Litter Quality Is a Stronger Driver than Temperature of Early Microbial Decomposition in Oligotrophic Streams: a Microcosm Study

[EN]Litter decomposition is an ecological process of key importance for forest headwater stream functioning, with repercussions for the global carbon cycle. The process is directly and indirectly mediated by microbial decomposers, mostly aquatic hyphomycetes, and influenced by environmental and biological factors such as water temperature and litter quality. These two factors are forecasted to change globally within the next few decades, in ways that may have contrasting effects on microbial-induced litter decomposition: while warming is expected to enhance microbial performance, the reduction in litter quality due to increased atmospheric carbon dioxide and community composition alteration may have the opposite outcome. We explored this issue through a microcosm experiment focused on early microbial-mediated litter decomposition under stream oligotrophic conditions, by simultaneously manipulating water temperature (10 degrees C and 15 degrees C) and litter quality (12 broadleaf plant species classified into 4 categories based on initial concentrations of nitrogen and tannins). We assessed potential changes in microbial-mediated litter decomposition and the performance of fungal decomposers (i.e., microbial respiration, biomass accrual, and sporulation rate) and species richness. We found stronger effects of litter quality, which enhanced the performance of microbial decomposers and decomposition rates, than temperature, which barely influenced any of the studied variables. Our results suggest that poorer litter quality associated with global change will have a major repercussion on stream ecosystem functioning.This study was financed by the Portuguese Foundation for Science and Technology (FCT), through the strategic projects UIDP/04292/2020 granted to MARE and start-up funds from the Donana Biological Station (EBD-CSIC) and Ikerbasque to LB. Financial support granted by the FCT to VF (IF/00129/2014, CEECIND/02484/2018) is also acknowledged. Special thanks to A. LandeiraDabarca and C. Grela-Docal for their assistance in field and laboratory work and to L. Barmuta, J. Chara, C. Colon-Gaud, M.O. Gessner, J.F. Goncalves Jr., B. McKie, R. Pearson, C. Swan, and C. Yule, among others, collaborators from GLoBE network, for collecting the plant litter

Resource-allocation tradeoffs in caddisflies facing multiple stressors

The replacement of native forests by exotic tree monocultures, such as those of Eucalyptus, decreases the quality of leaf litter inputs to streams and often reduces riparian cover, which can elevate water temperature. The combined effects of these stressors on the survival and performance of detritivores may be important, as detritivore species loss leads to reduced litter breakdown, a key ecosystem process. Potential loss of cased caddisfly larvae is of particular concern because they are the predominant detritivores in many streams, they are sensitive to warming, and they expend energy on building and carrying their cases, which may be an added burden under times of stress. In a microcosm experiment, we tested whether (i) poor-quality Eucalyptus globulus litter impaired case construction by larvae of Sericostoma pyrenaicum (due to preferential allocation of the scarcer available energy to larval fitness) compared to high-quality Alnus glutinosa litter; (ii) whether this effect was enhanced by higher temperatures (15 vs. 10 degrees C) resulting in faster metabolism and greater energy expenditure; but (iii) reduced in the presence of chemical cues from a predatory fish (due to greater investment in more protective cases). We found that Eucalyptus had lethal and sublethal effects on larval caddisflies, increasing mortality, reducing growth, and impairing case construction, compared to larvae fed Alnus. Temperature did not reinforce the effects of exotic litter on case construction, but predator chemical cues triggered the construction of more protective cases (i.e., longer and better cemented) despite the lower resource quality, providing evidence for environmentally mediated resource-allocation tradeoffs

Amphibian loss alters periphyton structure and invertebrate growth in montane streams

