Limnetica 40

Wetlands act as nutrient sinks or sources playing a key role in downstream water quality and ecosystem productivity. Despite their importance, wetlands are highly threatened ecosystems and biogeochemical processes are changing in response to several external and internal constrains; however, these functional alterations have not been evaluated in the long term. In this paper we assess the nutrient dynamics (TOC, TN and TP) in Las Tablas de Daimiel in the long-term, through an input-output study using nutrient concentrations and flows measured monthly in the wetland from October 1980 to December 2018. Considering the entire period, on an annual basis Las Tablas de Daimiel was a net source of TOC (467 % of TOC inputs were exported) and net sink of TN and TP (83 % of TN and 92 % of phosphorus were retained and transformed in the wetland). For the longer period of records, mean export loading of TOC was 2177 g C/m²/y, while retention of TN and TP were 62.8 g N/m²/y and 5.7 g P/m²/y, respectively. However, since the 1980s nutrient concentration and nutrient import and export trends have changed dramatically as has their seasonality. High TOC export in Las Tablas are probably related to the short circuit existing in the hydrological functioning of the wetland after the 1980s in which outflows are usually disrupted to extend the wetland inundation for longer. TN and TP retentions begin to show signs of exhaustion but currently it is not possible to conclude if the wetland sediments have reached nutrient saturation. During the last 38 years the wetland has undergone similar environmental scenarios several times but the biogeochemical response of the system has never been repeated. This means the time scale of the environmental interactions and the ecosystem responses are unpredictable, and therefore long-term observations are key to understanding how nature will respond to upcoming global change.