Verónica Díaz Villanueva

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.

Biofilm formation at warming temperature: acceleration of microbial colonization and microbial interactive effects

River biofilms that grow on wet benthic surface are mainly composed of bacteria, algae, cyanobacteria and protozoa embedded in a polysaccharide matrix. The effects of increased river water temperature on biofilm formation were investigated. A laboratory experiment was designed employing two temperatures (11.1–13.2°C, night–day; 14.7–16.0°C, night–day) and two nutrient levels (0.054 mg P l−1, 0.75 mg N l−1; 0.54 mg P l−1, 7.5 mg N l−1). Biofilm formation at the higher temperature was faster, while the biomass of the mature biofilm was mainly determined by nutrient availability. The specific response of the three microbial groups that colonized the substrata (algae, bacteria and ciliates) was modulated by interactions between them. The greater bacterial growth rate and earlier bacterial colonization at the higher temperature and higher nutrient status was not translated into the accrual of higher bacterial biomass. This may result from ciliates grazing on the bacteria, as shown by an earlier increase in peritrichia at higher temperatures, and especially at high nutrient conditions. Temperature and ciliate grazing might determine the growth of a distinctive bacterial community under warming conditions. Warmer conditions also produced a thicker biofilm, while functional responses were much less evident (increases in the heterotrophic utilization of polysaccharides and peptides, but no increase in primary production and respiration). Increasing the temperature of river water might lead to faster biofilm recolonization after disturbances, with a distinct biofilm community structure that might affect the trophic web. Warming effects would be expected to be more relevant under eutrophic conditions.

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.

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.

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.

THE ROLE OF MICROORGANISMS IN THE DIET OF VERGER CF. LIMNOPHILUS (TRICHOPTERA: LIMNEPHILIDAE) LARVAE IN A PATAGONIAN ANDEAN TEMPORARY POND

The importance of fungi and bacteria attached to leaf litter in the diet and growth of shredders in flowing waters is well-documented. This study focuses on the role of microorganisms colonizing submerged leaf litter in the diet and growth ofVerger cf.limnophilus (Trichoptera: Limnephilidae) larvae in a Patagonian Andean temporary pond (Fantasma pond, 41°07′S, 71°27′W). First, the feeding habits were analyzed through an experiment that compared consumption of CPOM and FPOM. Once we determined thatV. cf.limnophilus consumed CPOM, we performed an experiment to compare consumption and growth rates of larvae fed on non-autoclaved and autoclaved decaying leaves. Algae was the most abundant group to colonize leaf surface, comprising 74% of total biovolume. Consumption of non-autoclaved leaves was fourfold that of autoclaved treatments, which produced negative insect growth rates. AlthoughV. cf.limnophilus processed leaves by shredding, microorganisms living on the leaf litter were found to be an important food resource. As microbial biomass represents a small percentage of the ingested food (0.22%),V. cflimnophilus appears to process relatively large quantities of detritus to obtain sufficient resources for growth (100 mg leaves to grow 3 mg).

Leaf litter breakdown and benthic invertebrate colonization affected by seasonal drought in headwater lotic systems of Andean Patagonia

We aimed to establish the effect of seasonal drought on leaf litter breakdown and invertebrate communities. Differences in breakdown rates of Nothofagus pumilio were experimentally compared using the litter bag method (coarse and fine mesh size bags) in two first-order streams, one intermittent and one perennial, during two different hydrological periods. Colonizing fauna found in coarse mesh bags was quantified, identified and compared with benthic biota from the same streams. Leaf litter decay rates in low flow conditions revealed that breakdown was principally a consequence of microbial action in the intermittent stream. In contrast, breakdown in high flow conditions was caused by invertebrate feeding in both streams. Collector–gatherers constituted most of the abundance and biomass in bags from the intermittent stream, due to their rapid benthic recolonization. Shredders peaked at approximately 50% remaining leaf litter mass in both streams only during high flow, which coincided with general models of detritus breakdown in streams. Considering global warming scenarios, with drought and water temperature increases expected for many regions of the world, these studies on the consequences for the biota and ecological processes of small streams will allow the prediction of negative effects on such vulnerable ecosystems.

Seasonal patterns of organic matter stoichiometry along a mountain catchment

Organic matter (OM) nutrient content is expected to change as OM is transported from upstream forested reaches downstream to open land, and these changes could depend on seasonal patterns. The aim of this study was to analyze OM carbon nitrogen and phosphorus content, from headwater reaches to open reaches downstream. We sampled four sites along a mountain catchment over the course of one year and quantified fine particulate organic carbon (FPOC) in transport and dissolved organic carbon (DOC) concentration and elemental content. Both FPOC and DOC concentration decreased downstream, and changes in OM stoichiometry depended on the season. Stoichiometry of FPOM and DOM along the catchment was similar in autumn, with higher C:P and C:N ratios upstream, which could be associated with the relative importance of allochthonous versus autochthonous OM origin. Conversely in spring, C:N was very low in the forest sites. Our results highlight that headwaters running through pristine forests are a source of POM and DOM for downstream ecosystems, and that the quality of such resources depends on season.