Environmental warming increases the importance of high-turnover energy channels in stream food webs

Warming temperatures are altering communities and trophic networks across Earth's ecosystems. While the overall influence of warming on food webs is often context-dependent, increasing temperatures are predicted to change communities in two fundamental ways: (1) by reducing average body size and (2) by increasing individual metabolic rates. These warming-induced changes have the potential to influence the distribution of food web fluxes, food web stability, and the relative importance of deterministic and stochastic ecological processes shaping community assembly. Here, we quantified patterns and the relative distribution of organic matter fluxes through stream food webs spanning a broad natural temperature gradient (5-27°C). We then related these patterns to species and community trait distributions of mean body size and population biomass turnover (P:B) within and across streams. We predicted that (1) communities in warmer streams would exhibit smaller body size and higher P:B and (2) organic matter fluxes within warmer communities would increasingly skew toward smaller, higher P:B populations. Across the temperature gradient, warmer communities were characterized by smaller body size (~9% per °C) and higher P:B (~7% faster turnover per °C) populations on average. Additionally, organic matter fluxes within warmer streams were increasingly skewed toward higher P:B populations, demonstrating that warming can restructure organic matter fluxes in both an absolute and relative sense. With warming, the relative distribution of organic matter fluxes was decreasingly likely to arise through the random sorting of species, suggesting stronger selection for traits driving high turnover with increasing temperature. Our study suggests that a warming world will favor energy fluxes through "smaller and faster" populations, and that these changes may be more predictable than previously thought.

The prey availability and diet of juvenile Atlantic salmon (Salmo salar L.) in low-productivity rivers in northern Europe

The availability of resources varies across a species distributional range, and a low-productivity area can make a species more vulnerable. We investigated the invertebrate composition and prey choice of juvenile Atlantic salmon (Salmo salar L.) in low-productivity rivers in northeast Iceland, which is one of the species' most northerly distributions. By sampling benthic and drift invertebrate populations, we found that prey availability was similar within and between rivers. Gut content samples showed that the main prey choice for juvenile S. salar was the Chironomidae. The type of food items consumed varied across different weight groups of S. salar, with smaller juveniles having more diverse diet. S. salar did not have a selection preference for chironomids, which indicates that they were eating the highly available prey in their environment, rather than hunting high biomass items such as terrestrial invertebrates and large Dipterans. Estimates of dietary niche showed that S. salar in these low-productivity rivers relied on consuming what was most readily available, the chironomids, and that they must share resources with other salmonid species. This may be due to the low diversity of freshwater invertebrates (fewer prey options), whereas S. salar in nutrient-rich rivers could rely more on terrestrial invertebrates as an additional subsidy in their diet. In conclusion, with limited prey choices, juvenile S. salar in nutrient-poor rivers, especially in a biogeographically isolated region with low species diversity, may increase in vulnerability and decrease in adaptability to environmental change. Management methods that increase benthic prey abundance and diversity are recommended for conserving the S. salar population in a nutrient-poor river.

Regional impacts of warming on biodiversity and biomass in high latitude stream ecosystems across the Northern Hemisphere

Warming can have profound impacts on ecological communities. However, explorations of how differences in biogeography and productivity might reshape the effect of warming have been limited to theoretical or proxy-based approaches: for instance, studies of latitudinal temperature gradients are often conflated with other drivers (e.g., species richness). Here, we overcome these limitations by using local geothermal temperature gradients across multiple high-latitude stream ecosystems. Each suite of streams (6-11 warmed by 1-15°C above ambient) is set within one of five regions (37 streams total); because the heating comes from the bedrock and is not confounded by changes in chemistry, we can isolate the effect of temperature. We found a negative overall relationship between diatom and invertebrate species richness and temperature, but the strength of the relationship varied regionally, declining more strongly in regions with low terrestrial productivity. Total invertebrate biomass increased with temperature in all regions. The latter pattern combined with the former suggests that the increased biomass of tolerant species might compensate for the loss of sensitive species. Our results show that the impact of warming can be dependent on regional conditions, demonstrating that local variation should be included in future climate projections rather than simply assuming universal relationships.

