Isabel Rodrigues Fernandes

Intraspecific traits change biodiversity effects on ecosystem functioning under metal stress

Studies investigating the impacts of biodiversity loss on ecosystem processes have often reached different conclusions, probably because insufficient attention has been paid to some aspects including (1) which biodiversity measure (e.g., species number, species identity or trait) better explains ecosystem functioning, (2) the mechanisms underpinning biodiversity effects, and (3) how can environmental context modulates biodiversity effects. Here, we investigated how species number (one to three species) and traits of aquatic fungal decomposers (by replacement of a functional type from an unpolluted site by another from a metal-polluted site) affect fungal production (biomass acumulation) and plant litter decomposition in the presence and absence of metal stress. To examine the putative mechanisms that explain biodiversity effects, we determined the contribution of each fungal species to the total biomass produced in multicultures by real-time PCR. In the absence of metal, positive diversity effects were observed for fungal production and leaf decomposition as a result of species complementarity. Metal stress decreased diversity effects on leaf decomposition in assemblages containing the functional type from the unpolluted site, probably due to competitive interactions between fungi. However, dominance effect maintained positive diversity effects under metal stress in assemblages containing the functional type from the metal-polluted site. These findings emphasize the importance of intraspecific diversity in modulating diversity effects under metal stress, providing evidence that trait-based diversity measures should be incorporated when examining biodiversity effects.

Mixtures of zinc and phosphate affect leaf litter decomposition by aquatic fungi in streams

To better understand the impacts of multiple stressors in freshwaters, we investigated the effects of mixtures of zinc and inorganic phosphorus on microbial decomposition of leaf litter. Alder leaves were colonized in a stream and placed in microcosms with stream water supplemented or not with 3 concentrations of zinc (Zn up to 9.8 mg/l) or phosphate (P-PO(4)(3-) up to 0.5 mg/l), alone and in all possible combinations. We measured leaf mass loss, and fungal biomass, reproduction and diversity. In control microcosms, 23 species of aquatic hyphomycetes were identified on leaves, and the exposure to the highest zinc concentration reduced diversity to 14 species. Articulosporatetracladia was the dominant species followed by Flagellospora sp. and Alatosporaacuminata. The exposure to phosphate increased the contribution of A.acuminata, but this species was negatively affected by zinc. Under high zinc stress, Varicosporiumelodeae increased its contribution to the total conidial production. The exposure to high zinc concentration, alone or in mixtures with phosphate, led to shifts in fungal community structure, as indicated by cluster analysis based on sporulation data and denaturing gradient gel electrophoresis (DGGE) fingerprints of fungal DNA. These changes were accompanied by a reduction in leaf decomposition, particularly in mixtures with high Zn concentration, in which leaf mass loss was 30% lower than in the control. This suggests that the co-occurrence of zinc and phosphate may have negative effects on stream ecosystem functioning. However, we did not detect decreased leaf-associated fungal biomass and sporulation, probably because a delay in fungal colonization occurred due to the presence of stressors.

Eutrophication modulates plant-litter diversity effects on litter decomposition in streams

Freshwater ecosystems are severely impacted by changes in riparian vegetation and eutrophication, but their interactive effects on litter decomposition and associated biota remain poorly understood. We placed 5 leaf species in coarse-mesh bags alone or in mixtures and immersed them in 6 low-order streams along a eutrophication gradient. Fungal and invertebrate assemblages were mainly structured by stream eutrophication. The quality of leaf species also structured fungal assemblages, whereas the number of leaf species structured invertebrate assemblages. Effects of leaf diversity on decomposition were synergistic and increased with the number of leaf species. However, the synergistic diversity effects were found only in streams with lower nutrient levels, a result suggesting that oligotrophic streams depend more than eutrophic streams on the number of plant litter species. On the other hand, leaf species identity affected leaf-litter decomposition and fungal and invertebrate biomasses on leaves. Initial leaf N concentration and leaf-litter decomposition were positively linearly related, and this relationship became stronger as eutrophication increased. This result suggests that leaf-litter decomposition depends more on the quality than the number of plant litter species in eutrophic streams. Overall, our results highlight that eutrophication modulates leaf diversity effects on leaf-litter decomposition with potential implications for stream ecosystem management.

