The role of salinity in the selection of biological traits of freshwater invertebrates

Salinity exerts significant pressure on freshwater faunas living in slightly salted water (i.e. water with salinity < 3 g L-1). Changes in seven biological and physiological characteristics of freshwater taxa belonging to natural invertebrate communities observed along a salinity gradient were investigated. Benthic macroinvertebrates were sampled from four stations on the Meurthe River, a six-order stream in northeastern France with a gradient ranging in concentrations from 0.21 g L-1 to 2.60 g L-1 of total dissolved solids over a distance of only 19 km. A fuzzy coding of seven traits allowed us to quantify the potential physiological requirements or biological traits of each species or genus observed. The consideration of whole assemblages highlighted trends in the use of each modality by communities along the salinity gradient. A change in many invertebrate traits was observed. Ovoviviparity was important at the station with the highest salinity, the internal development of eggs in females most probably contributing to a better protection of the young. In contrast, this reproductive type, leading to physiological constraints and limited fecundity, was less adopted in less salted sites. A modification in benthic community feeding habits along the salinity gradient was also discovered. An increase in deposit feeders at the expense of scrapers and piercers was observed. This pattern is in accordance with an energy transfer in the ecosystem from water column (i.e. suspended organic material) to river bed (deposited organic material) along the salinity gradient.