Ricardo Albariño

A global experiment suggests climate warming will not accelerate litter decomposition in streams but might reduce carbon sequestration

The decomposition of plant litter is one of the most important ecosystem processes in the biosphere and is particularly sensitive to climate warming. Aquatic ecosystems are well suited to studying warming effects on decomposition because the otherwise confounding influence of moisture is constant. By using a latitudinal temperature gradient in an unprecedented global experiment in streams, we found that climate warming will likely hasten microbial litter decomposition and produce an equivalent decline in detritivore-mediated decomposition rates. As a result, overall decomposition rates should remain unchanged. Nevertheless, the process would be profoundly altered, because the shift in importance from detritivores to microbes in warm climates would likely increase CO(2) production and decrease the generation and sequestration of recalcitrant organic particles. In view of recent estimates showing that inland waters are a significant component of the global carbon cycle, this implies consequences for global biogeochemistry and a possible positive climate feedback.

Global distribution of a key trophic guild contrasts with common latitudinal diversity patterns

Most hypotheses explaining the general gradient of higher diversity toward the equator are implicit or explicit about greater species packing in the tropics. However, global patterns of diversity within guilds, including trophic guilds (i.e., groups of organisms that use similar food resources), are poorly known. We explored global diversity patterns of a key trophic guild in stream ecosystems, the detritivore shredders. This was motivated by the fundamental ecological role of shredders as decomposers of leaf litter and by some records pointing to low shredder diversity and abundance in the tropics, which contrasts with diversity patterns of most major taxa for which broad-scale latitudinal patterns haven been examined. Given this evidence, we hypothesized that shredders are more abundant and diverse in temperate than in tropical streams, and that this pattern is related to the higher temperatures and lower availability of high-quality leaf litter in the tropics. Our comprehensive global survey (129 stream sites from 14 regions on six continents) corroborated the expected latitudinal pattern and showed that shredder distribution (abundance, diversity and assemblage composition) was explained by a combination of factors, including water temperature (some taxa were restricted to cool waters) and biogeography (some taxa were more diverse in particular biogeographic realms). In contrast to our hypothesis, shredder diversity was unrelated to leaf toughness, but it was inversely related to litter diversity. Our findings markedly contrast with global trends of diversity for most taxa, and with the general rule of higher consumer diversity at higher levels of resource diversity. Moreover, they highlight the emerging role of temperature in understanding global patterns of diversity, which is of great relevance in the face of projected global warming.

Impact of exotic rainbow trout on the benthic macroinvertebrate community from Andean-Patagonian headwater streams.

: Introduction of salmonids is a common and widespread practice in rivers and lakes of Patagonia, but their impacts remain poorly understood. We analyse the effect of exotic introduced salmonids (rainbow trout Oncorhynchus mykiss) on the benthic macroinvertebrate community of low order streams. We conducted a field survey in three headwater streams (Challhuaco, Cascada and Pescadero streams) in the northern Andean-Patagonian region (around 41 ° S and 1500m above sea level). The streams are canopied by deciduous Nothofagus pumi-lio forest. On each system, we established fishless and contiguous fish sites separated by waterfalls that limit fish access to upper sections. At each site we determined benthic macroinvertebrate size, taxonomic and functional structure in addition to trout gut contents. In the presence of trout, we observed significant shifts in invertebrate body size towards smaller individuals, thus a decrease in total macroinvertebrate biomass was observed. We found that large taxa (Klapopteryx kuscheli, Tipula sp.) and active swimming species (Metamonius anceps and Hyalella curvispina) were reduced in abundance or were absent in reaches with trout. At the same time, we found that trout positively selected large size classes of invertebrates. As a consequence of species losses, benthic community structure changed drastically in the presence of trout : shredders were the feeding group most affected negatively (-68 %) followed by scrapers. The reduction of shredders may suggest a potential effect on Nothofagus leaf litter breakdown and hence, supply of FPOM to downstream reaches.