Amphibians are declining worldwide due to a combination of stressors such as climate change, invasive species, habitat loss, pollution and emergent diseases. Although their losses are likely to have important ecological consequences on the structure and functioning of freshwater ecosystems, this issue has been scarcely explored. We conducted an experiment in three montane streams-where primary production is the main source of energy and carbon-to assess the effects of amphibian disappearance (i.e. presence or absence of the common midwife toad Alytes obstetricans, a common species found in pools of these streams) on several aspects of ecosystem functioning and structure: periphyton biomass and chlorophyll a concentration, algal assemblage structure, and growth of macroinvertebrate grazers. We compared four types of experimental enclosures: (i) without macroinvertebrates or amphibians; (ii) with larvae of the caddisfly Allogamus laureatus; (iii) with A. obstetricans tadpoles; and (iv) with both A. laureatus larvae and A. obstetricans tadpoles. The absence of tadpoles increased periphyton biomass, but did not cause differences on inorganic sediment accrual. The algal assemblage had a higher diversity in the absence of tadpoles, and their characteristic taxa differed from the assemblages in presence of tadpoles. A. laureatus presented higher mass in presence of tadpoles; however, tadpole length was not affected by presence of macroinvertebrates. Our results suggest that presence of tadpoles is a driver of periphyton accrual and assemblage structure, acting as top-down control and with key potential consequences on the functioning of montane stream ecosystems.This study was funded by the Spanish Ministry for Science, Innovation and Universities and FEDER (BioLoss project, Ref. RTI2018-095023-B-I00 to L.B.) and the Basque Government (Ref. IT951-16 to the Stream Ecology Group at the UPV/EHU). A.A. was supported by UPV/EHU predoctoral fellowships

Diets of leaf litter-associated invertebrates in three tropical streams

Shredders play a major ecological role in temperate streams, but their numerical importance is highly variable within the tropics. Detailed studies on the diets of tropical stream invertebrates are advisable to be able to better describe and understand this variation. Here, we examined the diets of invertebrates collected from the leaf litter of three tropical streams in Colombia, using gut content analysis. Fine and coarse particulate organic matter were the main food resources for invertebrates, which could be divided into four main dietary groups: predators, shredders, specialist collectors and generalist collectors. While the specialist collectors were the most numerically abundant group (54%), shredder biomass accounted for 63% of total invertebrate biomass, suggesting that shredders play a significant ecological role in the study streams. We describe the diets of 12 out of 47 taxa that were previously unknown, which indicates that knowledge about the feeding ecology of tropical stream invertebrates is still incipient. © 2012 EDP Sciences.Peer Reviewe

Effects of gamma irradiation on instream leaf litter decomposition

Leaf litter decomposition is a key process in stream ecosystems, the rates of which can vary with changes in litter quality or its colonization by microorganisms. Decomposition in streams is increasingly used to compare ecosystem functioning globally, often requiring the distribution of litter across countries. It is important to understand whether litter sterilization, which is required by some countries, can alter the rates of decomposition and associated processes. We examined whether litter sterilization with gamma irradiation (25 kGy) influenced decomposition rates, litter stoichiometry, and colonization by invertebrates after weeks of instream incubation within coarse-mesh and fine-mesh litterbags. We used nine plant species from three families that varied widely in litter chemistry but found mostly consistent responses, with no differences in decomposition rates or numbers of invertebrates found at the end of the incubation period. However, litter stoichiometry differed between irradiated and control litter, with greater nutrient losses (mostly phosphorus) in the former. Therefore, the effects of irradiation on litter chemistry should be taken into account in studies focused on stoichiometry but not necessarily in those focused on decomposition rates, at least within the experimental timescale considered here

Key plant species and detritivores drive diversity effects on instream leaf litter decomposition more than functional diversity: A microcosm study

Anthropogenic impacts on freshwater ecosystems cause critical losses of biodiversity that can in turn impair key processes such as decomposition and nutrient cycling. Forest streams are mainly subsidized by terrestrial organic detritus, so their functioning and conservation status can be altered by changes in forest biodiversity and composition, particularly if these changes involve the replacement of functional groups or the loss of key species. We examined this issue using a microcosm experiment where we manipulated plant functional diversity (FD) (monocultures and low-FD and high-FD mixtures, resulting from different combinations of deciduous and evergreen Quercus species) and the presence of a key species (Alnus glutinosa), all in presence and absence of detritivores, and assessed effects on litter decomposition, nutrient cycling, and fungal and detritivore biomass. We found (i) positive diversity effects on detritivore-mediated decomposition, litter nutrient losses and detritivore biomass exclusively when A. glutinosa was present; and (ii) negative effects on the same processes when microbially mediated and on fungal biomass. Most positive trends could be explained by the higher litter palatability and litter trait variability obtained with the inclusion of alder leaves in the mixture. Our results support the hypothesis of a consistent slowing down of the decomposition process as a result of plant biodiversity loss, and hence effects on stream ecosystem functioning, especially when a key (N-fixing) species is lost; and underscore the importance of detritivores as drivers of plant diversity effects in the studied ecosystem processes.This study was funded by the 2014–2020 FEDER Operative Program Andalusia (RIOVEGEST project, Ref. FEDER-UAL18 -RNM -B006 – B, to J.J.C). Additional support was provided by the Spanish Ministry for Science, Innovation and Universities and FEDER (BioLoss project, Ref. RTI2018-095023- B-I00, to L.B.). Rubio-Ríos was supported by an FPU grant of the Spanish Ministry of Education, Culture and Sports (reference FPU16/03734)

