their metabolic activity transforms a considerable fraction of the carbon that they convey. In addition, nitrogen is cycled, sometimes in tight unison with carbon, with appreciable amounts being 'denitrified' between catchment and coast. However, shortfalls in our knowledge about the significance of exchange and interaction between rivers and their catchments, particularly the significance of interactions mediated through hyporheic sediments, are still apparent. From humble beginnings of quantifying the consumption of oxygen by small samples of gravel, to an integrated measurement of reach scale transformations of carbon and nitrogen, our understanding of the cycling of these two macro elements in rivers has improved markedly in the past few decades. However, recent discoveries of novel metabolic pathways in both the nitrogen and carbon cycle across a spectrum of aquatic ecosystems, highlights the need for new directions and a truly multidisciplinary approach to quantifying the flux of carbon and nitrogen through rivers.

Caddisflies as biomonitors identifying thresholds of toxic metal bioavailability that affect the stream benthos.

It has been proposed that bioaccumulated concentrations of toxic metals in tolerant biomonitors be used as indicators of metal bioavailability that could be calibrated against the ecological response to metals of sensitive biotic assemblages. Our hypothesis was that metal concentrations in caddisfly larvae Hydropsyche siltalai and Plectrocnemia conspersa, as tolerant biomonitors, indicate metal bioavailability in contaminated streams, and can be calibrated against metal-specific ecological responses of mayflies. Bioaccumulated concentrations of Cu, As, Zn and Pb in H. siltalai from SW English streams were related to the mayfly assemblage. Mayflies were always sparse where bioavailabilities were high and were abundant and diverse where bioavailabilities of all metals were low, a pattern particularly evident when the combined abundance of heptageniid and ephemerellid mayflies was the response variable. The results offer promise that bioaccumulated concentrations of metals in tolerant biomonitors can be used to diagnose ecological impacts on stream benthos from metal stressors.

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.

Foraging in a patchy environment by a predatory net-spinning caddis larva: A test of optimal foraging theory.

The predatory larvae of the caddis Plectrocnemia conspersa (Curtis) cause significant prey depletion in a habitat in which prey are patchily distributed. Optimal foraging theory predicts that under these circumstances a predator should stay in any given patch until the prey capture rate there drops to a value equal to the average for the habitat as a whole. This was tested using a combination of field and laboratory data and the results were in broad agreement with the prediction. A second prediction is that the marginal capture rate should be higher in a habitat richer in prey and this was not supported. It is argued that by using a simple rule-of-thumb (constant giving-up-time) P. conspersa is able to approach the optimal solution for much of the time.

The influence of substrate on the functional response of Plectrocnemia conspersa (Curtis) larvae (Trichoptera: Polycentropodidae).

1. Experiments were carried out on the effects of substrate type on the functional response of Plectrocnemia conspersa Curtis for two of its naturally occurring prey species. 2. For both prey species predation rate is depressed on substrates which correspond to those favoured by the prey in the field. 3. The difference between predation rate on simple and on certain more complex substrates is due to the provision of refuges for prey on the latter types. 4. Differences in predation rate on the two prey species are due to their different behaviour when caught in the predators net. 5. The influence of substrate on the functional response of P. conspersa is discussed in the context of the stability of predator-prey interactions and the availability of prey to predators.