A functional approach to the ecology of Atlantic Basque streams

Most research on stream ecology is focused on structural characteristics of stream ecosystems, while less effort is being dedicated to the study of their functional attributes. The laboratory of stream ecology, at the University of the Basque Country, has been researching streams from an ecosystem perspective, including both structural and functional properties. Here we review the research done so far. Basque streams running to the Atlantic Ocean are short, steep, and flashy, and tend to show large spatial variations as a result of changes in geology and soil use. Where they exist, riparian forests limit the growth of primary producers and are a source of coarse particulate organic matter, an important food resource for consumers. The trophic structure of benthic macroinvertebrate communities changes downstream with resource abundance, although temporal variations of both resources and consumers are not coupled. Gut content analyses show the diet of some species to change with instar development, and thus, warn against the indiscriminate assignation of trophic categories. Floods are important disturbances, affecting primary producers, consumers, and ecosystem processes. Other important disturbances are changes in riparian vegetation, which can profoundly affect the food resources of stream communities. This basic knowledge has been used to develop new tools to assess stream functional impairment, based in two pivotal functions: litter breakdown and whole stream metabolism. Both eutrophication and changes in riparian vegetation affect the use of leaf litter, and thus impact stream functioning making litter breakdown a promising tool for stream monitoring. On the other hand, whole stream metabolism is affected by many human impacts, and can be measured continuously in modern gauging stations, thus allowing for almost real-time monitoring of ecosystem functioning. We hope these and other functional properties will be built into routine monitoring schemes, which will therefore look at both the structural and functional sides of stream ecosystems.La mayor parte de los trabajos sobre ecología de ríos se ha centrado en características estructurales, y se ha dedicado menos esfuerzo a estudiar los atributos funcionales de los ecosistemas fluviales. El laboratorio de ecología de ríos, en la Universidad del País Vasco, ha estado estudiando arroyos desde una perspectiva ecosistémica, incluyendo propiedades estructurales y funcionales, y mostramos aquí una revisión de los trabajos llevados a cabo. Los ríos vascos que drenan al Océano Atlántico son cortos, de fuerte pendiente, y torrenciales, y tienden a mostrar fuertes variaciones espaciales a consecuencia de cambios en geología y usos del suelo. Cuando existen, los bosques riparios limitan la capacidad de crecimiento de los productores primarios, y son fuente de materia orgánica particulada gruesa, un importante recurso alimentario para los consumidores. La estructura trófica de las comunidades de macroinvertebrados bentónicos cambia aguas abajo con la abundancia de recursos, aunque las variaciones temporales de recursos y consumidores no están sincronizadas. El análisis de contenidos digestivos muestra que la dieta de algunas especies cambia con el grado de desarrollo larvario, y en consecuencia, previene contra la asignación indiscriminada de categorías tróficas. Las riadas son perturbaciones importantes que afectan tanto a productores primarios como a consumidores, o a procesos a nivel de ecosistema. Otras perturbaciones importantes son los cambios en la vegetación riparia, que pueden afectar profundamente los recursos alimentarios de las comunidades fluviales. Este conocimiento básico se ha utilizado para desarrollar nuevas herramientas de evaluación del estado de los ríos, basadas en dos funciones claves: la descomposición de hojarasca y el metabolismo del río. Tanto la eutrofización como los cambios en la vegetación de ribera afectan el uso de la hojarasca y, por tanto, impactan en el funcionamiento del ecosistema, lo que hace de la descomposición de hojarasca una herramienta prometedora para el seguimiento de los ríos. Por otra parte, el metabolismo del ecosistema fluvial es afectado por muchos impactos humanos, y puede ser medido en continuo en estaciones de aforo modernas, lo que permite seguir el funcionamiento del ecosistema casi a tiempo real. Esperamos que estas y otras propiedades funcionales se integrarán en los protocolos de seguimiento rutinario, que así tendrán en cuenta tanto el lado estructural como el funcional de los ecosistemas fluviales

The effect of sieve mesh size on the description of macroinvertebrate communities

Considerable time and effort is required to estimate the abundance and biomass of benthic macroinvertebrates, and often variable mesh size sieves are used to clean collected samples. We test whether the use of a mesh with a 1 mm pore size is adequate to obtain a valid description of a benthic macroinvertebrate community. Stream benthic surber samples were collected from 24 headwater streams. The densities, biotic indices and biological traits ofmacroinvertebrates retained in al mm mesh ('> 1 mm' fraction) were compared to the same descriptors for the of macroinvertebrates retained in a 0.5 mm mesh sieve ('total'). We found that, if only the large fraction (> 1 rnm) is examined, the cornmunity descriptors are affected. Nevertheless, the observed changes were proportional and predictable for all of the variables describing invertebrate communities. Statistical differenti­ation of the tested metrics between sites was similar for both mesh sizes. Depending on the aim of the study (e.g., environmental impact assessments), the use of a 1 mm mesh sieve would be sufficient in describing macroinvertebrate communities.Para las estimas de abundancia y biomasa, el procesado de las muestras en laboratorio de invertebrados bentónicos requiere un tiempo y esfuerzo considerable y generalmente implica el uso de tamices de diferente luz de malla para lavar la muestra. Nuestro trabajo trata de comprobar si es suficiente para una descripción válida de la comunidad el uso de un tamiz de malla de 1 mm de poro en el procesado de muestras. Con tal propósito se recogieron muestras bentónicas de río en 24 tramos de cabecera y se compararon densidades, índices bióticos y rasgos biológicos de los invertebrados que eran retenidos en un tamiz de luz de malla de 1 mm (fracción '>1 mm') con los obtenidos usando una de 0.5 mm ('total'). Nuestro estudio revela que el análisis exclusivo de la malla gruesa afecta a los descriptores de la comunidad. Sin embargo, los cambios observados son proporcionales para todas las variables y se pueden predecir bien con ecuaciones lineales. Por otro lado, la diferenciación estadística entre estaciones es similar usando ambos tipos de malla, lo que en definitiva sugiere que, dependiendo del objetivo del estudio (por ejemplo, evaluación de impacto ambiental), el examen de la fauna retenida en un tamiz de 1 mm de poro puede ser suficiente para la descripción de las comunidades de macroinvertebrados

