Thème :
Long term nutrient inputs from the largest Algerian coastal river (the Mafragh River-estuary)
Présentation :
The North African rivers generally carry high nitrogen (N), phosphorus (P) and silica (Si) loads, in response to natural and anthropogenic factors. Over the past several decades, the N and P fluxes from the Mediterranean rivers may have increased by more than 5-fold and 3-fold, respectively. These high nutrient loads, besides sustaining the high productivity of coastal waters, also promote eutrophication that is often manifested in noxious and harmful algal blooms, hypoxia, degradation of pelagic and benthic habitats, and changes in the structure of coastal food webs. In addition to N and P, Si plays a crucial role in controlling diatom abundance and the overall structure of coastal food webs. Diatoms need Si to build their external frustules, and their growth generally becomes Si-limited when Si:N molar ratio falls below 1:1. In contrast to the increasing trends in riverine N and P delivery, the levels of dissolved Si have dramatically decreased in many coastal regions worldwide. Importantly, N and P sequestration in reservoirs can be counterbalanced by human activities, but there is no similar compensation for Si. The decline in Si transport to the coastal ocean can in turn change the ratio of nutrients available for phytoplankton growth and thus affect the coastal food webs.
The Mafragh watershed is considered to be among the wettest areas in Algeria, having a an annual mean precipitation of 740 mm. The resulting runoff is intercepted by a series of three dams (total storage capacity of 300 × 10 million m3) and a large flood plain containing marshlands (about 170 km2). The mean discharge at Mafragh River (MR) outlet is only 24 m3 s-1. However, the MR discharge can vary significantly from over 2000 m3 s-1 during the peak flows to zero during the extended dry period. Indeed, the MR outlet is disconnected from the adjacent ocean for several months (usually during June-November), due to the dam retention and unregulated water use. In addition, the microtidal regime of the estuary (tidal range ~ 20 cm) facilitates the closing of the MR outlet. The MR has a large brackish section (20 km in Kebir East branch and 8 km in Bounamoussa branch), which typically occurs during March-May, following the December-February peak in freshwater discharge. The MR watershed is mostly forested (57.3%) while agricultural areas in the middle and lower sections occupy about 25% of the watershed, although the area is sparsely populated (about 0.5 × million inhabitants). Another notable characteristics of the MR catchment is the presence of multiple successive water bodies (the so-called cascading filters) along the aquatic continuum (reservoirs, wetlands, marchlands and a large brackish section), which can retain nutrients and other materials.
As stated above, despite a few published works on Mafragh River and other contiguous rivers, the data on nutrient fluxes from Algerian rivers are still lacking. This study describes the first long-term effort (2002–2017) to examine the nutrient inputs to Annaba Bay from the Mafragh River. The objectives are to
(i) assess nutrient inputs to Annaba Bay from riverine source,
(ii) explore seasonal and inter-annual variability in nutrient inputs, and,
(iii) fill the gaps in freshwater discharge and nutrient input inventories (DON and Si in particular) for the Southwestern Mediterranean Sea.
Thème :
Effects of river daming on water and nutrient inputs into Algerian coastal waters
Présentation :
In recent decades, human activity has significantly changed the hydrology and chemistry of catchments by retaining rivers water in dams and by introducing into the sea large masses of fertilizers from agricultural, industrial and household wastes.
In the southwestern Mediterranean catchments, water river discharge is low and decreases with irrigation and climate change. To fulfill the growing water needs, many dams have been built all around the area. However, dams may have a strong impact on the water and nutrient river discharge due to silicate and phosphorus retention within sediments.
Nowadays in Algeria, the intercepted runoff in dams reaches about 5.2 billion m3, and because of the growing irrigation needs and population intake, it is yet planned to store in the few coming years about 7.5 billion m3. Urban and agricultural nutrient inputs and water residence time within dams also lead to a change on the nutrients ratios, as the Redfield ratio. River nutrient discharge and nutrient ratios are known to play a special role in supporting the Mediterranean production where the productive areas are limited to the adjacent coast.
However, there are few studies for the southern Mediterranean basins and data on river nutrient loading to the Mediterranean watersheds are also scarce and are missing in North-African countries and can bias the general picture. Moreover, data on the distribution and fluxes of dissolved nitogen, phosphate and silicon in coastal catchments of Algeria are inexistent. Dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) are important components of riverine inputs and coastal catchments, but they are rarely considered within nutrient loadings pool. Therfore, DON needs to be considered into coastal nitrogen loading budgets, because of its rapid bioavailability as well as its atmospheric deposition in watersheds that forms significant fraction within total bioavailable dissolved nitrogen.
