Biofilm responses to flow intermittency in mediterranean rivers

Resumen

Streams that at times cease to flow at spatiotemporal scale along their course are called temporary streams. Currently, global change is altering fluvial ecosystem function and structure, as well as the ecosystem services that they provide. To effectively protect fluvial ecosystems adapting to global change, a detailed understanding of the effects that changes in hydrological regimes produce on their biodiversity and functioning is needed. Organisms inhabiting temporary streams are directly affected by their hydrological regime, including the stream biofilm. Biofilms are associations of heterotrophic and autotrophic microorganisms co-habiting in a matrix of polysaccharides, exudates and detritus. They are of particular relevance in temporary streams because of their diversity, abundance, and key role in ecosystem processes. Therefore, understanding biofilm response to hydrological regime variability is a vital step in order to understand the implications of increasing non-flow periods on fluvial ecosystems. Accordingly, the overall objective of this thesis was to investigate the duration and frequency of the non-flow period on fluvial ecosystems through stream biofilms, focusing on both the structure and functioning of their photoautotrophic community, in order to understand and predict non-flow period effects on temporary and new-temporary streams. To achieve this objective, detailed studies characterizing and analysing the effects of temporal components of the non-flow period on stream biofilms were conducted at different scales. In Paper I, II and III of this thesis, I analysed how duration and frequency of the non-flow period influenced biofilm structure, physiology and functioning in 33 Mediterranean streams. In Paper IV of this thesis, I analysed the resistance and resilience of biofilms from permanent and temporary streams to dry conditions. In Paper I, II, and III results highlight the non-flow duration above their frequency as an ecosystem driver in temporary streams. The exponential negative relationships between both the physiology and functioning of biofilm with the duration of the non-flow period also suggest an ecological zone-type threshold, with a transition from an aquatic to a terrestrial state after approximately 20-50 dry days. This shift was accelerated by the solar radiation and high stream temperatures to which biota was exposed during non-flow periods, indicating the importance of valley-floor form and riverine vegetation as a protective structure for temporary stream biofilms. Community analysis also highlighted the dominance of aerophyte and sub-aerophyte genera of cyanobacteria in temporary streams, whereas diatom genera characterized permanent stream communities, which reinforces the idea of a change of state from aquatic to terrestrial. The observed cause-response relationship suggests that photoautotrophic organisms in temporary streams play an essentially singular role, and that when the photoautotrophic α-diversity of temporary stream communities crosses a threshold, there is a sharp decline in autochthonous production. Paper IV presents evidences that the hydrological history of temporary streams generates a pool of resistant species to dry conditions better than species pool from permanent streams. Overall, the results of this thesis demonstrate duration of the non-flow period as a key influence on the structure and functioning of biofilms in permanent and temporary streams. The results also suggest the importance of maintaining photoautotrophic stream biodiversity to preserve stream ecosystems functioning and highlight the importance of valley-floor form and riverine vegetation to protect these communities. Looking ahead, understanding the spatiotemporal effects of the non-flow period on stream biofilms is crucial to improve the management of fluvial ecosystems in response to global change.