Trade-offs in plankton species richness arising from drought: insights from long-term data of a National Park wetland (central Spain)

Wetlands are considered vulnerable ecosystems of both high species richness and socio-economic value. In semi-arid regions, these ecosystems often experience long drought periods that are usually aggravated by local water overexploitation. Drought leads to: (i) reduced flooding area, (ii) isolation of water bodies, (iii) increased areas of dry sediments and shoreline length, and (iv) increased ionic concentration. These processes affect aquatic populations in a species-specific way and can have antagonistic effects on taxon richness. Here, we highlight long-term (1997-2008) trade-off effects on plankton species richness linked to drought in a semi-arid wetland (Las Tablas de Daimiel National Park, central Spain). Annual average phytoplankton species richness increased from wet- (1997-1998) to dry years (2001-2002) and taxon richness diminished again when drought was more severe (2007-2008). Zooplankton changes were more complex depending on taxonomic groups and the body size of the organisms, total species loss being related to hydrological conditions. Half of the algal species recorded in 2007-2008 and one-tenth of total zooplankton taxa were new comers in the wetland, because salinization, eutrophication and submerged macrophytes occurring in different sites enhanced species turnover and mitigated homogenization of beta diversity. Maintenance of one water body with a constant water level and macrophytes was the key to preventing the collapse of plankton richness. Our study has demonstrated that plankton can be very useful for tracking environmental changes of wetlands, thus giving the environmental manager another tool to enhance the conservation of wetlands and their biota.