The effects of climatic fluctuations and extreme events on running water ecosystems

Most research on the effects of environmental change in freshwaters has focused on incremental changes in average conditions, rather than fluctuations or extreme events such as heatwaves, cold snaps, droughts, floods or wildfires, which may have even more profound consequences. Such events are commonly predicted to increase in frequency, intensity and duration with global climate change, with many systems being exposed to conditions with no recent historical precedent. We propose a mechanistic framework for predicting potential impacts of environmental fluctuations on running water ecosystems by scaling up effects of fluctuations from individuals to entire ecosystems. This framework requires integration of four key components: effects of the environment on individual metabolism, metabolic and biomechanical constraints on fluctuating species interactions, assembly dynamics of local food webs and mapping the dynamics of the meta-community onto ecosystem function. We illustrate the framework by developing a mathematical model of environmental fluctuations on dynamically assembling food webs. We highlight (currently limited) empirical evidence for emerging insights and theoretical predictions. For example, widely supported predictions about the effects of environmental fluctuations are: high vulnerability of species with high per capita metabolic demands such as large-bodied ones at the top of food webs; simplification of food web network structure and impaired energetic transfer efficiency; reduced resilience and top-down relative to bottom-up regulation of food web and ecosystem processes. We conclude by identifying key questions and challenges that need to be addressed to develop more accurate and predictive bio-assessments of the effects of fluctuations, and implications of fluctuations for management practices in an increasingly uncertain world

A laboratory study on feeding plasticity of the shredder Sericostoma vittatum Rambur (Sericostomatidae)

Abstract Since litter input and availability of leaves in many streams is highly seasonal in Portugal, we investigated whether Sericostoma vittatum, a typical shredder, was able to grow using alternative food sources. To test this hypothesis we fed S. vittatum with Alnus glutinosa (alder, CPOM, coarse particulate organic matter), leaf powder from A. glutinosa and Acacia dealbata and FPOM (fine particulate organic matter) from a 5th and a >6th order river, the macrophyte Myriophyllum aquaticum and biofilm. Growth in S. vittatum was significantly influenced by the food item given (ANOVA, P = 0.0082). The food item promoting the highest growth was A. glutinosa, in the form of FPOM (6.48% day-1) and CPOM (4.24% day-1); all other forms of FPOM and biofilm provided relatively low growth rates (0.77–1.77% day-1). The macrophyte M. aquaticum was also used as food source by S. vittatum and promoted intermediate growth (1.96% day-1). Neither nitrogen, phosphorus nor caloric content was correlated with growth. However, since higher growth was achieved with alder, in the form of CPOM and FPOM, we concluded that the chemical content of food was more important for S. vittatum than the physical form of such food. This may partially explain why shredders are able to survive when leaves are scarce in streams