Community structure and ecosystem functions are determined by physical and chemical conditions, food resources and biotic interactions. Mediterranean-climate streams are intrinsically variable in streamflow, with marked flow reduction in summer and floods in autumn and spring according to regional precipitation patterns. Riverine species are adapted to
cope with natural and predictable disturbances, and their responses are in a large part determined by their physiological and behavioral adaptations. As for fish, swimming performance can determine their ability to use habitats differing in water velocity, to obtain
food and avoid predation, etc. Today, the intense use of water by human demands causes longer low-flow periods in streams, thereby producing a certain degree of water stress. In addition, nutrient enrichment and water pollution are also common anthropogenic stressors that threaten ecological integrity of freshwater ecosystems. Metal pollution is of particular concern due to its high potential toxicity, perdurability and bioaccumulation through the food chain. At the bottom of the food chain, periphyton communities serve as food and energy sources for many organisms (e.g. macroinvertebrate grazers), so any stressor affecting periphyton is likely to indirectly affect organisms of a higher trophic level (“bottom-up control”). On the contrary,
periphytic algal biomass can be affected by shifts in grazing pressure, e.g. resulting from predator-induced changes in grazer density (“top-down control”). Periphyton communities are also involved in important ecosystem functions (e.g. nutrient cycling), and are important bioindicators for habitat degradation. Field mesocosm experiments, from which we can modify the density of certain consumers (e.g. fishes), have become an invaluable tool in ecotoxicology
and ecology for increasing mechanistic understanding of stressor effects.
This thesis aimed to assess the responses of stream biota to flow alterations, in terms of water velocity increments and reductions of discharge, and to metal pollution. The investigations performed encompassed (i) respirometry studies using swim-tunnel respirometers to evaluate swimming capabilities, as indicative of water velocity tolerance, and
physiological responses (energetic costs of swimming) of several freshwater fishes, as well as the drivers (morphological features) of variation within and among species; and (ii) field observational and mesocosm studies were carried out to evaluate the role of the threatened fish
B. meridionalis as modulator of the response of stream ecosystem structure and function to anthropogenic stressors, such as hydrologic alterations (e.g. resulting from the water diversion)
and metal pollution.
Our results showed that there exist a high intra-and interspecific variation in absolute critical swimming speed and metabolic traits in fish. Generally, swimming performance and aerobic metabolism scaled positively with body size. The study that compared the invasive mosquitofish (Gambusia holbrooki) with the native Spanish toothcarp (Aphanius iberus)
revealed that mosquitofish can swim faster (i.e. higher critical swimming speed) and more efficiently than Spanish toothcarp, as it showed lower cost of transport at a comparable workload. Both sexes of mosquitofish showed a better hydrodynamic body shape, i.e. streamlined body and thiner caudal peduncle, both of which were positively correlated with
swimming performance. The second study revealed that, after accounting for body size effects, critical swimming speed was independently affected by total length or maximal metabolic rate within certain fish species. At the interspecific level, variation in critical swimming speed was partly explained by the variation in the interrelated traits of maximal metabolic rate, and fineness and muscle ratios, when accounting for up to eight freshwater fish species.
Field observational and mesocosm studies showed that the top-down control by grazers was largely responsible for the variation in structural responses of periphyton, such as biomass and nutrient content, and in functional responses, such as nutrient uptake rates. Generally, when
grazer densities were higher the periphyton biomass (ash-free dry mass or chlorophyll-a) was lower, whereas the nitrogen content of periphyton increased (i.e. lower C:N). The higher phosphate uptake capacity by periphyton was likely due to a more rejuvenated and active periphyton community. In addition, the variation in macroinvertebrate and fish densities were
directly related to the impacts of natural (floods) and anthropogenic stressors. In metal-polluted reaches, we found that direct negative effects of metals on grazers led to indirect positive effects
on periphyton biomass.
Our studies also provided evidence about the important functional role of the Mediterranean barbel (Barbus meridionalis) in streams because shifts in their densities can lead to large ecosystem changes. The absence of this predatory fish triggered trophic-cascade responses on periphyton biomass in low impacted streams, and led to modify phosphate uptake rates by periphyton through biotic interactions. On the contrary, this benthic fish markedly reduced periphyton biomass via bioturbation impacts, and the greatest consumption effects on macroinvertebrates were found in larvae of Chironomidae.
Finally, it was discussed (i) the importance of fish swimming capabilities, metabolism and morphological traits in influencing patterns of species distributions in Mediterranean streams with hydrologic alterations; and (ii) the context dependency of stream community responses to anthropogenic stressors. A number of management actions and future research prospects in relation to our findings were also raised throughout this thesis.