Spring plankton community structure and distribution in the north and south coasts of Sfax (Tunisia) after north coast restoration

Effects of Eutrophication on Plankton Abundance and Composition in the Gulf of Gabès (Mediterranean Sea, Tunisia)

Phytoplankton and Copepoda were investigated in the industrialized Gabès coast (Mediterranean Sea, Tunisia) to understand consequences of the Eutrophication Grade on the community composition. In the past 40 years, intensive agrochemical activities have developed in Gabès, discharging wastewater along the coast. In parallel, Gabès City has experienced a rapid demographic expansion (population: 131,000) that further increased sewage discharged into the sea. The present study was carried out in the Gulf of Gabès in March 2014. The abiotic analysis of seawater showed high concentrations of nutrients and eutrophication in all the studied fifteen stations. A growing eutrophic gradient was revealed from Zarrat to Gannouche. During this study, 42 phytoplankton taxa and 24 Copepoda taxa were identified. Bacillariophyta were the most abundant group, ranging from 67.7% to 89.2% of total phytoplankton specimens. Chaetoceros costatus, Euglena acusformis, and Thalassiosira sp. showed a positive correlation with Eutrophication Index (profited of nutrient availability). Therefore, the Shannon–Weaver diversity index of phytoplankton and Copepoda showed negative correlations with Eutrophication Index. The relatively high H′ values for phytoplankton suggest that the eutrophicated waters of Gulf of Gabès are not a hostile environment for them. Among Copepoda, Oithona similis, and Euterpina acutifrons seem to be insensible species to eutrophication.

Concentrations of trace metals in phytoplankton and zooplankton in the Gulf of Gabès, Tunisia

The concentrations of four trace metals (Cd, Cu, Pb, and Zn) were investigated for the first time in phytoplankton, zooplankton, and the seawater samples collected from the coast of Gabès, Tunisia, Mediterranean Sea. For over 40 years, this coast has witnessed significant anthropogenic impacts form fertilizer processing. Results obtained for Cd, Cu, Pb, and Zn in seawater far exceed the concentration reported for other Mediterranean coastal waters, highlighting the Gulf of Gabès as a pollution hotspot. The average metals concentration was in the order Zn > Pb > Cu > Cd in water, and phytoplankton, whereas Pb > Zn > Cu > Cd in zooplankton. The biomagnification in phytoplankton and zooplankton for Zn, Pb, Cu, and Cd was 116, 56, 38, 31, and 127, 157, 30 and 27. The biomagnification of Zn and Pb was higher in zooplankton than phytoplankton, while Cu and Cd were higher in phytoplankton.

Response of benthic macrofauna to multiple anthropogenic pressures in the shallow coastal zone south of Sfax (Tunisia, central Mediterranean Sea)

Anthropogenic activities including coastal industries, urbanization, extensive agriculture and aquaculture as well as their cumulative impacts represent major sources of perturbation of marine coastal systems. Macrobenthic communities are useful ecological indicators for monitoring the health status of marine environments (or polluted environments). The present study reports, for the first time, the response of benthic macrofauna sampled during two years survey (2015-2016) to multiple anthropogenic pressures on the coastal zone south of Sfax (Tunisia). A total of 12 stations were monitored seasonally at locations downstream from the main potential sources of disturbance. 106 macrobenthos taxa, belonging to six animal phyla and 70 families, were identified with a dominance of polychaetes (42%), crustaceans (35%) and molluscs (18%). We used an ANOVA test and cluster analysis to identify spatial gradient linked to environmental and anthropogenic factors, including depth, sedimentary texture and anthropogenic activities (i.e. phosphogypsum discharges).The macrofauna present lowest species number and abundance on stations undergoing anthropogenic inputs, which are extremely polluted by heavy metals (Cd, Cu, F and N) and excess of organic matter. Univariate parameters reveal a general trend of increasing species diversity with increasing distance from the pollution source. The polluted stations are strongly dominated by carnivores, and selective deposit feeders, and more closely linked to the availability of trophic resources than to anthropogenic constraints. The seasonal changes in macrobenthic abundance, diversity indices and community structure are mainly linked to the biological cycle (e.g. recruitment events) of the dominant species. Biotic indices (AMBI and BO2A) classified the coastal zone south of Sfax as moderate and good ecological status. This study suggests that initiating a long-term monitoring programme would improve our understanding of the temporal changes of macrobenthic communities of this ecosystem, contributing to the assessment of effective management and conservation measures in this disturbed area.

