“Light and flow stability are the factors that most influence river production”
The scientist Maite Arroita Azkarate researches river metabolism. Also called “heartbeat” of rivers, it uses it to know how ecosystems breathe. In fact, although there are clear criteria for the classification of terrestrial ecosystems, the same is not true of river ecosystems, and its objective is to resolve it. Arroita has identified in his latest study, in collaboration with US researchers, the main factors that condition the metabolism of river ecosystems. Research will allow better knowledge and management of rivers.
To start with, what is river metabolism?
The metabolism of an ecosystem encompasses the biological activity of all the organisms that inhabit it. There are many processes, but they can be summarized in two big ones: primary production -- through photosynthesis, how much solar energy is consolidated and how much organic matter is produced, and respiration -- how much organic matter is consumed. So river metabolism tells us what kind of energy flow there is. This is important, in short, because this energy flow is the support of life in rivers and trophic networks.
The main river producers are benthic algae, which grow on the surface of substrates, but also planktonic. [Briophytes (mosses)]. And in some rivers there are also macrophytes, water plants: water buttons, lentils, water lilies (floating or rooted)…
The goal of your research has been to better understand river metabolism and overcome a deficiency, right?
The problem is that in terrestrial ecosystems, this metabolism, the magnitude of production and respiration are well known. The seasonality of these ecosystems is also known, that is, how production changes according to the time of year. Factors affecting metabolism and seasonality are also known: on the one hand, temperature is the key and, on the other hand, water availability or precipitation. These two factors fundamentally determine the characteristics and physiognomy of the community. So, knowing these two factors, we can differentiate the biomes that appear in terrestrial ecosystems: depending on the temperature and the flow of water, we know whether it's going to be a desert, a tropical forest, a tundra... And based on that, we know, about what kind of vegetation there is going to be, when they're going to produce and how much; and based on that, we can predict what kind of biomes live in them.
The problem is that this classification has not been carried out in the rivers. Data is missing to be able to do something similar. And in our research, we've seen that the factors that condition river metabolism are not the same that condition the metabolism of terrestrial ecosystems. They're very different, and we didn't know them. Their determination is key to deep knowledge and good river management.
What factors have you identified?
We have seen that light and flow stability are the main factors affecting primary river production. Not so much precipitation or total amount of water, as on earth, but if the amount of water in the river remains stable.
For its determination, the seasonality of river metabolism has been studied through a large database, maximal productions and breaths and when they occur. And we've compared them to data from terrestrial ecosystems. The results show much lower metabolism compared to terrestrial ecosystems. When terrestrial ecosystems have a strong time – summer – biomass accumulates because production is higher than consumption. Nothing similar has been observed in the rivers. There is no significant increase: in most cases a slight repuncture is observed in spring-summer, other times at the beginning of the year… In the case of breathing, there are also many peaks in autumn. But there has been no clear seasonality.
And we've seen that in rivers, unlike soil, consumption is higher than production. This means that rivers are heterotrophic ecosystems throughout the year. So to sustain these ecosystems, you need the organic matter that comes from the outside. In our rivers it reaches them through the woods on the shore.
How does this relate to light and flow stability?
Light and flow stability are the main factors that condition photosynthesis. And in the case of consumption or breathing, the same rate of photosynthesis and the stability of the flow. In primary river production, the main process is photosynthesis, fixing solar energy. And if there's no solar energy, there's no photosynthesis either.
The availability of light varies greatly in river ecosystems. It varies with latitude, topography, coastal forest coverage and also with time. It even depends on water turbidity. We've seen that the lightest rivers have four times more production than the darkest ones. In the lights, in addition, the peaks occur in summer and in the dark in spring. In fact, in the poorly lit rivers the river forest grows in summer has great influence. As the leaves leave, less light reaches the rivers and limits primary production.
And in the case of the flow rate, what happens is that the rivers are very frequent alterations, like torrential flows or floods. It is true that there may be alterations at different levels: sometimes, after a storm, turbidity increases and limits the availability of light and, therefore, photosynthesis; but the most intense floods, which also move sediments, also move living beings along with sediments. Thus, the biomass that grows in rivers is very limited, since almost all living beings are dragged through the river. Somehow, the system does reset. Therefore, it is observed that production is three times higher in more stable rivers than in highly variable rivers.
Will your results help to respond to the existing shortage and classify river ecosystems?
The design of a classification like that used in terrestrial ecosystems can be quite ambitious, but it has given some clue. In the case of the flow, for example, there are rivers that function according to the snowy: the rise occurs when the snow melts and then they are more stable. In our latitudes, rivers that have small peaks at any time are more frequent, as they can rain at any time. There are Mediterranean rivers, with great winter flow, but very dry in summer. Etc. Therefore, a classification could be made according to the flow rate.
In the case of light it is more difficult, as it is affected by a lot of factors. Depending on the latitude you could make a certain classification, but the topography also affects you a lot. It's not the same as a river inside a canyon, which has no topographical obstacle. Or even small rivers usually receive less light, due to the greater influence of the riverside forest. So in that sense, it would be very difficult to classify under the light. So it would be hard to make a map of biomes like the one that terrestrial ecosystems have, but it could serve to take the first steps. For example, knowing the light that the river receives and its hydrology, it will help predict the magnitude and seasonality of the energy flows of that river; how much and when it will occur/breathe.
What has influenced the lack of effective classifications for river ecosystems so far?
I think it has been a technological issue, a lack of data. Primary producers of rivers are benthic algae, very small algae attached to substrates. The changes are constant and monitoring of biomass is very difficult. Therefore, river metabolism is estimated from the oxygen concentration of the water, since photosynthesis and respiration directly affect the dissolved oxygen concentration. This has been measured for a long time, but one-time measurements or experiments are done in a few days. In order to measure seasonality, complete annual data are needed, and also with high frequency. For example, those taken every 10 minutes.
In the last decade, long measurements of water quality have become more frequent since the start of the nets. In this project, carried out in the last two years with colleagues from the United States, a great effort has been made to measure the oxygen data of these networks and extract the metabolism from these data.
And can these results affect river management?
Well, our goal was more to identify the main patterns, which will allow us to predict, based on these factors, when and how much that biomass will produce. And depending on that, what kind of life there will be in that river. In a way, understanding the seasonality of rivers and the magnitude of production will help us know who will live there.
But it is true that in the context of an emergency or global change it can also have implications, since in addition to increasing temperature, rainfall is changing a lot: very dry summers, floods in other seasons... If this variability increases, there may be a limitation of river production and a decrease in river biomass. The decrease in energy flow would affect the trophic network. If less energy is introduced, there will be less development. This can affect biodiversity. Thus, it is essential to try to mitigate the effects of rain avenues, maintaining permeable soils and riverside forests in good ecological condition.
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