Spring aquatic plant survey
In spring 2024, our plant specialists, Udo and Claudia Schwarzer, conducted the aquatic plant survey at Paul de Toirões to:
- Evaluate plant species and communities at the intervention site.
- Access species abundance and distribution throughout the quarry.
- Analyse water quality.
- Compare data collected last autumn with the data collected in spring.
Both the autumn 2023 and spring 2024 assessments confirmed the presence of several red-listed and rare species, including Utricularia australis, Potamogeton trichoides, Littorella uniflora, and Nitella mucronata - a macroalga not recorded in Portugal since 1942. Comparing with results from the Autumn survey, in spring we found a few more red-listed species to add to our list: Isoetes histrix, Isoetes setaceum and Elatine macropoda.
In addition to this, water samples were taken and analysed for various parameters, including pH, conductivity, ORP, temperature, carbonate hardness, phosphate, and chlorides. An ROV and divers also explored the Big Blue Lake to assess its depth and morphology.
The surprises of the Big Blue Lake
The results were far from what we had expected. Our partners at Rewilding Portugal had even placed friendly bets on the lake's depth, some estimating it might be close to 30 meters deep. Imagine our surprise when we discovered the lake is mostly 1.40 meters deep, reaching 2.40 meters in some parts.
We also uncovered the secret behind the lake’s distinctive white-bluish coloration. The lake's bottom is composed of white clay that forms a floating mud, which, along with the shallow depth, gives the lake its opaque hue. This lake also exhibited higher pH and conductivity than the other water bodies, likely due to the presence of chlorides. Chlorides are usually absent or low in natural waters, suggesting either anthropogenic pollution or extraordinary mineralization, possibly from thermal water. Confirmation of this hypothesis would require a drone equipped with a thermal camera to detect temperature anomalies in the lake water.
The invasive Louisiana Crayfish
Another significant discovery at the Big Blue Lake was the high density of invasive Louisiana crayfish. This could explain the near absence of rooted underwater plants, except for the floating carpets of Ranunculus peltatus (a species we had not yet found at the quarry), which can survive unrooted and withstand constant crayfish attacks.
These results were truly fascinating and continue to highlight the diversity of aquatic habitats that are present at the old quarry. The survey emphasised the ecological importance of Paúl de Toirões as a diverse and dynamic pondscape, home to rare species and essential natural processes. Despite some surprising findings, these insights are crucial and will serve as a foundation for our future conservation and rewilding initiatives.
Aquatic plant survey
Here is a selection of species found during the field visit in May 2024.