Xisca Timoner

Contraction, fragmentation and expansion dynamics determine nutrient availability in a Mediterranean forest stream

Temporary streams are a dominant surface water type in the Mediterranean region. As a consequence of their hydrologic regime, these ecosystems contract and fragment as they dry, and expand after rewetting. Global change leads to a rapid increase in the extent of temporary streams, and more and more permanent streams are turning temporary. Consequently, there is an urgent need to better understand the effects of flow intermittency on the biogeochemistry and ecology of stream ecosystems. Our aim was to investigate how stream nutrient availability varied in relation to ecosystem contraction, fragmentation and expansion due to hydrologic drying and rewetting. We quantified the temporal and spatial changes in dissolved nitrogen (N) and phosphorus (P) concentrations along a reach of a temporary Mediterranean forest stream during an entire contraction–fragmentation–expansion hydrologic cycle. We observed marked temporal changes in N and P concentrations, in the proportion of organic and inorganic forms as well as in stoichiometric ratios, reflecting shifts in the relative importance of in-stream nutrient processing and external nutrient sources. In addition, the spatial heterogeneity of N and P concentrations and their ratios increased substantially with ecosystem fragmentation, reflecting the high relevance of in-stream processes when advective transport was lost. Overall, changes were more pronounced for N than for P. This study emphasizes the significance of flow intermittency in regulating stream nutrient availability and its implications for temporary stream management. Moreover, our results point to potential biogeochemical responses of these ecosystems in more temperate regions under future water scarcity scenarios.

Microbial biofilm structure and organic matter use in mediterranean streams

River and stream biofilms in mediterranean fluvial ecosystems face both extreme seasonality as well as arrhythmic fluctuations. The hydrological extremes (droughts and floods) impose direct changes in water availability but also in the quantity and quality of organic matter and nutrients that sustain the microbial growth. This review analyzes how these ecological pulses might determine unique properties of biofilms developing in mediterranean streams. The paper brings together data from heterotrophic and autotrophic community structure, and extracellular enzyme activities in biofilms in mediterranean streams. Mediterranean stream biofilms show higher use of peptides during the favorable period for epilithic algae development (spring), and preferential use of cellulose and hemicellulose in autumn as a response to allochthonous input. The drying process causes the reduction in bacterial production and chlorophyll biomass, but the rapid recovery of both autotrophs and heterotrophs with rewetting indicates their adaptability to fluctuations. Bacteria surviving the drought are mainly associated with sediment and leaf litter which serve as “humid refuges”. Some algae and cyanobacteria show resistant strategies to cope with the drought stress. The resistance to these fluctuations is strongly linked to the streambed characteristics (e.g., sediment grain size, organic matter accumulation, nutrient content).

Biofilm functional responses to the rehydration of a dry intermittent stream

Intermittent water flow regimes characterize streams in many world regions, especially those with arid and semiarid climates. During cease to flow conditions, biofilms on streambed sediments may be exposed to desiccation. Environmental conditions and resource availability change with desiccation and may influence biofilm functioning and whole stream ecosystem processes. Rainfall events during the nonflow phase can rehydrate streambed sediments, but the effect of these pulses on biofilm functioning is unclear. This study aimed to analyze the effects of a rehydration event on biofilm functional diversity during the nonflow period in a subtropical Australian stream. Biofilms from three different stream pools on the same reach; one permanently water-covered and the other two differing in their desiccation time were studied. Biofilms initially differed owing to the time they were exposed to dry conditions but rehydration events significantly increased biofilm functional diversity, producing a “reset” effect on the desiccation exposure, as after that bacterial functioning decreased again because of the new dry conditions. The observed rapid biofilm responses to rehydration during flow intermittency might be essential in sustaining biofilm functional diversity in intermittent streams.

The Biota of Intermittent Rivers and Ephemeral Streams: Prokaryotes, Fungi, and Protozoans

Microbial diversity and function in intermittent rivers and ephemeral streams (IRES) are tightly linked to specific habitat availability and hydrological phases. The intensity and frequency of the different phases (especially drying and rewetting) affect community composition and key functions, mainly linked to biogeochemical processes. Resistance and resilience strategies are distinct among microorganism groups—bacteria, archaea, fungi, protozoans—and strongly depend on different types of microhabitat or refuge available. The biodiversity of prokaryotes in IRES is strongly affected by hydrology but microhabitat conditions and type of benthic substrate significantly affect their community composition. Fungi are very sensitive to drying but use several refuges, including the terrestrial habitat, and resistance strategies. Protozoans show a wide range of survival strategies and several species can resist harsh conditions such as anoxia in drying pools. Thus, they become especially relevant for ecosystem functions when other organisms are inhibited. This sensitivity causes “waves” of microbial functions and biodiversity to covary with hydrological phases, potentially affecting ecosystem functioning and higher trophic levels. Microbially mediated functions in IRES are perhaps the most critical to freshwater ecosystem services such as nitrogen and carbon cycling. Therefore, efforts to manage and restore IRES will depend on improved understanding of hydrological controls on microbial communities and functions across space and time.

The Biota of Intermittent Rivers and Ephemeral Streams: Algae and Vascular Plants

Intermittent rivers and ephemeral streams (IRES) support highly biodiverse primary producers, including algae, cyanobacteria, and aquatic and riparian plants. All these groups share common traits to cope with a harsh environment whose water table varies and where desiccation is common. Traits include morphological characteristics, life cycle adaptations, and physiological mechanisms to contend with hydric stress and increased water temperatures. Despite these environmental challenges, primary producers play ubiquitous roles in the fluxes of energy and material in IRES. They are the basis for ecosystem metabolism, expressed as episodic pulses of production that occur immediately following rewetting, when their abundant output of high-quality organic matter is critical for supporting consumers both in-stream and in the neighboring terrestrial system.

Ecophysiology of River Algae

Algae in rivers are affected by light, water turbulence, and nutrient availability. These environmental factors ultimately affect algae according to their habitat, growth form, and specific physiological abilities. Water flow imposes limitations in the diffusion and availability of gases and resources, also in relation to algal size and growth form. Algae adapt physiologically to light scarcity or excess via photosynthetic mechanisms, as well as by modifying their pigment composition. The algal ability to obtain and keep resources is mediated by enzymes, and its ability to use and store materials is specific of the different algal groups. Toxicants impose a limit to algal performance and may affect photosynthesis as well as nutrient uptake, amongst other effects on algal cells.