Mireia Bartrons and Sandra Brucet publish a study showing that bottom-up relationships are stronger than top-down trophic relationships in trophic networks in Europe lakes

All ecosystems, and aquatic ecosystems are no exception, are made up of different trophic levels, or in other words, several rings that make up a food chain. The predator-prey relationships between more than two of these levels are known as trophic cascades, and can significantly affect how ecosystems function. These cascades, which are one of the most extensively studied concepts in ecology, are the subject of the article Energy-based top-down and bottom-up relationships between fish community energy demand or production and phytoplankton across lakes at a continental scale, published in the journal Limnology and Oceanography, and led by researchers from the University of Vic - Central University of Catalonia (UVic-UCC).

The article is authored by Mireia Bartrons, a researcher in the Aquatic Ecology Research Group and lecturer in the Faculty of Science and Technology, and the ICREA research professor Sandra Brucet, the coordinator of this research group. Their study examines the magnitude of trophic cascades from the perspective of energy in lakes and to that end, they took samples from 227 European lakes. The results suggest that bottom-up effects on ecosystems are stronger than top-down effects. According to Sandra Brucet, this means that "the observed effects of primary producers (which are those that are food for other species but do not feed on any of them) on the fish that inhabit the lakes are substantially stronger than the effects that those fish may have on those primary producers."

Calculating energy demand and production
In order to reach these conclusions, the researchers used the Metabolic Theory of Ecology to estimate the metabolic metrics of each fish community, and to calculate the energy demand and production by each community. In each of the lakes studied, "we quantified the magnitude of the trophic cascades and we established the direction in which they had the greatest impact on the ecosystem," explains Mireia Bartrons. In addition, she adds, "effects that are generally neglected in other studies, such as the metabolic activity of fish, have been taken into account for the first time when comparing trophic cascades between lakes."

The study, led by UVic-UCC, was carried out in partnership with 12 European researchers and research centres. The research has been funded by the German Research Foundation in the framework of the project FISHSIZE: Analysing size-density relationships of aquatic communities in response to strength of predator-prey interactions and resource subsidy as well as partially funded by the MINECO (Spanish Ministry of Economic Affairs and Social Transformation) in the framework of the project led by the UVic-UCC SizeEcoFun: Size-based approaches to understand impacts on river ecosystem functioning.

New results - change of hypothesis
In lakes, energy relationships from the upper to the lower levels, i.e. top-down trophic cascades from predators to primary producers, can be the result of increasing the metabolic activity of the zooplanktivorous fish community, for example. These fish exert strong pressure on zooplankton and phytoplankton numbers increase as a result, because they are subject to less pressure from their predators. In the opposite scenario, the effect of production by the community of lower trophic levels on the higher levels, i.e. bottom-up trophic cascades, can also occur if for example, nutrients encourage phytoplankton production, which in turn encourages the growth of zooplankton, which will eventually lead to an increase in the number of fish.

"We expected that an increase in metabolic demand by fish would lead to greater pressure on the first trophic levels in the lakes," says Mireia Bartrons. However, the study's findings have shown that "this effect is very mild compared to what happens when the process is reversed: the impact of primary production, or in other words the first trophic levels of the trophic network or phytoplankton on higher trophic levels, or fish, is much greater," explains the researcher Mireia Bartrons.

The 227 European lakes analysed contained different types of natural habitat, including waters in polar regions, waters in temperate and humid areas, and coastal temperate waters. Sampling was carried out in Turkey, Germany, the United Kingdom, the Czech Republic, Norway, Sweden, Denmark, Estonia, Slovenia and Italy. Parameters including temperature, lake depth, chlorophyll A levels and the human impact on the lakes, such as the percentage of agricultural land in the basin, the population density and the modification of the lake's perimeter were used for the research.

The researchers believe that the results of the study are "robust" because "metabolic variables have been applied that characterise the feeding and production rates of the fish community for the first time in a research study in this field, while taking into account effects which have generally been neglected in other large-scale studies comparing various lakes."