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National Institute of Research and Development for Biological Sciences

Spelunking & Questing: What do cave dwellers eat?

This article is the result of an interview with our colleague Octavian, which we conducted on the occasion of the completion of his research project on food webs in rivers flowing through caves. We discussed the importance of this project, the challenges encountered along the way, and the fascinating discoveries about the underground life. We hope this interview will provide an interesting insight into the mysterious and little-known world of cave ecology.

How did the story of this project begin?

The story began ten years ago, when I was working as a postdoctoral researcher at the Rivers Laboratory, a research station affiliated with Queen Mary University of London, England. Between 2013 and 2015, I studied how food webs in rivers are influenced by nutrient concentrations in the water. These food webs include basal resources (e.g. algae and detritus), intermediate consumers (e.g. aquatic invertebrates) and higher order consumers (e.g. trout).

In 2017-2018, we studied two water caves, each crossed by an underground stream and part of the same system carstic (Ciur-Ponor-Ciur-Izbuc system in the Apuseni Mountains). In the Ciur-Ponor cave, we investigated how surface floods, after heavy rains, carry into the cave detritus and surface fauna. It was pioneering research, one of the few to examine how surface energy inputs influence life underground. The results were published in 2019 in International Journal of Speleology.

During a visit to the Ciur-Izbuc cave, while digging around at the bottom of the underground stream, I discovered some depigmented, opaque-white crustaceans. At first I thought they were of the genus Niphargus, a common inhabitant of water caves. But on closer examination, I noticed that some of them had a small black spot where the eyes should have been. A surprising observation because cave dwellers, as a rule, no longer have eyes, they are a regressed character after millions of years of adapting to life underground. I realised they were actually a surface crustacean, a distant relative of the nifargus, viz. Gammarus balcanicus. The creek running through the cave was populated at the surface, just a few hundred metres from the entrance, by this crustacean in the form of a large population. In the cave, however, there was only a sparse population, probably temporary but depigmented, thus differing from the external population and indicating that they had spent some time underground.

We monitored the biological cycle of these two populations for one year (2017-2018). I was not a little surprised when I discovered big differences between them: the crustaceans in the cave were larger, the females sexually matured later and produced smaller but more numerous eggs. The cave population was phosphorus limited and had a higher trophic position and a omnivory more pronounced than that on the surface. The results were published in 2020 in the journal Zoology.

These findings inspired me to develop a theoretical model of how rivers flowing through caves work. In collaboration with fellow mathematicians and my former boss at the Rivers Lab, I published the Epigea-Hypogean Continuum model in 2021 in the journal Aquatic Sciences.

This model was the basis for a grant proposal, funded in 2022 by UEFISCDI, for a "Young Teams" grant. With funding of around €90,000, we collaborated with colleagues from INCDSB and the Institute of Speleology "Emil Racoviță" for water chemistry analysis and field work. The complex analyses, carried out by colleagues from Koblenz-Landau University in Germany, included measuring nutrient content and stable isotopes carbon and nitrogen, essential for tracking mass fluxes in food webs. The project website can be accessed at here.

Sample collection work has been intensive, with campaigns every two months for one year (2022-2023). The Gaura Haiducilor cave, near Moldova Nouă, Caraș-Severin county, was ideal for testing the Epigea-Hypogean Continuum conceptual model because it is crossed by a stream that also has a surface course.

What is the main goal of your project?

The main goal of the project is to understand the food webs in rivers that have an underground route through caves. The first thing a person notices when walking along the water of such a river during the transition from daylight to darkness in the cave is the parallel decrease in density and diversity of aquatic invertebrates, as well as the amount of leaves, wood and fine-grained detritus (see Figure 1).

Figure 1. Variation in density and diversity at the cave entrance (S1-S3)

Also, the biofilm that forms on the stones is no longer greenish in colour, as can be seen on the surface, but greyish-brown, due to the disappearance of algae from this food component once the flowing water enters the cave. However, these visually easy to observe phenomena are more difficult to explain when we want to understand to what extent these basal resources (e.g. wood, leaves, biofilm) condition life in the cave. This requires gathering quantitative information to measure mass and nutrient flows from food to and through consumers, but with a food web approach.

S1, S2 and S3 are representative stations used for testing this conceptual model. Station S1 is located at the surface before the river enters the cave. Station S2 is located in the section of river just beyond the cave entrance, and station S3 is located deep in the cave, approximately 1 km from S2.

What types of basic energy resources are studied in this project?

Four broad types of basal resources are recognised, each with its own specificity. Firstly, the biofilm that develops on rocks. This biofilm on the surface is greenish in colour due to algae that produce photosynthesis and contain chlorophyll. In reality, it contains not only algae, but also a consortium of bacteria, fungi and protozoa. Taken together, this food resource is considered qualitatively superior to the other three types of basal resources, which in turn are grouped under the generic form of detritus, i.e. wood, leaves and fine-grained organic matter. The detritus is carried by floods into the cave, so its quantity decreases as we move away from the mouth of the cave downstream. The first type of detritus to fall on such a transect are wood, which is usually transported relatively short distances from the river cave entrance. Then follow the leaves and often only fine-grained detritus reaches the cave bottoms, which is usually transported in the water mass in an upstream-downstream direction.

