Chlorophyll a, nutrients and phytoplanktonic community in a continental ecosystem highly influenced by marine waters

The Júcar River, characterized by a very irregular freshwater flow, discharges into the Spanish Mediterranean coastal waters. However, the flow at its mouth is usually insignificant due to the overexploitation of upstream waters, except during intense rain periods. The presence or absence of freshwater generates two different scenarios/conditions in this ecosystem. The vertical gradient of salinity due to freshwater flows generates differences in nutrient content and phytoplankton composition and abundance along the water column. Nutrients, in general, increase their concentration in the superficial layers due to the river inputs. However, orthosilicic acid, nitrite and nitrate decrease with depth and when getting closer to the river mouth. The main freshwater phytoplankton groups are Bacillariophyceae, Chlorophyceae and Cyanobacteria. Whether or not freshwater flow reaches the estuary, the water column is almost entirely formed by marine water. Under this condition, nutrient concentrations are lower, and Cryptophyceae, Prymnesiophyceae, Dinophyceae and picocyanobacteria dominate phytoplankton composition. This paper presents the results of two campaigns with different environmental conditions (with and without freshwater flow respectively). Physicochemical and biological variables were studied at three stations placed along the estuary. ► The Júcar River is characterized by a very irregular flow. ► The flow at its mouth is usually insignificant except during intense rainy periods. ► The presence or absence of freshwater generates two different scenarios. ► With freshwater flow there are vertical gradients of salinity and nutrients. ► The river inputs change phytoplankton composition and abundance along water column.