Invertebrate communities in springs across a gradient in thermal regimes

In many respects, freshwater springs can be considered as unique ecosystems on the fringe of aquatic habitats. This integrates their uniqueness in terms of stability of environmental metrics. The main objective of our study was to evaluate how environmental variables may shape invertebrate diversity and community composition in different freshwater spring types and habitats within. In order to do so, we sampled invertebrates from 49 springs in Iceland, where we included both limnocrene and rheocrene springs. At each site, samples were taken from the benthic substrate of the spring (“surface”) and the upwelling groundwater at the spring source (“source”). To collect invertebrates from the spring sources we used a modified method of “electrobugging” and Surber sampler for collecting invertebrates from the surface. In total, 54 invertebrate taxa were identified, mostly Chironomidae (Diptera). Chironomid larvae also dominated in terms of abundance (67%), followed by Ostracoda (12%) and Copepoda (9%). The species composition in the surface samples differed considerably between rheocrene and limnocrene springs and was characterised by several indicator species. Alpha diversity was greater at the surface of springs than at the source, but the beta diversity was higher at the source. Diversity, as summarized by taxa richness and Shannon diversity, was negatively correlated with temperature at the surface. At the source, on the other hand, Shannon diversity increased with temperature. The community assembly in springs appears to be greatly affected by water temperature, with the source community of hot springs being more niche-assembled (i.e., affected by mechanisms of tolerance and adaptation) than the source community of cold springs, which is more dispersal-assembled (i.e., by mechanisms of drift and colonization).

Resource supply governs the apparent temperature dependence of animal production in stream ecosystems

Rising global temperatures are changing how energy and materials move through ecosystems, with potential consequences for the role of animals in these processes. We tested a central prediction of the metabolic scaling framework-the temperature independence of animal community production-using a series of geothermally heated streams and a comprehensive empirical analysis. We show that the apparent temperature sensitivity of animal production was consistent with theory for individuals (Ep_ind  = 0.64 vs. 0.65 eV), but strongly amplified relative to theoretical expectations for communities, both among (Ep_among  = 0.67 vs. 0 eV) and within (Ep_within  = 1.52 vs. 0 eV) streams. After accounting for spatial and temporal variation in resources, we show that the apparent positive effect of temperature was driven by resource supply, providing strong empirical support for the temperature independence of invertebrate production and the necessary inclusion of resources in metabolic scaling efforts.

Thermal niche diversity and trophic redundancy drive neutral effects of warming on energy flux through a stream food web

Climate warming is predicted to alter routing and flows of energy through food webs because of the critical and varied effects of temperature on physiological rates, community structure, and trophic dynamics. Few studies, however, have experimentally assessed the net effect of warming on energy flux and food web dynamics in natural intact communities. Here, we test how warming affects energy flux and the trophic basis of production in a natural invertebrate food web by experimentally heating a stream reach in southwest Iceland by ~4°C for 2 yr and comparing its response to an unheated reference stream. Previous results from this experiment showed that warming led to shifts in the structure of the invertebrate assemblage, with estimated increases in total metabolic demand but no change in annual secondary production. We hypothesized that elevated metabolic demand and invariant secondary production would combine to increase total consumption of organic matter in the food web, if diet composition did not change appreciably with warming. Dietary composition of primary consumers indeed varied little between streams and among years, with gut contents primarily consisting of diatoms (72.9%) and amorphous detritus (19.5%). Diatoms dominated the trophic basis of production of primary consumers in both study streams, contributing 79-86% to secondary production. Although warming increased the flux of filamentous algae within the food web, total resource consumption did not increase as predicted. The neutral net effect of warming on total energy flow through the food web was a result of taxon-level variation in responses to warming, a neutral effect on total invertebrate production, and strong trophic redundancy within the invertebrate assemblage. Thus, food webs characterized by a high degree of trophic redundancy may be more resistant to the effects of climate warming than those with more diverse and specialized consumers.

Invertebrate biodiversity in cold groundwater fissures in Iceland

Iceland has an abundance of fissures that are parallel to the Mid-Atlantic Ridge where bedrock cracks as a result of continental rifting. Some fissures penetrate the aquifer and expose the groundwater within the bedrock, becoming springs. As such, groundwater fissures have uniform and constant physical and chemical environment but they can differ greatly in morphology. In addition, there is often great variation in depth within fissures and substrate types contrast between vertical rock wall and more heterogenous horizontal bottom. The variation in morphological environment may create dissimilar habitats with unique characteristics and/or influence distribution of resources. Our objective was to study macrozoobenthos communities in cold groundwater fissures in Iceland in relation to physical habitat by comparing invertebrate diversity and density both between fissures with different morphological characteristics as well as between substrate types and depths within fissures. Samples were collected in two fissures in SW Iceland, Silfra and Flosagjá. Assemblages were similar between fissures except for higher densities of cladocerans in Flosagjá fissure. Within fissures, there was significant difference in Shannon diversity between substrate types in Flosagjá, and ostracods were found in significantly higher densities on the bottom. The distribution of all other taxa groups was homogenous in both fissures regardless of depth gradient and substrate. Invertebrates were found to be living within and around a biofilm that covered the entire substrate. These biofilm mats are made from Cyanobacteria and benthic diatoms, which are successful under low light conditions and may minimize any effect of the heterogeneous habitat creating a uniform and suitable microhabitat for invertebrates regardless of depth and substrate type.

