By studying a small watercourse in the Great Plain, researchers at the Danube Research Institute of the ELKH Centre for Ecological Research (CER) have found that fast-flowing and high-intensity disturbances, such as heavy rains of short duration, have a significant effect on the diversity of benthic algal assemblages and their ability to create biofilm, even during periods of relatively stable water flows. This negative effect is exacerbated during periods of severe disturbances, which result in the dominance of species with extreme characteristics. At the same time, the researchers observed that extreme weather events did not reduce the functional richness of the communities. This means that benthic algal communities are able to retain their function, providing that the unfavorable periods are not of significant length. This study was published in the journal Ecohydrology in late February.

Global climate change is one of the greatest ecological challenges of our time, and one additional huge challenge is to explore how the frequently occurring extreme weather events shape communities in surface waters. Extreme weather leads to unpredictable changes in the water supply, such as frequent dehydration of riverbeds and flash floods in the case of small watercourses. These anomalies place an enormous burden on both macroscopic and microscopic communities.

Benthic algae (i.e. living on solid surfaces in water) play a key role in the material flow of surface waters, by producing oxygen, and forming a living biofilm that stabilizes the substrate. Because they are sensitive to changes in their environment, unpredictable water level fluctuations and increasingly frequent and prolonged dehydration of small watercourses cause fundamental structural changes in algal communities. The biodiversity of these communities decreases, which ultimately affects the entire food web by altering the habitats of other groups of organisms and/or influencing the quality and quantity of available food sources.

In a colonization experiment performed in a lowland watercourse, the researchers investigated the effect of individual weather events on biofilm-creating algae in three different hydrological situations: during a period of stable water, a period of moderate disturbance and a period of severe disturbance. The algal colonization was monitored for 84 days. By the end of this field experiment, the small watercourse was completely dry.

Algal biofilm formed on the bed of the lowland watercourse of the Great Plain and on the inserted wooden slides in the period of stable water level (Photo: Áron Lukács)

Algal biofilm formed on the bed of the lowland watercourse of the Great Plain and on the inserted wooden slides in the period of fluctuating water level (Photo: Áron Lukács)

The results revealed a clear difference between the two periods in the composition of the biofilm-creating algal community.

Summary diagram of the results of the colonization experiment

The period of moderate disturbance and stable water level was mostly preferred by large, weakly attached filamentous and colony-forming, and large-sized mobile algae. At the same time, a short period of heavy rainfall followed by a slight decrease in water level, which did not affect significantly the water level itself, shifted the composition of the algal community towards the proliferation of medium and small, typically unicellular species. During the highly disturbed period, the numbers of small, or mobile species continued to rise, while those of filamentous and large colonial species declined drastically. Surprisingly, the number of taxa did not decrease during the severely disturbed period, meaning that in addition to the species that may have been eliminated, there were always others that still appeared in the community. In contrast, taxonomic and phylogenetic diversity itself was much lower during this period than during the period of moderate disturbance with a stable water level. This means that closely-related species formed the community, within which, as a result of the disturbance highly tolerant, adaptive species became dominant.

Community performances depend not only on the number of species in the habitats, but also on the characteristics of the existing species. These characteristics determine the role and function of the given species within the community. It is also valid for the present study that the more functionally diverse a community is, the greater its resilience. Researchers have previously hypothesized that extreme weather events will significantly reduce the functional diversity of a community, thereby increase its vulnerability. However, the results of this study only partially supported this assumption. Functional divergence, which emphasizes the rise of taxa with extreme properties in the community, was clearly greater during the period of severe disturbance. On the other hand, in terms of their functional richness, there was no difference between the two periods.

These results show that due to the drastic changes in water level experienced in this study, the function that a given algal community can perform effectively could also be changed. At the same time, even at stable water level, moderately disturbed periods did not disappear from the system, only their proportions can change. This means that if extreme weather events are persistent, it could exacerbate their negative effects on the algal communities, increasing their vulnerability. Yet, if the adverse effect is not permanent, the community can regenerate and does not lose its function. However, the latest climate scenarios predict an increase in the number and intensity of extreme climate events in the Carpathian Basin. This, in turn, is expected to lead to a significant transformation of the taxonomic and functional composition of the biofilm-creating algal communities, which will also affect the functioning and health of the habitats and their assemblages.

Publication:

Á Lukács., I Bácsi., Zs Nemes-Kókai., G Borics., G Várbíró., E T-Krasznai., V B-Béres. (2021) Strong influence of climatic extremes on diversity of benthic algae and cyanobacteria in a lowland intermittent stream. Ecohydrology e2286.