Why is it important to understand the influence of these resources on food webs in streams draining into caves?

The answer to this question lies in the field of basic research. The main focus of aquatic ecology is on understanding the structure and functioning of aquatic ecosystems, including rivers. From their sources to their discharge into the sea, rivers are relatively well understood today. However, subterranean rivers are a widespread habitat type worldwide and, in some countries such as Romania, which has more than 11,000 caves, are very common.

What is the Epigean - Hypogean Continuum conceptual model and why is it important?

Two key theories have emerged in the last two decades: stoichiometric ecology and metabolic theory. I do not wish to explain here how these two fundamental concepts have been related to rivers flowing through caves; for details, I am forced to refer readers to a theoretical article published in 2021 in the journal Aquatic Sciences, where the Epigean - Hypogean Continuum model is explained at length. However, I can mention that, unfortunately, groundwater ecology lags far behind zoology, which still prevails in the national and international scientific world in the study of these ecosystems. How life is structured underground and how it is influenced by the surface are aspects that are not well understood at the moment. The subject has been addressed in recent decades, but the results are not sufficient to form a true global picture. For this reason, several working hypotheses have been tested.

What are the main working assumptions of the project?

  • Working hypothesis 1: Reducing the quantity, quality and diversity of basal resources as the river flows from surface to subsurface leads to decreased nutrient fluxes through food webs.
  • Working hypothesis 2: The reduction in the quantity, quality and diversity of basal resources as the river flows from the surface to the subsurface leads to decreases in the abundance, biomass and secondary production of invertebrate communities.
  • Working hypothesis 3: Reduced basal energy inputs lead to increased trophic position, omnivory, size and degree of overlap of trophic niches for species with comparable body size on the surface-subterranean transect.
  • Working hypothesis 4: Bottom-up effects, exerted from the bottom up as the river flows from the surface to the subsurface, strongly alter the topology of food webs, which become shorter and with lower diversity at the base of the pyramid.

What impact could the results of this project have on our understanding of cave ecology?

The results are key to understanding cave ecology because the flow of rivers from the surface to the subsurface provides a natural gradient by which we can measure how declining basal resource stocks condition food webs and nutrient and energy flow. This project introduces a new, albeit seemingly simple, approach to measure for the first time the flow of nutrients and mass through surface and subsurface food webs within the same ecosystem: subterranean flowing rivers.

What are the practical implications of this project for the conservation of underground ecosystems?

Underground ecosystems are among the most endangered habitat types. However, the current paradigm of thinking is limited to the endangered species criterion, given their high degree of endemism. This project will contribute to the implementation of a complementary conceptual approach: species cannot exist without trophic interactions, so the protection of a cave in the future should also be thought of in terms of their role in ecosystem functioning, not just as fragile objects that should not be touched because of their high degree of endemism.

What discoveries have been made about cave-dwelling creatures and how are they influenced by their underground environment?

As this approach was based on intensive field sampling campaigns, on average one every two months, the results are preliminary at this stage. However, a preliminary result indicates that the main influence on the condition of life in the studied cave is triggered by a decrease in carbon stock, not necessarily nitrogen or phosphorus. In other words, the quantity and diversity of basal resources decreases significantly as the stream flows through the cave, but their quality (measured as nitrogen and phosphorus concentrations) remains similar. Moreover, trophic interactions in the cave were strongly conditioned by this phenomenon. Currently (June 2024), we are working on a first manuscript based on data collected from the field.

What are the biggest challenges the research team encountered during the course of this project and how were these obstacles overcome?

The biggest challenges faced by our research team were the complex and time-consuming administrative issues. We hope that in the future these processes will be streamlined so that research staff can devote more time to actual scientific activities and less to administrative tasks.

How can the findings of this project be applied to other areas or sectors, such as biodiversity conservation or natural resource management?

Underground aquatic ecosystems are increasingly affected by anthropogenic activities such as overexploitation of water resources, pollution and land use, with detrimental but poorly understood effects on hypogeous biodiversity. With this in mind, we stress that future biodiversity conservation projects would be more effective if they included this integrative food web approach in their agenda.

What are the next steps of this project and what scientific challenges or questions will be addressed in the future?

The next steps of this project will be to publish the results of the current project and to continue the research in future projects. One of the crucial environmental factors involved in conditioning subsurface life and nutrient fluxes is the content of dissolved nutrients in the water, such as the chemical species carbon, nitrogen and phosphorus. Thus, a logical progression for the future is to apply the working hypotheses outlined in the current proposal to several aquatic caves, covering gradients of water nutrient loading, ranging from low, medium and finally high (e.g. organically polluted) caves. Extending the study from a single cave to multiple water caves will allow a deeper understanding of this conceptual model.