Seasonal variability of picophytoplankton under contrasting environments in northern Tunisian coasts, southwestern Mediterranean Sea

We investigated at the single cell level during 16months (June 2012 to September 2013) the temporal distribution of picophytoplankton (picoeukaryotes, Synechococcus and Prochlorococcus) communities in two contrasted ecosystems: the Bay of Bizerte characterised by an oligotrophic regime typical of the Mediterranean Sea and the Bizerte Lagoon that exhibits a mesotrophic/eutrophic state. We aimed at depicting seasonal variations and quantifying the relationships between the environmental factors and the structure and abundance of picophytoplankton communities. Results showed that picophytoplankton groups were able to grow under a wide range of environmental conditions varying seasonally, although their abundances and contributions to the total chlorophyll biomass significantly varied and showed importance in the Bay of Bizerte. Synechococcus was the most abundant group reaching 225∗10³cells·cm^-3 in the Bay and 278∗10³cells·cm^-3 in the lagoon. This group was present all over the year in both ecosystems. Structural equation model results pointed out a different configuration regarding the picophytoplankton environmental drivers. The complexity of the configuration, i.e. number of significant links within the system, decreased under enhanced eutrophication conditions. The less exposure to anthropogenic stress, i.e. in the Bay of Bizerte, highlight a larger role of nutrient and hydrological conditions on the seasonal variations of picophytoplankton, whereas a negative effect of eutrophication on picophytoplankton communities was unveiled in the Bizerte Lagoon. We stress that such influence may be exacerbated under expected scenarios of Mediterranean warming conditions and nutrient release in coastal ecosystems.

Assessing ultraphytoplankton and heterotrophic prokaryote composition by flow cytometry in a Mediterranean lagoon

In the eutrophic Ghar El Melh Lagoon (GML, Tunisia), the distribution of heterotrophic prokaryotes, pico- and nanophytoplankton was studied at five stations in November 2012 at the single cell level, along with environmental factors. Flow cytometry analysis of ultraplankton (<10 μm) resolved (i) two heterotrophic prokaryote groups, low and high nucleic acid contents (LNA and HNA, respectively), and (ii) eight to nine ultraphytoplankton groups (cryptophyte-like cells, two nanoeukaryote subgroups, two picoeukaryote subgroups and three Synechococcus-like cells subgroups). Prochlorococcus was not detected. According to redundancy analysis (RDA), a significant difference was found in the distribution of the ultraplankton between stations (F = 2.61, p < 0.05); maximum proliferations of heterotrophic prokaryotes were observed in the inner parts of the lagoon at stations 3, 4 and 5 affected by urban, agricultural and industrial discharges. Ultraphytoplankton concentrations were the highest near the outlet of the lagoon at stations 1 and 2 influenced by freshwater outflow and oligotrophic Mediterranean water inflow, respectively. At station 1, the large ultraphytoplankton concentration derives from the high abundance of cryptophyte-like cells favoured by the freshwater outflow whereas at station 2, the input of oligotrophic Mediterranean water enhanced the abundance of Synechococcus and picoeukaryotes at the expense of nanoeukaryotes. Two trophic regimes were thus differentiated in GML.

Microbial composition and ecological features of phototrophic biofilms proliferating in the Moidons Caves (France): investigation at the single-cell level

The authors investigated the microbial composition of phototrophic biofilms proliferating in a show cave using flow cytometry for the first time in such a context. Results are based on several biofilms sampled in the Moidons Caves (France) and concern both heterotrophic prokaryotes and autotrophic microorganisms. Heterotrophic microorganisms with low nucleic acid content were dominant in biofilms, as can be expected from the oligotrophic conditions prevailing within the cave. Analysis of the biofilm autotrophic components revealed the presence of several taxa, particularly the unicellular green algae Chlorella minutissima, specifically well adapted to this cave. Relationships between flow cytometry results and environmental variables determined in the cave were established and discussed so as to better understand biofilm proliferation processes in caves.

