Chlorophyll a and phytoplankton maximum at the halocline of Ebro River estuary

Abstract

The Ebro River flows into the Mediterranean coast of Spain and its last stretch behaves most of the time as a highly stratified estuary. Four field campaigns were carried out during years 1999-2000 to study water quality within the estuary. In this paper, the results of two of these field campaigns are shown. These results are based on the development of a new sampling technique, which allows obtaining samples at the halocline with a high resolution. As a consequence, concentration peaks for different nutrients (ammonium, phosphorus, and orthosilicic acid), chlorophyll and some phytoplankton groups could be observed at the interface area. In the summer samplings, chlorophyll peaks showed two different patterns at the halocline along the estuary. At the stations located close to the estuary head, these peaks were always found in the shallower zone of the interface, above of the observed peaks of ammonium, soluble reactive phosphorus (SRP) and orthosilicic acid. This chlorophyll accumulation in the shallower zone seems to come from surface layer phytoplankton settling, being temporally retained there due to the sharp increase of fluid density. The second pattern is observed close to the mouth, where chlorophyll peaks spatially coincide with those of nutrients or even are located below these, suggesting a growing zone due to nutrient abundance. Phytoplankton counts confirm this hypothesis since peaks of multicellular prokaryotes, diatoms and chlorophyceae are observed at the halocline, with greater densities in the freshwater layer than in the saltwater one, indicating that this accumulation comes from the surface layer. Nevertheless other groups such as unicellular prokaryotes and cryptophyceae presenting larger concentration at the halocline deeper layer than in the shallower one seem to employ the interface as a proliferation zone taking advantage of the more favourable nutritive conditions due to mineralization occurring there.