Sedimentological evidence of European spread of hypoxia in freshwaters caused by local anthropogenic pressures

The development of seasonal or persistent hypoxia in many aquatic environments severely stresses ecosystems, causing a decline of fisheries, a loss of biodiversity, and an alteration of food webs, including mass mortality of... [ view full abstract ]

The development of seasonal or persistent hypoxia in many aquatic environments severely stresses ecosystems, causing a decline of fisheries, a loss of biodiversity, and an alteration of food webs, including mass mortality of fauna. In lakes, long-term instrumental monitoring surveys remain limited, preventing a regional reconstruction of hypoxia’s dynamic and pinpointing long-term causes of these changes in Europe. Nevertheless, hypoxic conditions are recorded in lakes when varve sediments start to be preserved once thresholds in oxygen-depleted conditions are crossed. Here, we compiled i) the records of 959 dated lake records across Europe and ii) the time when varves started to be preserved in varved lakes as an indication of the regional evolution of hypoxia over the last 2,000 years. Then, we compared these data with anthropogenic and environmental variables compiled for each of these 959 watersheds. Our results show that 108 sites were naturally hypoxic during the last 2,000 years but that afterward continental hypoxia started spreading in Europe around AD 1900, mainly because of local growth in population density, human footprint and land uses, leading to eutrophication. No significant correlation was found with changes in precipitation or temperature. Furthermore, no sign of general return to past well-oxygenated conditions are observed despite implementation of local restoration programs and implementation of policies limiting nutrients yields since several decades in Europe. This highlights the low resilience of lacustrine systems in the context of the added likely stress due to global warming and population increase.