Effects of dilution stress on the functioning of a saline Mediterranean stream

The effects of seasonality and dilution stress on the functioning of Rambla Salada, a hypersaline Mediterranean stream in SE Spain, were evaluated. The stream is subject to diffuse freshwater inputs from the drainage of intensively irrigated agriculture in the catchment and periodic losses of water through an irrigation channel. Metabolic rates and the biomass of primary producers and consumers were estimated over a 2-year period. During the first year several dilution events occurred, while during the second year the salinity recovery reached predisturbance levels. Functional indicators were compared in the disturbance and recovery salinity periods. Primary production and respiration rates in the Rambla Salada ranged between 0.07-21.05 and 0.19-17.39 g O-2 m(-2) day(-1), respectively. The mean values for these variables were 7.35 and 5.48 g O-2 m(-2) day(-1), respectively. Mean net daily metabolism rate was 1.87 +/- A 0.52 g O-2 m(-2) day(-1) and mean production/respiration ratio was 2.48 +/- A 1.1, reflecting autotrophic metabolism. The metabolic rates showed the typical seasonal pattern of Mediterranean open canopy streams. Therefore, gross primary production (GPP) and ecosystem respiration (ER) registered maximum values in summer, intermediate values in spring and autumn and minimum values in winter. The metabolic rates and biomass of consumers were greater in the disturbance period than in the recovery period. However, they did not show significant differences between periods due to their important dependence on seasonal cycle. Seasonality accounted for much of the temporal variability in GPP and ER (76% and 83% in the multiregression models, respectively). Light availability seems to be the most important factor for GPP and ER in the Rambla Salada. Autotrophic biomass responded more to variations in discharge and conductivity than to seasonal variations. In fact, it was severely affected by freshwater inputs after which the epipelic biomass decreased significantly and Cladophora glomerata proliferated rapidly. Epipelic algal biomass was the most sensitive parameter to dilution disturbance.