C-N mismatch in the leaf litter-shredder relationship of an Andean Patagonian stream detritivore

Abstract Aquatic shredders depend mostly on terrestrial leaf litter as a food resource, and differences between the C:N ratio of their food resource and their bodies may affect nutrient excretion and the composition of their feces. Laboratory experiments were used to test how the stonefly Klapopteryx kuscheli rebalances the high C:N ratio of its food. An experiment was designed in which K. kuscheli was fed 5 different leaf litters across a gradient of C:N ratios and the elemental ratios of the food, insect bodies, and feces were analyzed. Klapopteryx kuscheli regulated its internal elemental composition by changing its excretion of N depending on the N content of its food. An inverse relationship was found between N content of most foods and excreted NH4+. However, K. kuscheli feces were rich in N and the insect excreted very small amounts of NH4+ when fed Nothofagus pumilio, probably because of refractory N-based compounds in the leaves of this species. Homeostatic regulation of N content by invertebrate shredders may influence N dynamics in small nutrient-poor streams.

Grazing impact of two aquatic invertebrates on periphyton from an Andean-Patagonian stream

The mayfly Meridialaris chiloeensis and the snail Chilina dombeiana were observed to coexist and develop abundant populations in several Andean streams. In this study we examined and compared the mouthpart morphologies and the grazing mechanisms of these two species. In addition, through field experiments we analysed the grazing effect on periphyton composition and biomass. Results showed that the herbivores contrasted in their mouthpart morphology and foraging behaviour but would play a similar ecological role, since they both can be considered as scrapers. Experimental results indicated that the individual mayfly effect on chlorophyll-a and - ash free dry mass was lower than that of the individual snail. However, considering the spring and autumn abundances of both populations in a natural environment, their grazing impact might be similar or even higher for the mayfly. In addition, M. chi- loeensis depressed the rosette forming algae and favoured the prostrate ones. As a re- sult, the mayfly grazing produced a community dominated by Nitzschia palea instead - of Achnanthes minutissima that dominated the grazer-free controls, while the snail changed the taxonomic composition very little.

Food Quality, Larval Consumption, and Growth of Klapopteryx kuscheli (Plecoptera: Austroperlidae) from a South Andes Stream

ABSTRACT The effect of food quality, measured as leaf resistance and carbon:nitrogen ratio, on the consumption and growth of the plecopteran Klapopteryx kuscheli was analyzed through an in situ experiment. Leaves from seven different tree species were used in feeding trials with intermediate sized K. kuscheli larvae. We used leaves from both deciduous and evergreen species, which exhibited a wide range in C:N ratio as well as leaf resistance. After 30 days, leaf consumption and larval growth were estimated for each of the 119 experimental units. Results showed that K. kuscheli was unable to consume tough leaves (leaf resistance higher than 300 g mm−2). Among deciduous species, consumption was proportional to C:N ratio showing a compensative strategy on leaves with higher C:N ratio. Nevertheless, consumption rate did not compensate for growth as ingestion was inversely related to food quality. Native deciduous plant species were of low quality for larval growth and efficient food conversion, and this may have ecological implications for both secondary production and life cycle length in detritivore-shredders inhabiting Andean low-order canopied streams.

Feeding habit of Notoperla archiplatae (Plecoptera) larvae in a North Patagonia Andean stream, Argentina

The feeding habit of Notoperla archiplatae (Plecoptera) larvae was assessed by means of the observation of mouthparts morphology and gut contents analysis. Larvae of different sizes were collected from the Ñireco stream, in the North Patagonian Andes. Mouthparts morphology was indicative of a phytophagous diet. Diet was composed mainly of diatoms; therefore, periphyton was sampled and compared with gut contents through an electivity index in order to determine if N. archiplatae selectively consumed certain diatom species. This analysis showed that the insects preferred erect diatoms, especially those with an arborescent habitus. Based on mouthpart morphology and gut content, N. archiplatae was assigned to the scrapers functional feeding group.The feeding habit of Notoperla archiplatae(Plecoptera) larvae was assessed by means of the observation of mouthparts morphology and gut contents analysis. Larvae of different sizes were collected from the Nireco stream, in the North Patagonian Andes. Mouthparts morphology was indicative of a phytophagous diet. Diet was composed mainly of diatoms; therefore, periphyton was sampled and compared with gut contents through an electivity index in order to determine if N. archiplatae selectively consumed certain diatom species. This analysis showed that the insects preferred erect diatoms, especially those with an arborescent habitus. Based on mouthpart morphology and gut content, N. archiplatae was assigned to the scrapers functional feeding group.