Considering the severe lack of biogeochemical data for coastal river, this project aims to evaluate nutrient fluxes and dam effects on the biogeochemistry of dissolved nitogen, phosphate and silicon in three representative coastal catchments of Algeria. The three catchments have a total surface of 11 160 km2 with a population of about 2 million people. They are mainly submitted to household and agricultural wastes and heavily managed by several dams that retain significant part of the precipitation wealth. The largest dam is Beni-Haroun (about one billion m3 storage capacity) built on the Kebir-Rhumel River. The catchment of the Kebir western River (1900 km2) is very weakly populated, with only 30 inhabitants per km2 and it is managed by Zit Emba dam that retains about 120 million m3.
The Zardaza dam (50 million m3) built on the Safsaf River (1250 km2).
Data analysis will focus on the effects of dams on nutrient retention and dissolved organic nutrient production and budgeting. This study will consider, for the first time, the role of dams in producing DON and DOP and challenges to reveal that function.
Thème :
North African coastal rivers: discharge and nutrient inputs to the receiving coasts (The Seybouse River-estuary case study)
Présentation :
North African coastal rivers are experiencing an exceptional reduction in river water flows due to dam storage, extraction for irrigation and climate change. Moreover, inadequate treatments of domestic and industrial wastes added to inefficacious urban development plans in North Africa would have increased pressures on river systems, leading to eutrophication, pollution andaquatic water quality alteration. Precipitation in the Mediterranean region is characterized by irregularity and often very stormy patterns. During these flash flood events, freshwater discharge into the sea can be multiplied by a factor of 10, delivering a major fraction of the annual loads of inorganic and organic nutrients plus other materials into the coastal zone. This will have evident consequences on the biogeochemical cycling of nutrients as well as on the role of estuaries and coastal areas as buffer zones between terrestrial and marine systems.
Information on water discharges in North African Rivers is hugely lacking, despite their importance in evaluating anthropogenic pressures on the catchments and the receiving marine coastal waters. The database for North Africa is poorly documented, despite the fact that many of these rivers have large water, nutrient, and sediment inputs and are particularly susceptible to natural and anthropogenic changes. Numerous studies outlined that in northern Africa, information on both river alteration and its effects on the coastal waters is fragmented and mostly gathered in unpublished reports. These significant gaps in the Mediterranean rivers discharges and nutrient inputs prevent a true representation of the global picture of freshwater and nutrient inputs in the Mediterranean Sea.
The Seybouse River (165 km length) has a small salt wedge estuary of about 8 km with a micro-tidal and semidiurnal regime. The extent of this part strongly varies according to the importance of the river flow, and function as an atypical estuary, evolving throughout the year over three phases: river, estuarine and saline lake phases. As the tidal range is very small (≈20 cm), the duration of each phase may strongly vary depending on the river input and the duration of the dry season. During an extended dry period, the estuary's outlet might be nearly closed and disconnected from tidal inlets for several months. In contrast, during high freshwater input episodes (river phase), often occurring between November–March, the salt wedge could be entirely discharged into the sea. On a wet year the estuary is connected to the sea, being dominated by the tidal intrusion, even during the dry period (April– October). In this dry period, the estuary presents intense vertical stratification. The extent of the estuarine phase, out of the flood episodes, depends both on dam's retention and direct water abstraction for agricultural needs.
Algerian estuaries are still less well-known, only a few of them have been explored for water and nutrient inputs and assessed the role of reservoirs in water and nutrient retentions in some coastal catchments. Until now, no study has considered the flow and associated nutrient inputs into the estuary and the latter inputs into the adjacent coastal waters. In addition, although silica is indicated as a proxy of freshwater influence and a limiting factor for diatom growth. Studies on Algerian estuaries have rarely monitored silica level and load. Similarly, the dissolved organic N and P forms have also been omitted in most of those studies, despite the importance and the role they play as bioavailable nutrients in N and P cycling in the highly dynamic coastal ecosystems.
To fulfill these crucial gaps, we assessed water discharges and nutrient loads from two Seybouse river gauging stations over 15-years study period (2002- 2017). During this period, heavy rainfall episods occurred in February 2012 and February 2015 which have heavily affecting the lowerreach of the river estuary.
The main objectives of the present study are
(1) to characterize the nutrients (organic and inorganic fractions) and loads of the Seybouse River lower-reach entering its estuarine part,
(2) to assess the influence of these nutrients and their molar ratio variations in the estuarine section, and
(3) to evaluate the potential influence of the outflow on the coastal eutrophication of Annaba Bay.
Thème :
The zooplankton from the southeastern Mediterranean Sea: composition, abundance and biomasse
Présentation :
Coastal marine areas are of important ecological, economic and social interest and are very sensitive to an¬thropogenic activities. Fluctuations of rivers and sewage discharge into coastal zone induce high temporal and spatial variability of environmental factors and food conditions, which controls the metabolism, feeding behav¬ior and production of the planktonic populations. The zooplankton plays a key role in the marine food web, which constitute the major diet component of numerous peligic fishes (sardines and anchovy) and other marine consumers. Studies on the dynamic of Mediterranean zooplankton are sparse or lacking, and still far from be¬ing understood, and ba¬sic exploratory research is still needed. Concerning the zooplankton biomass, only few reviews were performed in the western and the eastern basins, and comparisons remain uneasy because of the lack of standardized sam¬pling methods and data treatment. Moreover, most investigations were carried out in various areas with specific objectives and various strategies, types of nets and mesh sizes, and different sampling depths.
