Study on the preferential vertical distribution of phytoplankton in Alfacs Bay (NW Mediterranean)

Abstract

Phytoplankton was sampled at various depths during 24 hours in the estuarine Alfacs bay (NW Mediterranean Sea, Spain) to determine the preferential vertical distribution of 8 species representative of microplankton assemblages at this location (4 dinoflagellates, 2 coccolithophorids and 2 diatoms). The hypothesis that diel vertical migrations are undertaken was tested for each target species. Results indicate that vertical distribution is primarily determined by the superimposition of two water masses (upper estuarine mixed layer and bottom marine layer) and by the oscillation of the pycnocline as an interface between them. Cell concentrations were maximal close above or below the pycnocline for most species. 2 out of 4 dinoflagellates showed evidence of upwards migration across the pycnocline at night, a pattern that contrasts with documented dinoflagellate diel vertical migration patterns in literature. 1 dinoflagellate, 2 coccolithophorids and 1 diatom showed downwards displacement of cells below the pycnocline at only one sampling time at night that coincided with a short-lived peak of fluorescence below the pycnocline. The potential effects of a surface freshwater runoff observed at night upon the aforementioned migrations and the importance of environmental gradients on the establishment of diel vertical migrations are discussed. It is hypothesized that the lack of a nutricline in the bay at this time of the year could have prevented dinoflagellate cells from displaying the nighttime descent typical of motile dinoflagellates in water-stratified environments