Temperature-driven selection on metabolic traits increases the strength of an algal-grazer interaction in naturally warmed streams

Trophic interactions are important determinants of the structure and functioning of ecosystems. Because the metabolism and consumption rates of ectotherms increase sharply with temperature, there are major concerns that global warming will increase the strength of trophic interactions, destabilizing food webs, and altering ecosystem structure and function. We used geothermally warmed streams that span an 11°C temperature gradient to investigate the interplay between temperature-driven selection on traits related to metabolism and resource acquisition, and the interaction strength between the keystone gastropod grazer, Radix balthica, and a common algal resource. Populations from a warm stream (~28°C) had higher maximal metabolic rates and optimal temperatures than their counterparts from a cold stream (~17°C). We found that metabolic rates of the population originating from the warmer stream were higher across all measurement temperatures. A reciprocal transplant experiment demonstrated that the interaction strengths between the grazer and its algal resource were highest for both populations when transplanted into the warm stream. In line with the thermal dependence of respiration, interaction strengths involving grazers from the warm stream were always higher than those with grazers from the cold stream. These results imply that increases in metabolism and resource consumption mediated by the direct, thermodynamic effects of higher temperatures on physiological rates are not mitigated by metabolic compensation in the long term, and suggest that warming could increase the strength of algal-grazer interactions with likely knock-on effects for the biodiversity and productivity of aquatic ecosystems.

Increased resource use efficiency amplifies positive response of aquatic primary production to experimental warming

Climate warming is affecting the structure and function of river ecosystems, including their role in transforming and transporting carbon (C), nitrogen (N), and phosphorus (P). Predicting how river ecosystems respond to warming has been hindered by a dearth of information about how otherwise well-studied physiological responses to temperature scale from organismal to ecosystem levels. We conducted an ecosystem-level temperature manipulation to quantify how coupling of stream ecosystem metabolism and nutrient uptake responded to a realistic warming scenario. A ~3.3°C increase in mean water temperature altered coupling of C, N, and P fluxes in ways inconsistent with single-species laboratory experiments. Net primary production tripled during the year of experimental warming, while whole-stream N and P uptake rates did not change, resulting in 289% and 281% increases in autotrophic dissolved inorganic N and P use efficiency (UE), respectively. Increased ecosystem production was a product of unexpectedly large increases in mass-specific net primary production and autotroph biomass, supported by (i) combined increases in resource availability (via N mineralization and N₂ fixation) and (ii) elevated resource use efficiency, the latter associated with changes in community structure. These large changes in C and nutrient cycling could not have been predicted from the physiological effects of temperature alone. Our experiment provides clear ecosystem-level evidence that warming can shift the balance between C and nutrient cycling in rivers, demonstrating that warming will alter the important role of in-stream processes in C, N, and P transformations. Moreover, our results reveal a key role for nutrient supply and use efficiency in mediating responses of primary producers to climate warming.

Interactive effects of temperature and habitat complexity on freshwater communities

Warming can lead to increased growth of plants or algae at the base of the food web, which may increase the overall complexity of habitat available for other organisms. Temperature and habitat complexity have both been shown to alter the structure and functioning of communities, but they may also have interactive effects, for example, if the shade provided by additional habitat negates the positive effect of temperature on understory plant or algal growth. This study explored the interactive effects of these two major environmental factors in a manipulative field experiment, by assessing changes in ecosystem functioning (primary production and decomposition) and community structure in the presence and absence of artificial plants along a natural stream temperature gradient of 5-18°C. There was no effect of temperature or habitat complexity on benthic primary production, but epiphytic production increased with temperature in the more complex habitat. Cellulose decomposition rate increased with temperature, but was unaffected by habitat complexity. Macroinvertebrate communities were less similar to each other as temperature increased, while habitat complexity only altered community composition in the coldest streams. There was also an overall increase in macroinvertebrate abundance, body mass, and biomass in the warmest streams, driven by increasing dominance of snails and blackfly larvae. Presence of habitat complexity, however, dampened the strength of this temperature effect on the abundance of macroinvertebrates in the benthos. The interactive effects that were observed suggest that habitat complexity can modify the effects of temperature on important ecosystem functions and community structure, which may alter energy flow through the food web. Given that warming is likely to increase habitat complexity, particularly at higher latitudes, more studies should investigate these two major environmental factors in combination to improve our ability to predict the impacts of future global change.

