Resumen de Effects of multiple stressors on river biofilms: from community composition to ecosystem processes using experimental mesocosms
Human activity worldwide exposes aquatic ecosystems to multiple anthropogenic stressors. Freshwater ecosystems (e.g. rivers and streams) are of special concern because of their notable sensitivity to stressors and relevance for global biodiversity and human well-being. Multiple-stressor effects on freshwater ecosystems depend on stressor nature, level and spatial/temporal scale, and their combined effects do not always match the predictions built upon knowledge about individual effects, producing the so-called non-additive responses. Non-additive responses include synergisms, which refers to combined effects surpassing the sum of individual effects, and antagonisms, when the opposite occurs (i.e. one stressor mitigating the effect of another). Among the many stressors that threat freshwater ecosystems, those derived from land-use change include the release of many pollutants into rivers and streams flowing through urban and agricultural areas. Also, climatic stressors such as warming, and others related to human action such as hydrological stress, affect river ecosystems on a global scale by modifying biodiversity patterns and ecosystem functioning. Among the many organisms exposed to multiple stressors in freshwater ecosystems, those attached to river and stream sediments (i.e. river biofilms) play a crucial role in virtually all major ecosystem processes and are frequently used as sentinels when assessing stressor impacts on freshwater ecosystems.
The thesis aims to identify the single and multiple-stressor effects of warming, hydrological stress and pollutant exposure on river biofilms. To that purpose, I used several experimental approaches, consisting on glass crystallizers (i.e. microcosms, Paper I) and artificial streams (i.e. mesocosms, Paper II, III and IV) to expose epilithic (i.e. growing on cobbles) and epipsammic (i.e. growing on fine sediments) river biofilms to single and multiple-stressor scenarios under controlled conditions. I included among the stressors individual pollutants (Paper I), as well as complex mixtures (Paper II, III and IV), climatic stressors such as warming (Paper I, III and IV), and hydrological stress (Paper I, II, III and IV). Stressor levels in the experimental designs were generally simplified to two; i.e. presence (treatment) vs. absence (control) of the stressor. I also employed a regressional experimental design to test different stressor levels (Paper II), and search for potential thresholds.
In all the above designs, I tested the river biofilm response both at the structural and the functional scale, employing response variables that ranged from photosynthetic and enzymatic activity to gene expression and bacterial community composition.
I detected that hydrological stress was the most influential stressor, specially impairing the biofilm community growing on cobbles (epilithic). Water warming had lesser effects, mostly affecting bacterial activity due to the dependence of metabolic activity on temperature, but showed limited effects on bacterial community composition (Paper IV). Pollutant exposure had contrasting results depending on the nature of the pollutant used. Single pollutants (Paper I; herbicide, antibiotic) as well as the pesticide mixture (Paper III, IV) shaped biofilm community structure and function according to their mode of action. The antibiotic erythromycin mostly impaired the bacterial community, while the herbicide diuron affected the phototrophs. The complex mixture used in Paper II (i.e. WWTP effluent) induced significant shifts in community structure at WWTP effluent proportions above 50 % of the total stream flow. I made a main objective of the thesis determining the type of response which might be produced when biofilms are affected by multiple stressors. Additive responses were prevalent in most cases, while non-additive responses accounted between 14.5 % (Paper I) and 29 % (Paper III) of all biofilm responses. Among significant interactions, antagonisms dominated in all cases, representing between 59 % (Paper III) and 89 % (Paper IV) of all biofilm responses, while synergisms were less dominant and relegated to the epilithic biofilm.
The results presented in this thesis show that single and multiple stressors affect both biofilm community structure and function, and emphasize that river biofilms show an adaptive nature when facing multiple-stressor scenarios.
