Resource preference of two stream detritivores in the laboratory largely differs from the supply of detritus below eucalypt plantations

Detritivores are pivotal in forest streams as they process detritus and promote secondary production. Many studies have addressed the preference of freshwater detritivores towards materials of differing quality. Nevertheless, few studies compare the resource preferences in the laboratory with the availability in the field. In the present study, feeding preferences of two stream detritivores (the caddisfly Sericostoma pyrenaicum and the amphipod Echinogammarus tarragonensis), over three native leaf species (alder, chestnut and oak) and an exotic species (eucalypt) were quantified in the laboratory. Preference for eucalypt leaves conditioned for 1, 2 and 3 weeks was also described. We then contrasted the preference patterns in the laboratory feeding experiments with a 15-month-long benthic standing stock time series of a stream below a native deciduous forest and another below a eucalypt plantation. Both detritivores preferred consuming alder leaves and more conditioned eucalypt leaves, although the amphipod was more selective than the caddisfly. The consumption preference in the laboratory was unmatched by the availability in the field, especially under eucalypt plantations and for the amphipod. Our results show that the strength of the preference for high-quality resources can differ among different taxa, which can modulate their response to land use changes.Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. This study was funded by the Spanish Dirección General de Enseñanza Superior e Investigación Científica (Project DGESIC PB98-0151) and finanacial support in terms of a predoctoral Grant to A. Otermin from Basque Government

Extreme temperature events alter stream ecosystem functioning

Extreme temperature events have increased in intensity, duration and frequency in the last century, with potential consequences on organisms and ecosystems. In many streams, leaf litter of terrestrial origin is a key resource for microorganisms and some detritivores, and its decomposition has a main role on ecosystem functioning and is often used as an indicator of ecological integrity. As litter is often exposed to atmospheric conditions before entering the stream, extreme warming and freezing events may alter its physicochemical structure and affect decomposition and associated detritivores. We tested this prediction in a microcosm experiment by exposing litter of three tree species (in single-species treatments and the 3-species mixture) to different temperature pre-treatments: heating (40 degrees C), freezing ( - 20 degrees C) and both (heating followed by freezing). We then examined changes in litter traits due to leaching (72 h), litter decomposition in the absence and presence of detritivores, and detritivore growth (28 d), with focus on mass and nutrient (nitrogen and phosphorus) changes. Nutrient leaching was promoted mostly by the heating pre-treatment, which apparently produced lower-quality litter. However, microbial activity mostly resulted in litter mass and nutrient gain, which were reinforced by the heating pre-treatment, while freezing had the opposite effect. When detritivores were present, decomposition showed high variation among litter types but, again, the heating and freezing pre-treatments tended to reduce and enhance nutrient loss, respectively. The greatest and more consistent effects occurred for detritivore growth, which was reduced by temperature pre-treatments, particularly in the highest-quality litter type. In general, the sequential application of heating and freezing pre-treatments showed no synergistic effect, and the litter mixture showed similar responses to single-species treatments. Our results demonstrate that short-term extreme temperatures can modify litter quality in riparian soils and have subsequent effects on its decomposition within the stream and associated fauna, potentially altering stream food webs, ecosystem functioning and biogeochemical cycles.This study was funded by the Spanish Ministry for Science, Innovation and Universities and FEDER (project BioLoss, Ref. RTI2018-095023-B400), Basque Goverment funds (Ref. IT951-16) and Initiation Fondecyt Project (Ref. 11170390). I. Diaz and U. Apodaka-Etxebarria contributed to the sample processing

No evidence of biodiversity effects on stream ecosystem functioning across green and brown food web pathways

[EN] Biodiversity loss is known to affect the two fundamental and opposite processes controlling carbon and nutrient cycles globally, that is, primary production and decomposition, which are driven by green and brown food web compartments, respectively. However, biodiversity in these two food web compartments has been mostly studied independently, and potential reciprocal effects of biodiversity loss on ecosystem processes remain unclear. We conducted a 35-day stream mesocosm experiment with two levels of algal diversity (natural and diluted periphyton communities) and three levels of litter diversity (no litter, monocultures of poplar, maple, and oak, and the three-spp. mixture) to simulate changes in biodiversity in both the green and brown pathways of an aquatic food web. We then measured multiple ecosystem processes pertaining to carbon cycling. We predicted that algal diversity would enhance decomposition and sporulation of fungal decomposers, while litter diversity would enhance algal growth and net primary production, due to the more diverse algal exudates or litter nutrients being released from more diverse mixtures. In contrast to this hypothesis, we only found biodiversity effects on an ecosystem process within the green pathway: there was a relationship between algal diversity and biofilm carrying capacity. Nevertheless, we found that this relationship was affected by the presence or absence of litter (algal diversity increased the carrying capacity in presence of litter and decreased it in its absence), which also influenced the algal community structure. Our mesocosm experiment did not evidence relationships between biodiversity and ecosystem processes across different food web compartments, but further studies in more realistic conditions would be necessary to confirm this result. If supported, the lack of biodiversity-ecosystem functioning relationships across compartments would facilitate the prediction of the impacts of biodiversity loss on ecosystems.Spanish Ministry for Science, Innovation and Universities and FEDER; U.S. National Science Foundation, Grant/Award Number: 133234

Land use drives detritivore size structure and decomposition through shifts in resource quality and quantity