Glacier shrinkage driving global changes in downstream systems

Glaciers cover ∼10% of the Earth's land surface, but they are shrinking rapidly across most parts of the world, leading to cascading impacts on downstream systems. Glaciers impart unique footprints on river flow at times when other water sources are low. Changes in river hydrology and morphology caused by climate-induced glacier loss are projected to be the greatest of any hydrological system, with major implications for riverine and near-shore marine environments. Here, we synthesize current evidence of how glacier shrinkage will alter hydrological regimes, sediment transport, and biogeochemical and contaminant fluxes from rivers to oceans. This will profoundly influence the natural environment, including many facets of biodiversity, and the ecosystem services that glacier-fed rivers provide to humans, particularly provision of water for agriculture, hydropower, and consumption. We conclude that human society must plan adaptation and mitigation measures for the full breadth of impacts in all affected regions caused by glacier shrinkage.

Metabolic compensation constrains the temperature dependence of gross primary production

Gross primary production (GPP) is the largest flux in the carbon cycle, yet its response to global warming is highly uncertain. The temperature dependence of GPP is directly linked to photosynthetic physiology, but the response of GPP to warming over longer timescales could also be shaped by ecological and evolutionary processes that drive variation in community structure and functional trait distributions. Here, we show that selection on photosynthetic traits within and across taxa dampens the effects of temperature on GPP across a catchment of geothermally heated streams. Autotrophs from cold streams had higher photosynthetic rates and after accounting for differences in biomass among sites, biomass-specific GPP was independent of temperature in spite of a 20 °C thermal gradient. Our results suggest that temperature compensation of photosynthetic rates constrains the long-term temperature dependence of GPP, and highlights the importance of considering physiological, ecological and evolutionary mechanisms when predicting how ecosystem-level processes respond to warming.

Experimental whole-stream warming alters community size structure

How ecological communities respond to predicted increases in temperature will determine the extent to which Earth's biodiversity and ecosystem functioning can be maintained into a warmer future. Warming is predicted to alter the structure of natural communities, but robust tests of such predictions require appropriate large-scale manipulations of intact, natural habitat that is open to dispersal processes via exchange with regional species pools. Here, we report results of a two-year whole-stream warming experiment that shifted invertebrate assemblage structure via unanticipated mechanisms, while still conforming to community-level metabolic theory. While warming by 3.8 °C decreased invertebrate abundance in the experimental stream by 60% relative to a reference stream, total invertebrate biomass was unchanged. Associated shifts in invertebrate assemblage structure were driven by the arrival of new taxa and a higher proportion of large, warm-adapted species (i.e., snails and predatory dipterans) relative to small-bodied, cold-adapted taxa (e.g., chironomids and oligochaetes). Experimental warming consequently shifted assemblage size spectra in ways that were unexpected, but consistent with thermal optima of taxa in the regional species pool. Higher temperatures increased community-level energy demand, which was presumably satisfied by higher primary production after warming. Our experiment demonstrates how warming reassembles communities within the constraints of energy supply via regional exchange of species that differ in thermal physiological traits. Similar responses will likely mediate impacts of anthropogenic warming on biodiversity and ecosystem function across all ecological communities.

Temperature effects on fish production across a natural thermal gradient

Global warming is widely predicted to reduce the biomass production of top predators, or even result in species loss. Several exceptions to this expectation have been identified, however, and it is vital that we understand the underlying mechanisms if we are to improve our ability to predict future trends. Here, we used a natural warming experiment in Iceland and quantitative theoretical predictions to investigate the success of brown trout as top predators across a stream temperature gradient (4-25 °C). Brown trout are at the northern limit of their geographic distribution in this system, with ambient stream temperatures below their optimum for maximal growth, and above it in the warmest streams. A five-month mark-recapture study revealed that population abundance, biomass, growth rate, and production of trout all increased with stream temperature. We identified two mechanisms that contributed to these responses: (1) trout became more selective in their diet as stream temperature increased, feeding higher in the food web and increasing in trophic position; and (2) trophic transfer through the food web was more efficient in the warmer streams. We found little evidence to support a third potential mechanism: that external subsidies would play a more important role in the diet of trout with increasing stream temperature. Resource availability was also amplified through the trophic levels with warming, as predicted by metabolic theory in nutrient-replete systems. These results highlight circumstances in which top predators can thrive in warmer environments and contribute to our knowledge of warming impacts on natural communities and ecosystem functioning.