Functional responses of stream biofilms to flow cessation, desiccation and rewetting

Abstract

Summary 1. Mediterranean climate regions are characterised by long summer droughts that usually involve flow intermittency in low‐ to mid‐order streams. Flow intermittency implies flow cessation, drying and subsequent rewetting of the streambed, and affects both autotrophic and heterotrophic processes. The balance between these processes, as well as the balance in the use of carbon (C), nitrogen (N) and phosphorus (P) may change because of the ongoing increase in stream flow intermittency caused by global change in many regions. It is therefore crucial to understand better the consequences of this phenomenon. 2. Our two initial hypotheses were (i) that flow intermittency would impact more on autotrophic than on heterotrophic processes in stream biofilms owing to the higher water dependence of autotrophs, as well as differences in the water storage capacity of the stream biofilm compartments where autotrophic and heterotrophic processes mainly occur (surface cobbles versus hyporheic sediments) and (ii) that the C‐N‐P use by biofilms would change during the dry period (terrestrial phase) owing to the extreme water stress conditions. These hypotheses were tested by analysing the functional response of the main stream biofilms (epilithic, epipsammic and hyporheic) during flow cessation, desiccation and rewetting in a Mediterranean forested stream. The autotrophic response was characterised through changes in the photon yield, whereas the heterotrophic response was characterised by changes in the extracellular enzyme activities. 3. Streambed desiccation had clear effects on the functioning of stream biofilms. Autotrophic biomass decreased by 80% with streambed desiccation, but recovered rapidly after flow resumption. Heterotrophs were more resistant to water stress, especially in the epipsammic and hyporheic biofilms where bacterial cell density decreased only by 20%. 4. Extracellular enzyme activities remained relatively high, and the balance in the C‐N‐P use by biofilms changed during the dry period. The C and P breakdown capacities were maintained during dry conditions, especially in the epipsammic and hyporheic biofilms, but the degradation of N compounds sharply decreased. Elemental molar ratios (C:N and C:P) of the different biofilms also changed with streambed desiccation. C:P ratios increased from 80 to 300, while the C:N ratios increased from 10 to 16. 5. Given the contrasting responses of autotrophic and heterotrophic processes in the different biofilms, our results suggest that the current increase in flow intermittency extent is likely to increase the relative importance of heterotrophic processes in stream ecosystems, as well as the relative contribution of the hyporheic biofilm to C‐N‐P use. Our results further suggest that the longer streams remain dry, the more the biofilm stoichiometry will change.

Seasonality of algal communities in small streams and ditches in temperate regions using delayed fluorescence

“…Dry-out events do not seem to considerably alter the seasonal pattern of either suspended algae or attached algae communities. Both communities resumed the typical seasonal pattern within a short time (approximately one month) as reported previously for biofilms by Timoner et al (2012).…”

Stream Biofilm Responses to Flow Intermittency: From Cells to Ecosystems

“…Timoner et al (2012) showed that drying in a Mediterranean stream caused a major decrease in autotrophic biomass, while the heterotrophic biomass was less affected. Algal biomass decreased up to 80% with streambed desiccation, but bacterial cell density decreased only by 20%.…”

“…The absence of water directly affects the performance of algal photosynthesis; however, some bacterial groups have the ability to remain active under low humidity conditions. The recovery of biomass may also follow different patterns for the different substrata; the epilithic biofilm can be the least resistant because it is the most exposed, but also the quicker to recover after flow return both in terms of autotrophs and heterotrophs (Timoner et al, 2012).…”

Bottom-up effects of streambed drying on consumer performance through changes in resource quality

“…The higher C:N ratios observed in this study are likely to be related to chlorophyll degradation, which have been positively related to a decrease in leucine-aminopeptidase activity during desiccation in seasonal summer drought (Timoner et al 2012). Although in some cases a rapid recovery (within hours) of algal biomass has been reported after flow resumption (Romaní et al 2013), it has also been related to nutrients wash from the dried streambed or watershed after flow resumption (Bernal et al 2002), which did not take place in our experiment.…”

“…Although algae and bacteria seem to recover rapidly after flow resumption (Romaní et al 2013), effects on fungal biomass vary according to drying intensity and duration (Langhans and Tockner 2006;Bruder et al 2011). Because conditioning by these microorganisms influences the chemical composition of resources (Graça 2001;Ylla et al 2010;Timoner et al 2012), one might expect differences in the quality of resources available to consumers. Indeed, a decrease in the polysaccharide, amino acid and lipid content of benthic resources has been observed in an intermittent stream upon rewetting (Ylla et al 2010) while greater periods of flow intermittence increased C:N ratios in leaves (Sanpera-Calbet et al 2016).…”