Asma Hamza

Copepod community structure related to environmental factors from a summer cruise in the Gulf of Gabès (Tunisia, eastern Mediterranean Sea)

We studied the summer spatial distribution of the copepod community in both the neritic and oceanic areas of the Gulf of Gabes (Tunisia, eastern Mediterranean Sea) coupled with environmental factors. Copepods were the most abundant zooplankton throughout the sampling period, contributing 7 8% of the total zooplankton. A total of 14 copepod families were identified in all stations, with an overwhelming abundance of Acartiidae and Oithonidae ( 39 .0 5 and 39 . 09 % of total abundance, respectively). Abundance of Acartia clausi and chlorophyll-a concentrations were negatively correlated with salinity, suggesting that this species probably escaped the high coastal salinity ( 3 8 psu). Significant correlation determined between A. clausi and tintinnids at 5 0 m isobaths indicates that these planktonic ciliates probably served as a substantial food link towards higher trophic levels of this area. Conversely, Oithona nana which was well adapted to high chlorophyll-a concentrations and high salinity along the coast, showed significant correlations with Dictyochophyceae, Dinophyceae, Bacillariophyceae and Euglenophyceae, suggesting that this small copepod was capable of feeding on a wide selection of phytoplankton preys.

Variability in the Structure of Planktonic Microalgae Assemblages in Water Column Associated with Posidonia oceanica (L.) Bed in Tunisia

Patterns of phytoplankton in areas with seagrass (Posidonia oceanica) and areas without seagrass were quantified in the coast of Chebba (East of Tunisia). Replicate samples were collected at August 2011, from four stations (separated by 500 of metres). The diversity of phytoplankton was high around P. oceanica meadows compared to area without seagrass. A possible explanation to this finding is that water motion and hydrodynamics forces cause leaves agitation allowing the passage of epiphytic species in the water column. Our results also show an increase of abundance of potentially toxic dinoflagellates around Posidonia bed such as Alexandrium minitum, Amphidinium carterae, Karenia selliformis, Coolia monatis, Karlodinium veneficum, Ostreopsis ovata, Prorocentrum concavum, P. minimum, P. rathymum, and P. lima. Installation of fish farms on Posidonia beds should be avoided, not only to preserve this vulnerable habitat, but also to avoid fish contamination by toxic species derived from the resuspension of epiphytic community on seagrass substrata to the water column.

Long-term microphytoplankton variability patterns using multivariate analyses: ecological and management implications

Microphytoplankton data along the southern Tunisian coasts have been monitored weekly since 1995. This study used the data collected during the period 2000–2007 to determine the temporal variability patterns and to characterize the geographical structures of microphytoplankton populations. The methodological approach consisted in isolating the temporal variability common to all sampled sites and in analyzing the “residual” site-specific component. Multivariate ordination methods were used to determine the seasonal and interannual variability. The temporal pattern shared by all sites was highlighted and the seasonal cycle of the most frequent phytoplankton communities contrasted the diatoms winter-spring species to the dinoflagellates summer-fall species. Multitable comparison allowed the identification of three areas exhibiting similar phytoplankton population variability. The results, despite the coastal location of sampling sites, indicated that hydrodynamic and geomorphologic properties of the different areas as well as anthropogenic activities play a key role in the structure of the phytoplankton communities.

A Comparison of Abundance and Diversity of Epiphytic Microalgal Assemblages on the Leaves of the Seagrasses Posidonia oceanica (L.) and Cymodocea nodosa (Ucria) Asch in Eastern Tunisia

We studied spatial patterns in assemblages of epiphytic microalgae on the leaves of two seagrass species with different morphologies and longevity, Cymodocea nodosa and Posidonia oceanica, which cooccur in Chebba in Eastern Tunisia. Epiphyte assemblages were described for each species in summer. Epiphyte microalgal assemblages were more abundant on the leaves of C. nodosa but more diversified on the leaves of P. oceanica. We suggest that the differences in species composition and abundance between those seagrass species may reflect an interaction of timescales of seagrass longevity with timescales of algal reproductive biology. Short-lived C. nodosa was dominated by fast growing species such as the cyanobacteria species Oscillatoria sp., while P. oceanica leaves were colonized by more mature and diversified species such as Prorocentrales. Local environmental conditions (hydrodynamics, light penetration), host characteristics (meadow type, shapes forms of leaves, life span, and growth rate), and grazing effect seem also to be responsible for these dissimilarities in epiphytic microalgae communities.

A Bayesian network approach to determine environmental factors controlling Karenia selliformis occurrences and blooms in the Gulf of Gabès, Tunisia

A Bayesian Network modeling framework is introduced to explore the effect of physical and meteorological factors on the dinoflagellate red tide forming Karenia selliformis in various sampling sites of the national phytoplankton monitoring program. The proposed models took into account the physical environment effects (salinity, temperature and tide amplitude), meteorological constraints (evaporation, air temperature, insolation, rainfall, atmospheric pressure and humidity), sampling months and sites on both Karenia selliformis occurrences and blooms. The models produced plausible results and enabled the identification of the factors that directly impacted on the species occurrences and concentration levels. The sampling sites dominated the species occurrences. The models show that the relationship between salinity and Karenia selliformis is more apparent when the species concentrations are focused on and that the bloom occurrences can be predicted based on salinity. Concentrations up to 105 cells L-1 were recorded when salinity exceeded 42.5 and dominated the shallow and weak water renewal areas.