Afforestation with Eucalyptus globulus and leaf litter decomposition in streams of northern Spain

To test the hypothesis that decomposition of leaf species in streams is influenced by afforestation with Eucalyptus globulus, we compared decay rates, nutrient levels, fungal biomass and macroinvertebrate assemblages on alder and eucalyptus leaf litter in three streams (two headwaters under different forests, and a mid reach) of the Agüera catchment (northern Spain). Whatever the reach, alder always decomposed significantly faster than eucalyptus. Litter contents in nitrogen and phosphorus rose during breakdown at the mid reach, but not at the headwaters. No differences in fungal biomass were found between alder and eucalyptus leaves at the headwater reaches; however, at the mid reach, eucalyptus showed the highest values. Alder litter, a high quality substratum, was readily colonized by shredders, and decayed rapidly at all sites. Eucalyptus, a low quality species, had lower nutrient contents and was less favoured by shredders. Under high nutrient levels (particularly phosphorus), however, it was readily colonized by fungi, thus shifting from medium to high breakdown rates. The potentially negative impact of afforestation with eucalyptus on streams can thus be reduced in situations of high concentrations of dissolved nutrients

Drought and detritivores determine leaf litter decomposition in calcareous streams of the Ebro catchment (Spain)

Drought, an important environmental factor affecting the functioning of stream ecosystems, is likely to become more prevalent in the Mediterranean region as a consequence of climate change and enhanced water demand. Drought can have profound impacts on leaf litter decomposition, a key ecosystem process in headwater streams, but there is still limited information on its effects at the regional scale. We measured leaf litter decomposition across a gradient of aridity in the Ebro River basin. We deployed coarse- and fine-mesh bags with alder and oak leaves in 11 Mediterranean calcareous streams spanning a range of over 400 km, and determined changes in discharge, water quality, leaf-associated macroinvertebrates, leaf quality and decomposition rates. The study streams were subject to different degrees of drought, specific discharge (L s− 1 km− 2) ranging from 0.62 to 9.99. One of the streams dried out during the experiment, another one reached residual flow, whereas the rest registered uninterrupted flow but with different degrees of flow variability. Decomposition rates differed among sites, being lowest in the 2 most water-stressed sites, but showed no general correlation with specific discharge. Microbial decomposition rates were not correlated with final nutrient content of litter nor to fungal biomass. Total decomposition rate of alder was positively correlated to the density and biomass of shredders; that of oak was not. Shredder density in alder bags showed a positive relationship with specific discharge during the decomposition experiment. Overall, the results point to a complex pattern of litter decomposition at the regional scale, as drought affects decomposition directly by emersion of bags and indirectly by affecting the functional composition and density of detritivores

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

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

Interactions between large and small detritivores influence how biodiversity impacts litter decomposition

Understanding how biodiversity loss influences plant litter decomposition—that is, the biologically mediated conversion of coarse to fine particulate organic matter—is crucial to predict changes in the functioning of many stream ecosystems, where detrital food webs are dominant. Rates of litter decomposition are influenced by detritivore diversity, but the mechanisms behind this relationship are uncertain.As differences in detritivore body size are a major determinant of interspecific interactions, they should be key for predicting effects of detritivore diversity on decomposition. To explore this question, we manipulated detritivore diversity and body size simultaneously in a microcosm experiment using two small (Leuctra geniculata and Lepidostoma hirtum) and two large detritivore species (Sericostoma pyrenaicum and Echinogammarus berilloni) in all possible 1‐, 2‐ and 4‐species combinations, and litter discs of Alnus glutinosa.We expected that larger species would facilitate smaller species through the production of smaller litter fragments, resulting in faster decomposition and greater growth of smaller species in polycultures containing species of different body size. To examine this hypothesis, we used a set of “diversity–interaction” models that explored how decomposition was affected by different interspecific interactions and the role of body size, and quantified the magnitude of such effect through ratios of decomposition rates and detritivore growth between polycultures and monocultures.We found a clear positive effect of detritivore diversity on decomposition, which was mainly explained by facilitation and niche partitioning. Facilitation of small animals by larger ones was evidenced by a 12% increase in decomposition rates in polycultures compared to monocultures and the higher growth (20%) of small species, which partly fed on fine particulate organic matter produced by larger animals. When the large species were together in polycultures, decomposition was enhanced by 19%, but there were no changes in growth; niche partitioning was a plausible mechanism behind the increase in decomposition rates, as both species fed on different parts of litter discs, only one species being able to eat less palatable parts.Our study demonstrates that interspecific differences in body size should be taken into account in diversity–decomposition studies. Future studies should also consider differences in species’ vulnerability to extinction depending on body size and how this might affect ecosystem functioning in different scenarios of detritivore diversity and more complex food webs.Detritivore body size is a major determinant of interspecific interactions and should be key for predicting effects of detritivore diversity on decomposition. Here, the authors show that detritivore diversity enhances litter decomposition mainly due to facilitation of smaller detritivores by larger ones and niche partitioning between large species.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145535/1/jane12876.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145535/2/jane12876_am.pd