Limnetica 36

Coastal lagoons of the Ebro Delta (Catalonia, Spain) have been subjected to historical reductions of their hydrological connections with the sea and to freshwater discharges from rice agriculture, with a more recent switch to freshwater inputs from the Ebro River to improve their trophic state. As a result, the seasonal salinity regime in three formerly connected coastal lagoons varies, being polyhaline to oligohaline from May to November but polyhaline in winter. We evaluate the effects of current water management on the abundance and community structure of phytoplankton and zooplankton within the three lagoons and assess the contribution of environmental variables in February and August. Temporal variation evidenced the typical increase in phytoplankton and zooplankton abundance from winter to summer. However, contrasting August salinities among lagoons showed opposite effects in the two communities, with higher abundance of total zooplankton and lower abundance of total phytoplankton at higher salinities. Besides, each lagoon displayed distinctive groups of taxa and showed important temporal differences. Temperature and salinity were the most important factors determining the structure of assemblages (ca. 60% and 72% of phytoplankton and zooplankton variance, respectively). Compared to the period of agricultural runoff ca. 30 yrs ago, our results show an important decrease in the overall abundance of phytoplankton (>50%), whereas zooplankton abundances are ca. 6 times higher. However, environmental variability associated with the salinity gradient appears to be responsible for observed changes in the abundance and structure of planktonic assemblages.