Drying shapes the ecological niche of aquatic fungi with implications on ecosystem functioning

Fungi are among the most abundant and diverse organisms on Earth and play pivotal roles in global carbon processing,nutrient cycling and foodwebs. Despite their abundant and functional importance, little is known about the patterns andmechanisms governing their community composition in intermittent rivers and ephemeral streams, which are the mostcommon fluvial ecosystems globally. Thus far, it is known that aquatic fungi have evolved various life-history strategiesand functional adaptations to cope with drying.Nevertheless, some of these adaptations have ametabolic cost and tradeoffsbetween growth, reproduction and dispersion thatmay affect ecosystem functioning. Thus, understanding their ecologicalstrategies along a gradient of drying is crucial to assess how species will respond to global change and to identifymeaningful taxa tomaintain ecosystemfunctions. By combining in situ hydrological information with a niche-based approach,we analysed the role of drying in explaining the spatial segregation of fungal species, and we determined theirspecialization and affinity over a gradient of drying. In addition, we estimated whether species niches are good predictorsof two key ecosystem processes: organic matter decomposition and fungal biomass accrual. Overall, we found thatannual drying duration and frequency were the most influential variables upon species niche differentiation across the15 studied streams. Our cluster analysis identified four drying niche-based groups with contrasting distributions and responsesover the drying gradient: drying-sensitive, partly tolerant to drying, generalist, and drying-resistant specialist. Inaddition, we found that species belonging to the drying specialist group showed a weak contribution to both ecosystemprocesses, suggesting trade-offs between drying resistance strategies and the energy invested in growth. Taken together,our results suggest that increased water scarcity may jeopardise the capacity of aquatic fungi to guarantee ecosystemfunctioning and to maintain biogeochemical cycles despite their ability to cope with drying

Drying niches of aquatic macroinvertebrates identify potential biomonitoring indicators in intermittent and ephemeral streams

Intermittent rivers and ephemeral streams (IRES) compose the majority of draining networks on Earth, supporting a unique fraction of biodiversity. Despite their high ecological value, IRES are increasingly threatened by global change and require appropriate biomonitoring and restoration tools. However, indices and indicators used in routine biomonitoring programs are often confounded by drying effects. This occurs because most pollutionsensitive taxa are lost over drying gradients, limiting the utility of current biomonitoring tools in IRES. To address this challenge, there is a need to evaluate which taxa should be used to calculate biomonitoring metrics and indicators over the different portions of the drying gradient. Here, using high-resolution drying data from 33 unpolluted streams, we explored the drying preferences of macroinvertebrates to identify their potential as biomonitoring indicators in IRES. To do this, we characterized macroinvertebrate drying niches and identified those with drying resistance and sensitivity preferences. Next, we evaluated the capacity of functional traits to predict drying specialization and affinity. Finally, to identify potential biomonitoring metrics and indicators for IRES, we evaluated how drying influenced the density, relative abundance and richness of taxa in drying sensitive and drying-resistant niches. Our results identified three macroinvertebrate groups with drying resistant niches (partly tolerant, generalist and specialist taxa) and one group of drying-sensitive taxa. We also found that functional traits had a limited capacity to represent differences in drying niches, with shredding trophic preferences and body size showing the strongest correlations. In addition, we observed that the density, relative abundance and richness of drying-resistant taxa were less influenced by drying intensity than those of drying-sensitive taxa. Finally, we found that some pollution-sensitive taxa with partial (e.g., Lepidostoma), moderate (e.g., Corduliidae) or high drying tolerance (e.g., Nemoura) can serve as potential indicators in IRES. Taken together, our results demonstrate that characterizing drying niches can be a useful strategy for developing biomonitoring tools in IRES and for highlighting the limitations of taxonomic and trait-based approaches

Multiple drying aspects shape dissolved organic matter composition in intermittent streams

Water availability is a fundamental driver of biogeochemical processing in highly dynamic ecosystems such as intermittent rivers and ephemeral streams (IRES), which are recognized as the most common fluvial ecosystem globally. Because of their global extent, IRES have a remarkable contribution to organic matter processing, which is expected to intensify as climate change and water extraction expand IRES extension. Nevertheless, the effect of the complexity of the drying process on river biogeochemistry remains unclear. This study investigated how drying aspects affect the dissolved organic carbon (DOC) concentration and composition in 35 streams along a wide flow-intermittence gradient in the NE Iberian Peninsula. To do that, four drying aspects: annual drying duration, annual frequency, duration of the last drying event, and time since the last drying event were characterized. Results showed that DOC concentration and the contribution of humic-like compounds were positively associated with intensifying drying conditions. In addition, protein-like compounds decreased over the drying gradient. More specifically, changes in DOC concentration were driven mainly by annual drying duration, whereas annual drying frequency and the duration of the last drying event jointly explained dissolved organic matter composition. These results suggest that the quantity and composition of dissolved organic matter in streams respond differently to the temporal aspects of the drying process. Our study can help to better anticipate changes in organic matter in the context of climate change

Diversity mediates the responses of invertebrate density to duration and frequency of rivers annual drying regime