Land use change and nutrient pollution are two pervasive stressors that can modify carbon cycling, as they influence the inputs and the transformation of detritus. Understanding their impact on stream food webs and on diversity is particularly pressing, as streams are largely fuelled by detrital material received from the adjacent riparian environment. Here we assess how a switch from native deciduous forest to Eucalyptus plantations and nutrient enrichment alter the size distribution of stream detritivore communities and decomposition rates of detritus. As expected, more detritus resulted in higher size-independent, or overall, abundance (i.e. higher intercept of size spectra). This change in overall abundance was mainly driven by a change of the relative contribution of large taxa (Amphipoda and Trichoptera), which changed from an average relative abundance of 55.5 to 77.2 % between the sites compared for resource quantity differences in our study. In contrast, detritus quality modified the relative abundance of large vs small individuals (i.e. size spectra slopes), with shallow slopes of size spectra (proportionately more large individuals) associated with sites with nutrient-richer waters and steeper slopes (proportionately fewer large individuals) associated with sites draining Eucalyptus plantations. Decomposition rates of alder leaves due to macroinvertebrates increased from 0.0003 to 0.0142 when relative contribution of large organisms increased (modelled slopes of size spectra: −1.00 and − 0.33, respectively), highlighting the importance of large sized individuals for ecosystem functioning. Our study reveals that land use change and nutrient pollution can greatly impair the transfer of energy through the detrital or ‘brown’ food web by means of intra- and inter-specific responses to quality and quantity of the detritus. These responses enable linking land use change and nutrient pollution to ecosystem productivity and carbon cycling.This work was carried out with financial support from the EU Commission within the RivFunction project (contract EVK1-CT-2001-00088). AL acknowledges the financial support by the mobility program Ikermugikortasuna-2019 of the Basque Government

Leaf Traits Drive Plant Diversity Effects On Litter Decomposition And FPOM Production In Streams

Biodiversity loss in riparian forests has the potential to alter rates of leaf litter decomposition in stream ecosystems. However, studies have reported the full range of positive, negative and no effects of plant diversity loss on decomposition, and there is currently no explanation for such inconsistent results. Furthermore, it is uncertain whether plant diversity loss affects other ecological processes related to decomposition, such as fine particulate organic matter production or detritivore growth, which precludes a thorough understanding of how detrital stream food webs are impacted by plant diversity loss. We used a microcosm experiment to examine the effects of plant diversity loss on litter decomposition, fine particulate organic matter production, and growth of a dominant leaf-shredding detritivore, using litter mixtures varying in species composition. We hypothesized that plant diversity loss would decrease the rates of all studied processes, but such effects would depend on the leaf traits present in litter mixtures (both their average values and their variability). Our findings partly supported our hypotheses, showing that plant diversity loss had a consistently negative effect on litter decomposition and fine particulate organic matter production (but not on detritivore growth) across litter mixtures, which was mediated by detritivores. Importantly, the magnitude of the diversity effect and the relative importance of different mechanisms underlying this effect (i.e., complementarity vs. selection) varied depending on the species composition of litter mixtures, mainly because of differences in litter nutritional quality and trait variability. Complementarity was prevalent but varied in size, with positive selection effects also occurring in some mixtures. Our results support the notion that loss of riparian plant species is detrimental to key stream ecosystem processes that drive detrital food webs, but that the magnitude of such effects largely depends on the the order of species loss.This study was funded by the ‘BIOFUNCTION’ project (CGL2014-52779-P) from the Spanish Ministry of Economy and Competitiveness (MINECO) and FEDER to LB and JPo, Basque Government funds (IT302-16) to JPo, and Ikerbasque start-up funds to LB. NLR and AM were supported by a predoctoral fellowship from the Basque Government and a postdoctoral contract from the University of the Basque Country, respectively. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Temperature Sensitivity of Microbial Litter Decomposition in Freshwaters: Role of Leaf Litter Quality and Environmental Characteristics

Ongoing global warming is expected to alter temperature-dependent processes. Nevertheless, how co-occurring local drivers will influence temperature sensitivity of plant litter decomposition in lotic ecosystems remains uncertain. Here, we examined the temperature sensitivity of microbial-mediated decomposition, microbial respiration, fungal biomass and leaf nutrients of two plant species varying in litter quality. We also assessed whether the type of microbial community and stream water characteristics influence such responses to temperature. We incubated alder (Alnus glutinosa) and eucalypt (Eucalyptus globulus) litter discs in three streams differing in autumn–winter water temperature (range 4.6–8.9 °C). Simultaneously, in laboratory microcosms, litter discs microbially conditioned in these streams were incubated at 5, 10 and 15 °C with water from the conditioning stream and with a water control from an additional stream. Both in the field and in the laboratory, higher temperatures enhanced litter decomposition rates, except for eucalypt in the field. Leaf quality modified the response of decomposition to temperature in the field, with eucalypt leaf litter showing a lower increase, whereas it did not in the laboratory. The origin of microbial community only affected the decomposition rates in the laboratory, but it did not modify the response to temperature. Water quality only defined the phosphorus content of the leaf litter or the fungal biomass, but it did not modify the response to temperature. Our results suggest that the acceleration in decomposition by global warming will be shaped by local factors, mainly by leaf litter quality, in headwater streams.Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. This research was financed by the Ministry of Science and Innovation of the Spanish Government (CGL2010-22129-C04-01). S. Monroy was supported by a predoctoral grant by the Spanish Ministry of Economy and Competitiveness (BES-2012–060743) and a postdoctoral grant from the University of the Basque Country (DOKBERRI 2018 I)