Abstract

The present work is devoted to the study of the phytoplankton of the Mellah lagoon with an emphasis on HABs species, in relation with the current ecological conditions. During 2016, a bi-weekly sampling in 3 stations was realized. For temperature, there is two major thermal periods distinct characterizing the waters of the lagoon. The first period with increasing temperatures ranging from January (12.37) to July (28.54°C) and a second, where the trend reverses during the rest of the cycle. Similarly, the evolution of salinity shows two phases, the first is decreasing from September to February (35.68 to 24.23) and a second one increasing corresponding to warming of water during the rest of year. Dissolved oxygen recorded extreme values in August (5.25 mg.L-1) and in January (8.74 mg.L-1), these variations are mainly related to climatic conditions and photosynthetic activity. The pH of the waters (7.75 - 8.55) is relatively constant and slightly alkaline. The transparency of the water is strongly influenced by the mixing of water (in winter) and in summer and autumn (phytoplankton blooms), up 38.50 mg.L-1. The levels of dissolved inorganic nitrogen (DIN) are low in the lagoon with concentrations ranging from 0.38 to 8.09 μmoles.L-1. Also for dissolved reactive phosphorus (DRP), the concentrations are often less than 0.54 μmole.L-1 during the cycle. The low nutrient enrichment of the waters of Mellah results in moderate primary production, less than most Mediterranean lagoons. Thus, chlorophyll biomass varies between 0.08 and 5.59 μg.L-1, with an average of 2.41 μg.L-1. All these chemical and biochemical characteristics classify Mellah among the meso-oligotrophic lagoons.
In the Mellah lagoon, the grain size is organized in a concentric manner, with an accumulation of fine particles in the central basin. Thus the banks are formed of pure sands and as one go towards the center the pelitic fraction increases. This study allowed us to highlight five sedimentary zones ranging from pure sands near banks to pure silts in the center, through slightly silted sands, sandy silts and slightly sandy silts. The distributions of organic matter and sediment water content are positively correlated with fine fractions (r = 0.93 and 0.97 respectively).
In total, 227 species of phytoplankton (160 diatoms and 53 dinoflagellates) were inventoried, with a clear dominance of diatoms (62.97%) compared to dinoflagellates (36.84%). Diatoms dominated in the spring, while dinoflagellates grew better in the summer and early in autmn. Inded, both groups show a marked seasonal trend. The phytoplankton dynamics of the Mellah lagoon are influenced by several environmental factors but especially temperature and salinity. For the first time, potentially toxic species have been identified, including 2 diatoms (Pseudo-nitzschia group delicatissima, Pseudo-nitzschia group seriata) and 5 dinoflagellates (Alexandrium minutum, Alexandrium tamarense/catenella, Dinophysis acuminata, Dinophysis sacculus, Prorocentrum lima). This HABs species, need to implement a monitoring program to detect the proliferation of toxic species order to prevent any human intoxication due to the consumption of contaminated shells.
Until now, no studies on dinocysts in the Mellah lagoon have been conducted. In total, 48 morphotypes of cysts were inventoried belonging to 13 orders, 18 families and 30 genera. Among this list, there are 42 dinocysts divided into 7 orders: Peridiniales (3 families, 7 genera and 14 taxa), Gonyaulacales (3 families, 7 genera and 15 taxa), Gymnodiniales (2 families, 5 genera and 9 taxa), Suessuales, Prorocentrales, Thoracosphaerales, Tovelliales (1 family, 1 genus and 1 taxon for each order). The other 6 orders (Chattonellales, Coscinodiscales, Lithodesmiales, Triceratiales, Oligotrichida and Tintinada) contains just one taxon for each. The distribution of cysts in the superficial sediments of the Mellah is very heterogeneous. Indeed, their density oscillates between 1 and 315 cysts.g-1 DS. Assemblies of cysts identified are dominated by a limited number of species: Alexandrium minutum (15.77%); Gonyaulax verior (9.75%); Alexandrium affine (7.02%); Scrippsiella trochoides (6.64%); Protoperidinium spp. (6.53%) and Alexandrium pseudogonyaulax (6.16%). There is a positive correlation between the density of cysts and (1) the fine fraction of sediments (r = 0.74), (2) sedimentary organic matter (r = 0.70), (3) water content in sediments (r = 0.71) and (4) depth (r = 0.61). Finally, the accumulation of some dormant species in sediments can cause efflorescence, thus a recurrence of the proliferation of toxic species; mainly in the lagoon the exploitation of the shells is practiced for several years.

Keywords: environmental factors, phytoplankton, dinocysts, HABs, diversity, dynamics, Mellah lagoon.

Information

Item Type:	Thesis
Divisions:	» Laboratory for Terrestrial and Aquatic Ecosystems » Faculty of Natural Sciences and Life
ePrint ID:	1983
Date Deposited:	2019-11-03
Further Information:	Google Scholar
URI:	https://www.univ-soukahras.dz/en/publication/article/1983

BibTex