Assessment of anthropogenic inputs in the surface waters of the southern coastal area of Sfax during spring (Tunisia, Southern Mediterranean Sea)

The coastal marine area of Sfax (Tunisia), which is well-known for its high productivity and fisheries, is also subjected to anthropogenic inputs from diverse industrial, urban and agriculture activities. We investigated the spatial distribution of physical, chemical and biogeochemical parameters in the surface waters of the southern coastal area of Sfax. Pertinent tracers of anthropogenic inputs were identified. Twenty stations were sampled during March 2013 in the vicinity of the coastal areas reserved for waste discharge. Phosphogypsum wastes dumped close to the beaches were the main source of PO4(3-), Cl(-) and SO4(2-) in seawater. The high content in total polyphenolic compounds was due to the olive oil treatment waste water released from margins. These inorganic and organic inputs in the surface waters were associated with elevated COD. The BOD5/COD (<0.5) and COD/BOD5 (>3) ratios highlighted a chemical pollution with organic load of a low biodegradability.

Spatial and seasonal variability of pico-, nano- and microphytoplankton at the bottom seawater in the north coast of Sfax, Eastern Mediterranean Sea

In the frame of the Taparura Project, we studied the distribution of pico-, nano- and microphytoplankton communities in relation to environmental variables at 18 stations sampled during four coastal cruises conducted between October 2009 and July 2010 at the bottom, on the north coast of Sfax (Tunisia, Eastern Mediterranean Sea). The restoration effect on coastal ultraphytoplankton (<10 μm) and microphytoplankton (<200 μm) was investigated using conventional flow cytometry and inverted microscopy. Flow cytometry analysis of ultraphytoplankton resolved six groups (Prochlorococcus, Synechococcus, nanoeukaryotes and three distinct subgroups within picoeukaryotes). In addition to these autotrophic groups, two unknown groups were characterised on the north coast. Picophytoplankton abundance shifted from a summer dominance of Synechococcus to a dominance of picoeukaryotes and Prochlorococcus during spring. Nanoeukaryotes were the most abundant in spring. Microphytoplankton was resolved into five groups, labelled Bacillariophyceae, Dinophyceae, Cyanobacteriae, Euglenophyceae and Chlorophyceae. A total of 90 microphytoplankton species were identified in all stations, with an overwhelming abundance of large diatoms, a typical trait of benthic communities (Coscinodiscus sp., Grammatophora sp., Navicula sp., Pleurosigma sp., Striatella unipunctata …). Results collected in this study are favouring a beneficial impact on the ecosystem of the Sfax north coast restoration achieved by the Taparura Project.

Seasonal distribution of ultraphytoplankton and heterotrophic prokaryotes in relation to abiotic variables on the north coast of Sfax after restoration

The Taparura project was set up to restore the north Sfax coast (Tunisia) by shutting down the northern phosphate plant responsible for chronic pollution and uncontrolled phosphogypsum dumping. The restoration effect on coastal ultraphytoplankton (<10 μm) and heterotrophic prokaryotes was investigated using conventional flow cytometry over four successive seasons during 2009-2010. Cell concentrations were generally higher than values reported for the open sea, both in the western and eastern Mediterranean basins. One striking point was that chl a concentration on the north Sfax coast was unchanged after restoration but was still one order of magnitude higher than in the Gulf of Gabès. Restoration of pH, following the shutdown of the phosphate processing plants on the north coast, appeared to reach normal levels for seawater during the study, whereas seawater acidification persisted on the south coast where plants are still in operation. The largest ultraphytoplankton biomass was from an unknown cell group, whose identity and role needs to be established.

Restoration impact of an uncontrolled phosphogypsum dump site on the seasonal distribution of abiotic variables, phytoplankton and zooplankton along the near shore of the south-western Mediterranean coast

'In connection with the Taparura Project, we studied the distribution of phytoplankton and zooplankton communities in relation to environmental variables at 18 stations sampled during four coastal cruises conducted between October 2009 and July 2010 on the north coast of Sfax (Tunisia, western Mediterranean Sea). The inshore location was largely dominated by diatoms (66 %) represented essentially by members of the genera Navicula, Grammatophora, and Licmophora. Dinophyceae were numerically the second largest group and showed an enhanced species richness. Cyanobacteriae developed in association with an important proliferation of colonial Trichodesmium erythraeum, contributing 39.4 % of total phytoplankton abundances. The results suggest that phytoplankters are generally adapted to specific environmental conditions. Copepods were the most abundant zooplankton group (82 %) of total zooplankton. A total of 21 copepod species were identified in all stations, with an overwhelming abundance of Oithona similis in autumn and summer, Euterpina acutifrons in winter, and Oncaea conifera in spring. The phosphogypsum restoration had been acutely necessary allowing dominant zooplankton species to exploit a wide range of food resources including phytoplankton and thus improving water quality.