Biotic and abiotic variables influencing plant litter breakdown in streams: A global study

Plant litter breakdown is a key ecological process in terrestrial and freshwater ecosystems. Streams and rivers, in particular, contribute substantially to global carbon fluxes. However, there is little information available on the relative roles of different drivers of plant litter breakdown in fresh waters, particularly at large scales. We present a global-scale study of litter breakdown in streams to compare the roles of biotic, climatic and other environmental factors on breakdown rates. We conducted an experiment in 24 streams encompassing latitudes from 47.8° N to 42.8° S, using litter mixtures of local species differing in quality and phylogenetic diversity (PD), and alder (Alnus glutinosa) to control for variation in litter traits. Our models revealed that breakdown of alder was driven by climate, with some influence of pH, whereas variation in breakdown of litter mixtures was explained mainly by litter quality and PD. Effects of litter quality and PD and stream pH were more positive at higher temperatures, indicating that different mechanisms may operate at different latitudes. These results reflect global variability caused by multiple factors, but unexplained variance points to the need for expanded global-scale comparisons.

Detritivores feeding on poor quality food are more sensitive to increased temperatures

As temperature increases the metabolic rates, the effect of warming on animals will also enhance animal-driven nutrient cycling with important consequences on ecosystem dynamics. We tested the effects of increased temperature (15 and 20°C, optimal and suboptimal temperatures, respectively) on metabolic rates of the shredder larvae Sericostoma vittatum fed on three diets, Alnus glutinosa (L.) Gaertn., Eucalyptus globulus Labill. and Quercus robur L. We measured P and N content in leaves, faeces and excreta and calculated C, N, and P assimilation efficiencies, and mass balances. Carbon assimilation efficiency (AE) was reduced at 20°C when larvae fed on Q. robur; nitrogen-AE was reduced at 20°C in all diets and phosphorus-AE was not affected by temperature. Larvae achieved a net N gain in all treatments, however, increased temperatures had a negative effect on N incorporation into body tissue. The mass balance of P was negatively affected by temperature; larvae fed on Q. robur and on E. globulus had null balances at 15°C and negative at 20°C. Our results showed that high temperature increased nutrient excretion and affected N:P ratios in excreta, thus changes in temperature may have severe consequences on larval mediated leaf litter processing and nutrient cycling. However, the type of diet seemed to modulate the way temperature affects larval metabolism regarding excretion rate and assimilation efficiencies. The extent to which optimal–suboptimal temperature variation will alter detritivore metabolism performance, internal nutrient balance and hence, cycling of elements in the environment seems crucial under global warming scenarios.

Invasive Salix fragilis alters benthic invertebrate communities and litter decomposition in northern Patagonian streams

Invasion by exotic trees into riparian areas has the potential to impact aquatic systems. We examined the effects of the exotic Salix fragilis (crack willow) on the structure and functioning of small streams in northern Patagonian Andes via a field survey of benthic invertebrates and leaf litter and an in situ experiment. We compared leaf decomposition of the native Ochetophila trinervis (chacay) and S. fragilis in reaches dominated by native vegetation versus reaches dominated by crack willow. We hypothesized that S. fragilis affects the quality of leaf litter entering the streams, changing the aquatic biota composition and litter decomposition. Our study showed that crack willow leaves decomposed slower than chacay, likely related to leaf properties (i.e., leaf toughness). Benthic leaf litter mass was similar between the two riparian vegetation types, though in stream reaches dominated by crack willow, leaves of this species represented 82% of the total leaf litter. Benthic invertebrate abundance and diversity were similar between reaches but species composition differed. Our study found little evidence for strong impacts of crack willow on those small streams. Further studies on other aspects of ecosystem functioning, such as primary production, would enhance our understanding of the impacts of crack willow on Patagonian streams.