Variability in the Structure of Phytoplankton Assemblages in relation to Human Disturbance in Southern Coast of Tunisia

We investigated the impact of industrial effluents on phytoplankton assemblage in southern Tunisia (Skhira). We specifically addressed changes in microalgae composition caused by this anthropogenic interference. A hierarchical sampling design was used to compare planktonic microalgae structure between one disturbed station and one control station. Samples were collected by scuba diving at 5 m depth in August 2012. A total of 76 microalgae taxa were identified. Dinoflagellates abundance was low in the disturbed station, especially Gonyaulacales and Prorocentrales due to P-limitation, whereas diatoms and cyanobacteria abundance were low in control station which is characterized by Si-limitation.

Seasonal dynamics of plankton communities coupled with environmental factors in a semi arid area: Sidi Saâd reservoir (Center of Tunisia)

In this study, we studied the influence of the physical-chemical and biological factors (bacterioplankton and phytoplankton abundances) for zooplankton dynamics in a Sidi Saâd reservoir in Centre of Tunisia. The samplings were carried out in spring, summer, autumn and winter (2005 to 2006) in the deepest station (surface, 5, 10 and 15 m). In this reservoir, the highest density of zooplankton abundance was recorded in summer (92.2 ind L -1 ; 0.43 × 10 3 μg L -1 ) and autumn (86.9 ind L -1 ; 0.23 × 10 3 μg L -1 ) at a depth of 10 m. The copepods (56% of total zooplankton abundance) and cladocera (42% of total zooplankton abundance) were the most abundant groups. The physico-chemical factors, especially the water temperature (r = 0.53, p = 0.027, n = 16) and dissolved oxygen (r = -0.59, p = 0.03, n = 16), influence directly the zooplankton community. It was suggested that in Sidi Saâd reservoir, both the top-down and bottom-up regulations account for the regulation of zooplankton. The phytoplankton was the factor responsible for the structure and seasonal dynamics of the zooplankton community, which are well related to changes in algae diversity and abundance, noting that cyanobacteria have major impacts. The “top-down” effect of planktivorous fish on the zooplankton is a significant factor affecting the plankton community’s dynamics in this reservoir. Key words : Sidi Saâd reservoir, water temperature, bacterioplankton, phytoplankton, zooplankton, top-down and bottom-up regulations.

Using general linear model, Bayesian Networks and Naive Bayes classifier for prediction of Karenia selliformis occurrences and blooms

Abstract The prediction of the dinoflagellate red tide forming Karenia selliformis is a relevant task to aid optimized management decisions in marine coastal water. The objective of the present study is to compare different modeling approaches for prediction of Karenia selliformis occurrences and blooms. A set of physical parameters (salinity, temperature and tide amplitude), meteorological constraints (evaporation, air temperature, insolation, rainfall, atmospheric pressure and humidity), sampling months and sampling sites are used. The model prediction included general linear model (GLM), Bayesian Network (BN) and the simplest BN type which is, Naive Bayes classifier (NB). The results showed that three models incriminated high salinity in Karenia selliformis blooms and the sampling sites, mainly Boughrara lagoon, in the occurrences. The BN performed better than linear models (NB and GLM) for both Karenia selliformis occurrences and blooms prediction. This later is related to the facts that BN considered the inter-independency between predictive variables and that the relationships between the variables and the outcome are often non-linear such us; the transition to bloom situations appeared to be triggered by a salinity threshold. This study is useful in the management of this ecosystem so as to use the best disposal options in the early prediction of the toxic blooms.

How could thermal stratification affect horizontal distribution of depth-integrated metazooplankton communities in the Gulf of Gabes (Tunisia)?

ABSTRACT The spatial and temporal variability of metazooplankton communities in relation to environmental and trophic factors were studied in the Gulf of Gabes during three periods of increasing thermal stratification in June, July and September. Non-metric multi-dimensional scaling (NMDS) revealed the time and space succession of three zooplankton communities in relation to the degree of thermal stratification. A co-inertia analysis showed the association between these communities and the trophic conditions (abundance and composition of microplankton) driven by stratification. Three divergent communities were evident. The first, characterized by annelid larvae and coastal copepods, was associated with shallow mixed and microplankton-rich water (June, coastal). The second, characterized by euphausiids, amphipods, doliolids and several deep-water copepods, corresponded to highly stratified microplankton-poor water (September). The third, characterized by the most ubiquitous and abundant species, was linked to intermediate water (June deep and July). Temperature and salinity conditions also explained the distribution of key species. Clausocalanus furcatus, Oithona plumifera and Triconia conifera were clearly associated with warm and highly stratified water (September). Enhanced thermal stratification led to higher surface salinity, explaining the emergence of euryhaline taxa such as Oithona plumifera.

Fluctuating asymmetry in grass goby Zosterisessor ophiocephalus Pallas, 1811 inhabiting polluted and unpolluted area in Tunisia.

Fluctuating asymmetry, a random deviation of bilateral traits, is assessed on grass goby Zosterisessor ophiocephalus Pallas, 1811 living in one disturbed site and in one unpolluted site Gulf of Gabes (Tunisia). Statistical analysis showed that assessed asymmetries are fluctuating. FA of orbital diameter, sub-orbital distance, pectoral fin and post-orbital length are higher in polluted site compared to control. Chemical pollution seems to be responsible for this high FA.