Abstract
The dial vertical migration is a vertical movement characterizing motile phytoplankton species. Some studies suggest that it is a positive photo-tactical response (Blasco, 1978) which only depends on light intensity others think that this migration represents an internal circadian rhythm (Jephson and Carlsson, 2009) which can be influenced by several factors including: physical factors mainly represented by temperature and chemical factors that are mainly nutrients, salinity and dissolved oxygen.
To better elucidate the vertical and circadian behaviour of phytoplankton and particularly of the species Planktothrix agardhii related to its environment, a campaign extending over 24 hours was carried out on 30 and 31 May 2012 in the dam reservoir Ghzala (Northern Tunisia) mainly operating for irrigation and fish farming.
Sampling took place at the intake tower of the dam with constant time step (4 hours: 12H, 16H, 20H, 00H, 04H, 08H)) and bathymetric step (1m over the entire water column from surface to bottom (16m). This sampling focused on the physical, chemical and biological parameters.
Oxygen, temperature and chlorophyll a parameters were measured in situ for 24 hours and throughout the water column using a multiparameter (model Lowrance HDS7). The nutrients were analysed in the laboratory using an auto analyser (Model AA3 Bran and Luebbe). Identification and enumeration of phytoplankton (Planktothrix agardhii) was carried out under an inverted microscope according to the method Utermohl (1958).
The analysis of circadian profiles of phytoplankton abundance shows that the 0-6m deep water section is the most prolific area where appears short movements top-down and bottom-up of the microflora, mainly represented by Cyclotella ocellata and Planktothrix agardhii with a very noticeable surface inhibition. The results of the statistical analysis using generalized linear models suggest that the vertical distribution of P. agardhii abundance depends on the bathymetry, temperature, pH, turbidity as well as orthophosphates and nitrates levels. Note also that a significant correlation was observed between chlorophyll a and phytoplankton abundance.