Abstract ID: 3.12947 | Withdrawn | Talk/Oral | TBA | TBA

The high-mountain alpine lake system of the Pyrenees comprises hundreds of soft-water, oligotrophic, shallow lakes that support a rich aquatic flora. Historically, these vegetated lakes—located between 1,600 and 2,500 m a.s.l.—were largely unaffected by human activities related to catchment land use. However, studies over the past decade indicate on-going global changes that may impact aquatic biodiversity. Building on a 1987 study that analyzed the flora of shallow Pyrenean lakes, this research examines changes in macrophyte biodiversity in thirty high-mountain lakes by comparing semiquantitative inventories from fixed transects over time. Our aim is to unveil potential biodiversity shifts and relate them to ongoing environmental changes. To achieve this, we assessed changes in species richness and abundance, as well as species-specific trends, across two time periods: an initial survey in 1987 and a follow-up conducted be- tween 2017 and 2022 (30 to 35 years apart). These community changes are analyzed in relation to concurrent shifts in lake water chem- istry while also evaluating the potential influence of introduced minnows (Phoxinus sp.). Our results reveal overall stability in regional macrophyte richness but notable shifts in species abundance and composition at the lake level. Oligotrophic mosses and floating-leaved macrophytes have declined, while some natopotamids, typically associated with more nutrient-rich waters, have expanded. These trends align with regional increases in conductivity and alkalinity, along with an overall reduction in nitrogen availability. Macrophyte community shifts were more pronounced in lakes where conductivity exceeded a threshold of 20 μS/cm. Contrary to our expectations, no direct link was found between minnow presence and water chemistry changes. However, a (non-significant) trend toward greater in- creases in conductivity was observed in lakes with Phoxinus sp., suggesting that these fish may act as exacerbators of regional trends rather than as the primary driver of change. These findings underscore the importance of long-term monitoring of plant communities to better understand the ecological consequences of environmental change in these pristine ecosystems. They also illustrate how biodiversity serves as an integrated record of shifting environmental conditions over time.