In the depths of the limestone, the source of a river

As part of Guy Fleisher's artistic residency, inspired by the natural soundscape of the River Lis in the Leiria region of Portugal, we've organised a discovery trail following the course of the river, to get a better understanding of the region's water cycle. We're taking you on a journey through April showers and sunny spells, to discover a contrasted geological and climatic ecosystem, marked by Oceanic and Mediterranean influences.

Our excursion begins, not as we might expect on the banks of the river, but in the mountains, and more specifically in the network of caves in the Serras de Aire and Candeeiros massif. Here we meet Danilo Guimarães, our guide at Grutas da Moeda, who explains that the Lis, like the surrounding rivers (Lena, Alviela), originates in the mountains, in huge freshwater reservoirs formed by the infiltration of rainwater. It is only further upstream, below the mountain, that this water rises from the ground to form the sources of the rivers.

Rain water, slightly acidified by carbon dioxide, dissolves the mountain's limestone rock, first creating lapiás ¹, then infiltrating and creating networks of underground caves. Over millions of years, a gigantic network of karstic galleries ² is formed at different levels, the lowest reaching depths of 300 m. It is at these depths that all the rainwater accumulates, forming one of the largest underground reservoirs of freshwater in Portugal and the Iberian Peninsula. During the dry season, the water remains at depth. During the rainy season, the pressure becomes too great, and the reservoir expels its surplus water through the cave systems until it resurfaces, giving rise to the surrounding rivers.

Mural paintings dating back to the Palaeolithic period bear witness to 40,000 years of human activity in the caves. We can assume that the caves were a refuge from the elements and wild animals. The temperature is very stable, around 17°C all year round, and the air is pure. The limestone rock, which naturally has a remarkable ability to absorb sound, envelops us in its precious silence. When you enter the caves, you feel as if you're in the earth's den, in a completely protected cocoon. Some of the cave walls are covered in calcium carbonate, creating calcite structures. Danilo points out the poetic dimension of this fact, water deprived of light that concentrates into crystals, themselves known for their energy and light-conducting properties.

The water in liquid form is also stored in excellent quality. At this stage of the hydrological cycle ³, the lack of light prevents the proliferation of algae. The water is highly concentrated in minerals and free from contamination. This is the same water that is drilled by the local water company to supply the city of Leiria. Exploring these caves gives us a better understanding of where the water we drink comes from. We go back to its origins to appreciate its sacred dimension and consider the disturbances that disrupt its cycle today.

Our journey continues at the Hydraulics, Resources and Environment Laboratory of the School of Technology and Management (ESTG) of the Polytechnic University of Leiria. Here we meet André Fonseca, the laboratory technician who organises various field projects to monitor the ecological state of the River Lis. André talks to us in particular about his work on benthic macroinvertebrates ^{4 5}.

These aquatic organisms include insects (larvae/nymphs and adults) as well as molluscs, annelids and crustaceans. They live in watercourses, on and in sediments, and their sensitivity to changes in environmental quality make them excellent bioindicators of water quality. Using a simplified approach, they can be classified into three groups according to their ability to tolerate water pollution: sensitive to pollution (group I), somewhat tolerant (group II) and tolerant to pollution (group III).

Our meeting with André took place in two stages: first we went with him on a field trip to take benthic samples from the banks of the River Lis, then we went to the laboratory to identify and document the macroinvertebrates collected in the samples. The distribution of the organisms we found according to their “sensitivity” to pollution enabled us to make an initial assessment of the quality of the water.

A majority of group III organisms means polluted water, while a greater diversity of organisms and the presence of group I organisms indicates better quality water. In our case, we found a majority of group II organisms, but also some “macros” from group I and III. The abundance and diversity found indicates us that the river Lis, at the sampling point, had moderate to good water quality.

We also found organisms that colonize different microhabitats and from various feeding groups. The trophic distribution ⁶ is also relevant because it provides insights into the ecological roles and interactions within the ecosystem, helping us understand the overall health and stability of the aquatic environment.

For example, if most organisms are predators (e.g. caddisflies of the Rhyacophilidae family), this might indicate a disruption in the balance between different feeding groups, suggesting a low productivity ecosystem ⁷. Additionally, when the water is cleaner, greater diversity means greater competition for food and therefore a better-controlled predator population. Another example is that if we notice the dominance of group III collectors like blackfly larvae in an ecosystem, this might be a strong indicator of organic pollution, eutrophication ⁸, and low oxygen levels, since these organisms thrive in waters rich in decomposing organic material, which serves as their primary food source.

In the past, the Lis was known to the locals as the rio lixo (‘rubbish river’) essentially because of the piggery wastewater discharges. The water quality in the Lis river basin still shows serious problems of pollution, mainly due to organic matter, nutrients like organic nitrogen, nitrites, nitrates and phosphorus, and fecal contamination ⁹. However, gradual upgrades in the wastewater sewage systems and treatment facilities, environmental protection laws and awareness-raising among producers have reduced water pollution, which is now of better quality.

Benthic macroinvertebrate assessment has been used to monitor water quality for a long time. Citizen science ¹⁰ projects, such as the Riverfly Monitoring Initiative, have been using volunteers to monitor the health of rivers and streams based on the macroinvertebrate population. One of these groups, the Citizen Crane project in south-west London, organises monthly sampling campaigns to identify the macroinvertebrates present in the river. A significant reduction in certain microorganisms is a reliable indication that can help identify a pollution incident. These kinds of citizen initiatives help to broaden the scope of scientific research while involving the public in environmental, social and health issues.