Analysis of hydrobiological pattern in the Bizerte lagoon (Tunisia)

Abstract

This study deals with the use of a multivariate analysis method in order to understand the functioning of the Bizerte lagoon ecosystem. A set of hydrobiological parameters was collected during 10 monthly campaigns in 2004. A variant of the EOF (Empirical Orthogonal Function) technique is used to examine hydrobiological variability modes in this lagoon. The permanent features characterising the lagoon are obtained by applying the EOF technique to data after removing the annual cycle. Two major modes were obtained. The first one shows a contrast between the northern sector of the lagoon, influenced by the Mediterranean Sea, and the southern one influenced by continental waters. This mode is mostly prominent for temperature and salinity. The second mode shows a contrast between shallow waters along the coastal zones and the deep central area of the lagoon. This contrast is more pronounced towards the southwestern edge of the lagoon, a region of important freshwater input from the Tinja channel, which drifts towards the downstream area of Menzel Bourguiba. This second mode may also correspond to northward water circulation under southerly wind blowing frequently in summer. The spatial and temporal variability in the lagoon is highlighted by applying the EOF to the raw data without removing the annual cycle. A weight series is obtained for each variable by extracting the corresponding data and then projecting them on the corresponding eigenvector. Using this technique, only one mode was found dominant with more than 52% of the total variance. It was also found that the north-south contrast of temperature, salinity and phosphorous and the coast-centre contrast of Chlorophyll a are enhanced in summer. Other variables such as ammonia and nitrate also show significant annual cycles but with local patterns.

Distribution of resting cysts of the potentially toxic dinoflagellate Alexandrium pseudogonyaulax in recently-deposited sediment within Bizerte Lagoon (Mediterranean coast, Tunisia)

“…In the Bizerte lagoon, water current circulation is mostly dictated by the predominating west-northwestern wind, which has an influence all throughout the year (Harzallah, 2003;Bejaoui and Harzallah, 2005;Bejaoui et al, 2008;Bejaoui 2009). Surface currents are strong at the northern, western and southern extremities of the lagoon, whereas they are relatively weak in the central part of the lagoon, which explains the accumulation of fine sediments in this part of the lagoon (Bejaoui, 2009).…”

“…At the bottom of the lagoon, where the influence of the surface wind is suppressed, the direction of the water flow tilts to the north-west direction and passes through the central part of the lagoon. An important feature of the water circulation in the Bizerte lagoon is the presence of three circulation gyratory cells, sited in the northern, southern and western sections of the lagoon (Harzallah, 2003;Bejaoui et al, 2008;Bejaoui, 2009).…”

“…It has been also suggested that that the mismatch between (Gracia et al, 2013). Previous studies (Harzallah, 2003;Bejaoui and Harzallah, 2005;Bejaoui et al, 2008;Bejaoui, 2009) indicated that Bizerte lagoon is characterized by low degrees of hydrodynamic energy at the bottom and mid-layers of the water column. Energy transmitted to the water layer contiguous to the sediment surface might not be high enough for resting cysts to be re-suspended out of the sediment in these parts of the lagoon.…”

“…Surface water currents are important, but, in the central and the deepest areas of the lagoon, the wind influence and surface currents weaken and the direction of water flow is reversed towards the northwest direction through the central portions of the lagoon (Harzallah, 2003;Bejaoui et al, 2008;Bejaoui, 2009). The water currents map for the lagoon highlights a two wind-current gyratory flow sweeping the entire western part of lagoon, which could promote re-suspension and spread of RCs and could explain the homogeneous pattern of cyst distribution observed in the western part of lagoon (Fig.…”

Do the levels of industrial pollutants influence the distribution and abundance of dinoflagellate cysts in the recently-deposited sediment of a Mediterranean coastal ecosystem?

“…It's economically important, holding nine shellfish farms (Mytilus galloprovincialis, Ostrea edulis and Crassostera gigas). In the Bizerte lagoon, the surface current is stronger than the bottom current at the northern, western and southern parts and is relatively weak in the central part of the lagoon (Bejaoui et al, 2008). The phytoplankton assemblage is mainly represented by 51 diatom taxa and 31 dinoflagellate taxa (Bellakhal-Fartouna and Daly Yahia-Kéfi, 2004).…”