Permanent salt evaporation ponds in a semi-arid Mediterranean region as model systems to study primary production processes under hypersaline conditions

A change from planktonic to benthic primary production was observed along an increasing salinity gradient in a permanent solar saltpan sequence (Las Salinas del Pinet) in a semi-arid Mediterranean region (Spain). The biomass and photosynthesis of the water column decreased greatly when salinity increased, while benthic production increased when cyanobacteria crusts developed. This produced a change from autotrophic to heterotrophic in water column productivity. However in the benthos, the changes from heterotrophic to autotrophic were seen throughout the pond sequence. Changes in phytoplankton composition in the studied saltern appeared more influenced by phytoplankton salinity tolerance since 88.0% of variation in the photosynthesis rates and 76.0% in the respiration rates were negatively and positively explained by increased salinity, respectively. However the changes in the benthos composition did not appear to be highly influenced by the benthos salinity tolerance since only half of the variation in the photosynthesis rates were explained by increased salinity. A lack of correlation between the respiration and photosynthesis benthic values can be explained by the oxygen consumed by heterotrophic organisms and by chemical demand. The nutrients system varied seasonally in this saltern, and this variability appeared to relate to the occurrence of the evaporative concentration of water and bacterial activity. This study highlighted that stressors such as water salinity should be considered in future hydrological management plans in order to preserve water resources, especially in warmer and drier climates.