Morphology and distribution of two brackish diatoms (Bacillariophyceae): Cyclotella atomus Hustedt and Thalassiosira guillardii Hasle in the estuary of Urdaibai (northern Spain)

The morphology of Cyclotella atomus and Thalassiosira guillardii has been s tudied by means of light and scanning electron microscopy from samples take n in the estuary of Urdaibai (northern Spain). These two centric diatoms re ach high densities in the upper part of the estuary when river discharge is low enough as to permit the development of phytoplankton blooms. Cyclotell a atomus was observed to form short chains during bloom conditions and show ed some degree of morphological variation of the valves. Valves of T. guill ardii showed a considerable variability in the number and disposition of bo th central and marginal fultoportulae. The distribution of these two specie s along the longitudinal axis of the estuary appears to be strongly influen ced by physical factors, mainly salinity and river discharge.