Since the pioneering works of Rose (1925) and Ber¬nard (1955), which were limited to the zooplankton from Algiers, very few studies have been performed in Alge¬rian coastal waters, and very little is known regarding the zooplankton community. Although there have been several studies on the phytoplankton, physical and chemical characteristics of Annaba Bay, there have only been two studies of the surface zooplankton from this area by Ounissi et al. (1998) and Khélifi-Touhami et al. (2007), who exam¬ined the copepod structure of Annaba Bay and the adja¬cent Tunisian coast. There are, however, no data on the annual distribution of zooplankton abundance, biomass, diversity and species composition. Among the published studies, there are differences in the sampling equipment, time of collection, and sampling depth of zooplankton abundance and biomass, which makes any comparison difficult.
Annaba Bay is a singular coastal area (southwestern Mediterranean Sea) that receives large diffusive inputs from the Seybouse and Mafragh estuaries and has also been subjected to severe pollution problems induced by direct domestic and industrial wastes. The estuaries’ plumes are relatively limited to only several ki¬lometers, but under the flooding period, the dilution zone due to their freshwater input can reach the entrance of the bay in the north western part. From the marine side, the entrance of Annaba Bay is submitted to the Modified Atlantic Water (MAW) intrusion, which may renew and disseminate the estua¬rine and land inputs.
The objectives of this project are
(i) to analyze the zooplankton composition and abundance of Annaba Bay, under estuarine influences and
(ii) to compare our findings with previous studies on the zooplankton struc¬ture of the bay that were conducted 20 and 10 years ago in order to reveal possible populations and environmental change and to
(iii) to develop a comprehensive leaflet for the Copepoda as the major component of the marine zooplankton
This study provide for the first time both com¬plete annual cycle of zooplankton in the northeastern Algerian coasts and information about the zooplankton biomass. In addition, this study may facilitate the com¬parison between different Mediterranean coastal areas, as the zooplankton was sampled by WP2 net (200 μm mesh size) and its biomass was expressed in dry weight, fol¬lowing the methods mostly used for marine zooplankton studies.
Thème :
Riverine and atmospheric nutrient and particulate material inputs to the southwestern Mediterranean Sea
Présentation :
Studies of river and estuarine modifications at global or regional scales remain lacking and assessing impacts of some river or estuarine syndromes on aquatic resources is considered a first priority. In the southwestern Mediterranean Sea, atmospheric inputs of nutrients are becoming the main external source due to declining river discharge in recent decades following reservoir construction and changes in irrigation.
Information on riverine and atmospheric inputs of materials is still scarce in the Mediterranean basin particularly for Algerian coastal catchments. Annaba’s coastal region is subject to atmospheric deposition under the main influences of European airflows (October-March) and African air masses with high dust loads (April-September). This region is among the wettest areas in Algeria and receives inputs from two important Algerian rivers, namely the Mafragh River and the Seybouse River. Although the wet atmospheric deposition of dissolved nutrients has been recognized as significant in the Mediterranean Sea, data on nutrient wet deposition on Algerian coasts are almost nonexistent. In addition, little is known about nutrient and material transport from Algerian coastal catchments.
The present project is based on an extensive riverine and rainwater data collection during six years in the Annaba coastal area of the southwestern Mediterranean. We present the distribution and fluxes of dissolved nutrients [silicates Si(OH)4), dissolved inorganic nitrogen components (ammonia: NH4; nitrate: NO3; nitrite: NO2), dissolved organic nitrogen (DON), total dissolved nitrogen (TDN), phosphate (PO4), polyphosphate (P2O5), dissolved organic phosphorus (DOP), total dissolved phosphorus (TDP)] and particulate material [suspended particulate material (SPM), Chlorophyll a (Chl a), particulate organic carbon (POC), Particulate organic nitrogen (PON), particulate organic phosphorus (POP) and particulate biogenic silica (BSi)] into the Annaba Bay from the Mafragh and Seybouse Rivers together with the wet atmospheric deposition of dissolved nutrients.
To our knowledge, this is the first assessment of the wet deposition of dissolved nutrients and of particulate biogenic silica, particulate organic carbon, suspended particulate matter and chlorophyll a in southwestern Mediterranean waters, including Algerian coastal waters.
Our main objectives were to:
(1) assess the relative contributions of riverine and wet atmospheric inputs of dissolved nutrients (Nitrogen, Phosphorus, Silicon) into Annaba Bay,
(2) determine the relative importance of the dissolved and particulate matter transferred into the bay through the two rivers outlets and
(3) assess differences in the material deliveries of the two rivers, which are differently affected by human activities.