Water availability is key to understanding ecosystems with dry and wet phases

Writing / Irene Vega

Many aquatic and terrestrial ecosystems alternate between wet and dry phases, as is the case with rivers and temporary lagoons, intertidal zones or flood plains. These environments are known as transitional ecosystems and, traditionally, the different phases have been studied separately, as if there were only one of them.

In order to have a more complete view of these ecosystems, a research team from the MNCN-CSIC and the URJC has carried out a study that identifies the presence of water as the global driving force that shapes life in these mixed systems. This work has brought together for the first time an interdisciplinary team of terrestrial and aquatic ecologists, together with microbiologists, to break down the barriers of traditional approaches.

“This biased approach has led to a poor understanding of these ecosystems, which limits our ability to conserve their biodiversity and the many benefits they provide to society, such as drinking water, climate stability, food production, and carbon and nutrient recycling,” says Pilar Hurtado, senior author of the paper and postdoctoral researcher in the Biodiversity and Conservation Area of ​​the URJC.

The study, published in the prestigious journal Ecology Letters, proposes considering the wet and dry phases as two inseparable components of the system that alternate cyclically. This periodic alternation between phases with and without water gives rise to a predictable succession of microscopic organisms that cover the substrate and are capable of maintaining the functions performed by these ecosystems. During the dry phase, organisms such as bacteria, fungi, algae, protozoa or lichens give rise to what is known as biological crusts. However, in the presence of water these organisms take the form of biofilms covering stones and fine substrate.

“When we walk along a stream or even through grassland, if we pay attention, we can see that the ground is covered by mosses, lichens and other tiny organisms that perform essential functions for soil fertility and nutrient recycling,” says Rebeca Arias-Real, first author of the paper and postdoctoral researcher at the MNCN-CSIC. “On the other hand, if we go into a river and touch the stones and the substrate, we will notice a gelatinous film, which is what is known as biofilm. Biological crust and biofilm are two sides of the same coin,” says Arias-Real.

“Unlike other ecosystems that experience irreversible changes following a disturbance, transitions from aquatic to terrestrial, or vice versa, do not impose a definitive change on the ecosystem. In fact, these reversible transitions between states occur naturally and periodically, from minutes to years, in response to phases with contrasting water availability,” says Pilar Hurtado. “Considering the two phases of these ecosystems together can help us better understand how climate change and other human impacts could modify biodiversity patterns on a global scale and understand the degree of threat to such essential functions as freshwater availability, climate stability or food production,” she adds.

This work lays the groundwork for improving the study and understanding of how global water availability could compromise biodiversity and people on a planet with increasingly extreme climatic conditions. It also opens up new lines of interdisciplinary research to address the challenges of a constantly changing world.