Predicting the impacts of global change on highly dynamic ecosystems requires a better understanding of how communities respond to disturbance duration, frequency and timing. Intermittent rivers and ephemeral streams are dynamic ecosystems that are recognized as the most common fluvial ecosystem globally. The complexity of the drying process can give rise to different annual and antecedent hydrological conditions, but their effect on aquatic communities remains unclear. Here, using aquatic invertebrates from 33 streams across a flow-intermittence gradient, we assessed how annual (drying duration and frequency) and recent drying characteristics (duration of the last dry period and flowing duration since the last rewetting) affect the density and diversity metrics of communities and trophic groups while controlling for other key abiotic factors (dissolved oxygen and altitude). We characterized invertebrate communities using taxonomy and functional traits to capture biological features that increase vulnerability to drying. In addition, using structural equation modelling (SEM), we evaluated pathways by which drying characteristics directly impact invertebrate density and whether diversity indirectly mediates such relationships. We show that drying frequency drove reductions in diversity at the community level and within trophic groups, whereas both the drying duration and frequency had a negative influence on density metrics. Reductions in taxonomic richness were linked to increased annual drying duration, whereas functional diversity declined in response to annual drying frequency. Filterer, predator and shredder trophic groups exhibited the strongest negative responses to drying. Recent drying characteristics had a minor effect on density and diversity metrics. Our SEM results demonstrated that diversity mediates the negative impacts of annual drying duration and frequency on invertebrate density through reductions in their taxonomic richness and functional diversity. Our results underscore the importance of considering multiple drying characteristics together with the interdependence of density and diversity to better anticipate drying responses in freshwater ecosystems. Keywords: biodiversity-ecosystem functioning, drying, flow intermittence, functional traits, intermittent rivers and ephemeral streams, Mediterranean stream

Quality and quantity of leaf-litter: both are important for feeding preferences and growth of an aquatic shredder

The study of leaf litter as a resource for shredders has emerged as a key topic in trophic links in ecology. However, thus far, most studies have emphasized the leaf quality as one of the main determinants of shredder behaviour and growth without simultaneously considering the leaf quantity availability. Nevertheless, the combined effects of leaf quantity and quality on shredder behaviour and growth is particularly crucial to further understand how ecosystem functioning may respond to the increasing flow intermittency due to climate change. In this study, we explore how changes in the leaf litter quality and quantity influence the feeding preferences and growth of an invertebrate shredder (Potamophylax latipennis). To do so, we used black poplar leaves conditioned in two streams with different flow regimens as a food resource. Afterwards, using a microcosm approach, we offered leaf discs that varied in terms of leaf quantity and quality to P. latipennis. Our results showed that flow intermittency had a negative effect on the quality of the food resource, and a lower quality had a negative effect on the consumption and growth rates of P. latipennis. Furthermore, we found that P. latipennis fed selectively on higher quality leaves even though the availability (quantity) of this resource was lower. In the context of climate change, with higher aridity/drier conditions/scenarios, our findings suggest that a decrease in the availability (quantity) of high-quality resources could potentially threaten links in global fluvial food webs and thus threaten ecosystem functioning

Subsurface zones in intermittent streams are hotspots of microbial decomposition during the non-flow period

The microbial decomposition of organic matter is a fundamental ecosystem process that transforms organic matter and fuels detritus-based food webs, influencing biogeochemical cycles such as C-cycling. The efficiency of this process can be compromised during the non-flow periods of intermittent and ephemeral streams (IRES). When water flow ceases, sediments represent the last wet habitat available to microorganisms and may play an important role in sustaining microbial decomposition. However, despite the increasing prevalence of IRES due to climate change and water abstraction, it is unclear to what degree the subsurface habitat can sustain microbial decomposition during non-flow periods. In order to gather information, we selected 20 streams across Catalonia (Spain) along a gradient of flow intermittency, where we measured microbial decomposition and fungal biomass by placing wood sticks in both the surface and subsurface zones (15 cm below the streambed) over the course of one hydrological year. Our results showed that microbial decomposition and fungal biomass were consistently greater in the subsurface zone than in the surface zone, when intermittency increased. Although flow intermittency was the main driver of both microbial decomposition and fungal biomass, phosphorus availability in the water, sediment C:N ratio and sediment grain size also played relevant roles in surface and subsurface organic matter processing. Thus, our findings demonstrate that although the OM processing in both zones decreases with increased intermittency, the subsurface zone made an important contribution during the non-flow periods in IRES. Therefore, subsurface activity during non-flow periods has the potential to affect and maintain ecosystem functioning

Effects of frequency and duration of flow intermittence on biodiversity and ecosystem functioning: insights form Mediterranean streams