Effects of Riparian Plant Diversity Loss on Aquatic Microbial Decomposers Become More Pronounced with Increasing Time

We examined the potential long-term impacts of riparian plant diversity loss on diversity and activity of aquatic microbial decomposers. Microbial assemblages were obtained in a mixed-forest stream by immersion of mesh bags containing three leaf species (alder, oak and eucalyptus), commonly found in riparian corridors of Iberian streams. Simulation of species loss was done in microcosms by including a set of all leaf species, retrieved from the stream, and non-colonized leaves of three, two or one leaf species. Leaves were renewed every month throughout six months, and microbial inoculum was ensured by a set of colonized leaves from the previous month. Microbial diversity, leaf mass loss and fungal biomass were assessed at the second and sixth months after plant species loss. Molecular diversity of fungi and bacteria, as the total number of operational taxonomic units per leaf diversity treatment, decreased with leaf diversity loss. Fungal biomass tended to decrease linearly with leaf species loss on oak and eucalyptus, suggesting more pronounced effects of leaf diversity on lower quality leaves. Decomposition of alder and eucalyptus leaves was affected by leaf species identity, mainly after longer times following diversity loss. Leaf decomposition of alder decreased when mixed with eucalyptus, while decomposition of eucalyptus decreased in mixtures with oak. Results suggest that the effects of leaf diversity on microbial decomposers depended on leaf species number and also on which species were lost from the system, especially after longer times. This may have implications for the management of riparian forests to maintain stream ecosystem functioning.

Plant litter diversity affects invertebrate shredder activity and the quality of fine particulate organic matter in streams

There is evidence that loss of riparian plant diversity alters the availability and quality of resources in streams, but little is known about how such effects change with time after loss of diversity. We used a microcosm approach with leaves of alder, oak and eucalypt previously colonised by microbes in a mixed forest stream to test how loss of litter diversity and time (2 and 6 months after loss of diversity) affect leaf consumption by invertebrate shredders, the elemental composition of shredder tissues, and the quality of fine particulate organic matter (FPOM). The number and identity of leaf species affected leaf consumption and FPOM production by shredders. Effects of leaf species diversity were positive and became more frequent with time after loss of diversity. Carbon (C) and nitrogen (N) composition of invertebrate tissues changed with the leaf identity. FPOM quality (C:N ratio) was positively correlated with leaf quality. Leaf consumption by the animals decreased linearly with the increase in C:N imbalance between leaf litter and invertebrate tissues. Our results suggest that changes in plant litter diversity affect the activity of shredders (leaf consumption and FPOM production), and the quality of food resources (FPOM and shredders) to higher trophic levels in streams; such effects are likely to become stronger with time after loss of plant diversity.

Spring stimulates leaf decomposition in moderately eutrophic streams

In forested headwater streams, decomposition of allochthonous organic matter is a fundamental process driven by aquatic microbes and invertebrate shredders. We examined how season and eutrophication affect leaf decomposition and the associated decomposer communities by immersing leaves of a late deciduous species (Quercus robur) in five streams in Portugal along a gradient of eutrophication in autumn and spring. We found hump-shaped relationships between leaf decomposition and total nitrogen and phosphorus in stream water in both seasons. Leaf decomposition and shredder biomass were higher during spring in streams with moderate levels of eutrophication. Fungal sporulation and biomass were stimulated at moderate levels of eutrophication and inhibited at low or high levels of eutrophication. Fungal assemblage composition shifted between seasons and along the gradient of eutrophication. Tricladium chaetocladium increased its contribution to total conidial production in spring, while Dimorphosporafoliicola was dominant in the most eutrophic streams where Articulosporatetracladia was almost absent. Invertebrate shredders were the primary decomposers of leaves in streams with moderate levels of eutrophication, particularly in the warmest season. Although the presence of late deciduous plant species, such as oak, in the riparian corridors may help to mitigate food depletion to freshwater decomposers in spring, our results suggest that moderate eutrophication can accelerate decomposition further reducing litter standing stocks in the warmer seasons.

New climatic targets against global warming: will the maximum 2 °C temperature rise affect estuarine benthic communities?

The Paris Agreement signed by 195 countries in 2015 sets out a global action plan to avoid dangerous climate change by limiting global warming to remain below 2 °C. Under that premise, in situ experiments were run to test the effects of 2 °C temperature increase on the benthic communities in a seagrass bed and adjacent bare sediment, from a temperate European estuary. Temperature was artificially increased in situ and diversity and ecosystem functioning components measured after 10 and 30 days. Despite some warmness effects on the analysed components, significant impacts were not verified on macro and microfauna structure, bioturbation or in the fluxes of nutrients. The effect of site/habitat seemed more important than the effects of the warmness, with the seagrass habitat providing more homogenous results and being less impacted by warmness than the adjacent bare sediment. The results reinforce that most ecological responses to global changes are context dependent and that ecosystem stability depends not only on biological diversity but also on the availability of different habitats and niches, highlighting the role of coastal wetlands. In the context of the Paris Agreement it seems that estuarine benthic ecosystems will be able to cope if global warming remains below 2 °C.