A common fungicide impairs stream ecosystem functioning through effects on aquatic hyphomycetes and detritivorous caddisflies

Fungicides can reach streams through runoff or adhered to leaf litter, and have the potential to adversely affect processes such as litter decomposition and associated communities. This study investigated the effects of chlorothalonil, a widely used fungicide, on litter decomposition, detritivorous invertebrates (larvae of the insect Sericostoma pyrenaicum) and aquatic hyphomycetes (AHs), using stream microcosms. We considered the single and combined effects of two exposure modes: waterborne fungicide (at two concentrations: 0.125 mu g L-1 and 1.25 mu g L-1) and litter previously sprayed with the fungicide (i.e., pre-treated litter, using the application dose concentration of 1250 mu g L-1). We also assessed whether fungicide effects on invertebrates, AHs and decomposition varied among litter types (i.e., different plant species), and whether plant diversity mitigated any of those effects. Invertebrate survival and AH sporulation rate and taxon richness were strongly reduced by most combinations of fungicide exposure modes; however, invertebrates were not affected by the low waterborne concentration, whereas AHs suffered the highest reduction at this concentration. Total decomposition was slowed down by both exposure modes, and microbial decomposition was reduced by litter pre-treatment, while the waterborne fungicide had different effects depending on plant species. In general, with the exception of microbial decomposition, responses varied little among litter types. Moreover, and contrary to our expectation, plant diversity did not modulate the fungicide effects. Our results highlight the severity of fungicide inputs to streams through effects on invertebrate and microbial communities and ecosystem functioning, even in streams with well-preserved, diverse riparian vegetation.We thank Richard Pearson and two anonymous reviewers for their comments on the manuscript. This study was derived from AC's PhD thesis and AA's MSc thesis. AC was supported by a fellowship of the National Secretariat of Science, Technology and Innovation (SENACYT) and the National Research System of Panama (SNI). SM was supported by a postdoctoral grant from the University of the Basque Country. Additional funding was obtained from the Spanish Ministry for Science, Innovation and Universities and FEDER (project BioLoss, RTI2018095023-B-I00) and Basque Government funds (IT951-16)

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

Litter Decomposition can be Reduced by Pesticide Effects on Detritivores and Decomposers: Implications for Tropical Stream Functioning

Understanding which factors affect the process of leaf litter decomposition is crucial if we are to predict changes in the functioning of stream ecosystems as a result of human activities. One major activity with known consequences on streams is agriculture, which is of particular concern in tropical regions, where forests are being rapidly replaced by crops. While pesticides are potential drivers of reduced decomposition rates observed in agricultural tropical streams, their specific effects on the performance of decomposers and detritivores are mostly unknown. We used a microcosm experiment to examine the individual and joint effects of an insecticide (chlorpyrifos) and a fungicide (chlorothalonil) on survival and growth of detritivores (Anchytarsus, Hyalella and Lepidostoma), aquatic hyphomycetes (AH) sporulation rate, taxon richness, assemblage structure, and leaf litter decomposition rates. Our results revealed detrimental effects on detritivore survival (which were mostly due to the insecticide and strongest for Hyalella), changes in AH assemblage structure, and reduced sporulation rate, taxon richness and microbial decomposition (mostly in response to the fungicide). Total decomposition was reduced especially when the pesticides were combined, suggesting that they operated differently and their effects were additive. Importantly, effects on decomposition were greater for single-species detritivore treatments than for the 3-species mixture, indicating that detritivore species loss may exacerbate the consequences of pesticides of stream ecosystem functioning.This work was supported by the National Secretariat for Science, Technology and Innovation (SENACYT; project APY-GC-2018B-052; contract no. 259e2018) and the Ministry of Economy and Finance of Panama (MEF; project 019910.001). AC was supported by a fellowship from SENACYT (contract no. 001e2015) and by the National Research System of Panama (SNI; PhD category; contract no. 186e2018). GC was supported by a fellowship from IFARHU- SENACYT (contract no. 270-2018-1011