Anthropogenic forcing in wetlands: a case study in NW Iberia

Abstract

Wetlands are exceptional ecosystems that contribute to biodiversity and play a key role in the hydrological cycle. Knowledge of their ecology is essential for proper conservation and management, but environmental policies still do not consider long-term ecological studies. Here we present the application of multi-proxy analyses to a 115 cm-thick core from La Molina mire (Alto de la Espina) located in NW Iberia, with a chronology spanning from the local Iron Age (ca. 2500 cal BP) to the present. The mire is located in an area intensively mined for gold during the Roman period, and close to a water-canalization system used for mining operations at that time. Our aim was to get insights into the development of the wetland by combining hydro-hygrophytes, non-pollen palynomorphs and geochemical analyses, supported by 14C datings and multivariate statistics (PCA). The results indicated a complex pattern of ecological succession. First, the data suggest the wetland was a minerogenic mire in the Iron Age. After that, and since Roman times, it was subjected to dramatic hydrological changes due to the rise of the water-table, fluctuating between the presence of open water and several phases of drawdown. Finally, by ca. 1350 years ago, the wetland experienced a rapid evolution towards ombrotrophy. High grazing pressure was also detected for the last 100 years. The significant change occurred during Early Roman Empire seems to have been the consequence of the direct use of the wetland as a water-reservoir of the canalisation system used for gold-mining. Thus, the current nature of the mire may be the result of an anthropogenic forcing; although multiple anthropogenic and natural causes were potentially linked to the shifts detected, which at least in three phases supposed a threshold response. Our investigation suggests that palaeoecological research is necessary to understand the origin of wetlands ecosystems, their long-term ecology and the role played by human activities.