[eng] Intermittent rivers and ephemeral streams (IRES) are watercourses that naturally and periodically cease to flow. They represent more than half of the global river network and are expanding due to global change. In this thesis, I investigate the mechanisms linking flow intermittence with biodiversity and ecosystem functioning, which sustain biogeochemical cycles and energy transfer in the system. Chapter 1 analyzes the effects of hydrology, micro-habitat (surface and subsurface zones) and biotic features on organic matter decomposition and fungal biomass, in 20 streams. In Chapter 2, I assess the effect of different flow intermittence metrics (i.e., annual intermittence regime and recent aquatic status) on aquatic biodiversity, including both taxonomic and functional-trait-based metrics, in 33 streams. Chapter 3 analyzes how aquatic hyphomycete richness and composition (beta diversity and its turnover and nestedness components) are affected by a flow intermittence gradient and how these community changes affect organic matter decomposition, in 15 streams and in a microcosm approach. Finally, in Chapter 4, I explore how changes in both leaf litter quality and quantity determine the feeding preferences and growth of an invertebrate shredder. The results of Chapter 1 show that the subsurface zone contributes to maintaining microbial decomposition during non-flow periods in IRES, mainly because of the levels of fungal biomass present in the subsurface zone. In Chapter 2, I conclude that a combination of flow intermittence metrics are needed to explain the high dynamism of the invertebrate community in IRES and potentially ecosystem functioning. Moreover, this chapter shows that hydrological variables outweigh non-hydrological factors in explaining invertebrate community variation, thereby supporting the use of the former in IRES classification and bio-monitoring routines. Chapter 3 reveals that the reduction of aquatic hyphomycete richness and species turnover as a result of flow intermittence, could have negative effects on organic matter decomposition. Finally, in Chapter 4, I provide evidence on how flow intermittence reduces the quality of leaf litter, in terms of fungal richness and composition, fungal biomass and lipid content. These changes in food quality influence the consumption rates and growth of shredders, which are able to feed selectively on higher quality leaves, even though its availability is lower. Taken together, these results will help to improve the biomonitoring and management of IRES and to a better prediction of ecosystem trajectories in response to global change

Drying Shapes Aquatic Fungal Community Assembly by Reducing Functional Diversity

Aquatic fungi are highly diverse organisms that play a critical role in global biogeochemical cycles. Yet it remains unclear which assembly processes determine their co-occurrence and assembly patterns over gradients of drying intensity, which is a common stressor in fluvial networks. Although aquatic fungi possess drying-specific adaptations, little is known about how functional similarity influences co-occurrence probability and which functional traits are sorted by drying. Using field data from 15 streams, we investigated how co-occurrence patterns and assembly processes responded to drying intensity. To do so, we determined fungal co-occurrence patterns, functional traits that best explain species co-occurrence likelihood, and community assembly mechanisms explaining changes in functional diversity over the drying gradient. Our results identified 24 species pairs with positive co-occurrence probabilities and 16 species pairs with negative associations. The co-occurrence probability was correlated with species differences in conidia shape and fungal endophytic capacity. Functional diversity reduction over the drying gradient is generally associated with non-random abiotic filtering. However, the assembly processes changed over the drying gradient, with random assembly prevailing at low drying intensity and abiotic filtering gaining more importance as drying intensifies. Collectively, our results can help anticipate the impacts of global change on fungal communities and ecosystem functioning.Thisworkwas funded byMCIN/AEI/10.13039/501100011033 through the project RIVSTRESS (PID2020-115708RB-C21). R.A.-R. held a post-doctoral grant “Margarita Salas” from the Spanish Ministry of Universities and the Next Generation EU-Recovery, Transformation and Resilience Plan. P.H. held a post-doctoral grant “Margarita Salas” from the Spanish Ministry of Universities and the Next Generation EU-Recovery, Transformation and Resilience Plan and a ‘Juan de la Cierva– Formación’ contract (Spanish Ministry of Science and Innovation, FJC2020-045923-I). C.G.-C. was supported by a Junior Leader Fellowship contract (LCF/BQ/PR22/11920005) funded by “la Caixa” Foundation (ID 100010434).Peer reviewe

Quality and quantity of leaf litter: Both are important for feeding preferences and growth of an aquatic shredder.

The study of leaf litter as a resource for shredders has emerged as a key topic in trophic links in ecology. However, thus far, most studies have emphasized the leaf quality as one of the main determinants of shredder behaviour and growth without simultaneously considering the leaf quantity availability. Nevertheless, the combined effects of leaf quantity and quality on shredder behaviour and growth is particularly crucial to further understand how ecosystem functioning may respond to the increasing flow intermittency due to climate change. In this study, we explore how changes in the leaf litter quality and quantity influence the feeding preferences and growth of an invertebrate shredder (Potamophylax latipennis). To do so, we used black poplar leaves conditioned in two streams with different flow regimens as a food resource. Afterwards, using a microcosm approach, we offered leaf discs that varied in terms of leaf quantity and quality to P. latipennis. Our results showed that flow intermittency had a negative effect on the quality of the food resource, and a lower quality had a negative effect on the consumption and growth rates of P. latipennis. Furthermore, we found that P. latipennis fed selectively on higher quality leaves even though the availability (quantity) of this resource was lower. In the context of climate change, with higher aridity/drier conditions/scenarios, our findings suggest that a decrease in the availability (quantity) of high-quality resources could potentially threaten links in global fluvial food webs and thus threaten ecosystem functioning

Multi‐model assessment of hydrological and environmental impacts on streambed microbes in Mediterranean catchments

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