18 pagesInternational audienceThe Middle East region suffers already from the gradual effects of climate change and will be among the most vulnerable regions in the future. As a result, productivity should undergo losses due to high temperatures, drought, floods and soil degradation which threaten food security of Levantine countries. Since water is the critical factor in the region, even slight changes in air temperature and rainfall patterns will have considerable impact. It has been proven that potential climate change may disrupt, on one hand, most ecosystems through changes in their physicochemical conditions, and on the other hand the species which are living in these ecosystems. Then, the biodiversity can be found challenging. In this study, the effects of climate change on population and phytoplankton communities of Lake Karaoun were investigated since 1992. The climate regime shifts have been shown to alter the lake ecosystem. In the past, Lake Karaoun was characterized by a highly diversified microflora dominated by diatoms and green algae. Recent climatic fluctuations, with culmination in 2008-2011 and temperatures exceeding 40˚C have upset this biodiversity. Blooms of cyanobacteria, specifically Microcystis aeruginosa and Aphanizomenon ovalisporum, have occurred and disturbed both the ecosystem and the functioning of the lake

Slim, et al.. On the successful use of a simplified model to simulate the succession of toxic cyanobacteria in a hypereutrophic reservoir with a highly fluctuating water level. Environmental Science and Pollution Research, Springer Verlag, 2017, 24 (26), pp.20934-20948. 10.1007/s11356-017-9723-9. hal-01592944 1 This article was published as: Ali Fadel, Bruno Lemaire, Brigitte Vinçon-Leite, Ali Atoui, Kamal Slim, et al. (2017). On the successful use of a simplified model to simulate the succession of toxic cyanobacteria in a hypereutrophic reservoir with a highly fluctuating water level. Environmental Science and Pollution Research, 24 (26), pp.20934 -20948. DOI : 10.1007/s11356-017-9723-9 On the successful use of a simplified model to simulate the succession of toxic cyanobacteria in a hypereutrophic reservoir with a highly fluctuating water level AbstractMany freshwater bodies worldwide that suffer from harmful algal blooms would benefit for their management from a simple ecological model that requires few field data, e.g., for early warning systems. Beyond a certain degree, adding processes to ecological models can reduce model predictive capabilities. In this work, we assess whether a simple ecological model without nutrients is able to describe the succession of cyanobacterial blooms of different species in a hypereutrophic reservoir and help understand the factors that determine these blooms. In our study site, Karaoun Reservoir, Lebanon, cyanobacteria Aphanizomenon ovalisporum and Microcystis aeruginosa alternatively bloom. A simple configuration of the model Dyresm-Caedym was used: both cyanobacteria were simulated, with constant vertical migration velocity for Aphanizomenon ovalisporum, with vertical migration velocity dependent on light for Microcystis aeruginosa and with growth limited by light and temperature and not by nutrients for both species. The model was calibrated on two successive years with contrasted bloom patterns and high variations in water level. It was able to reproduce the measurements: it showed a good performance for the water level (root mean square error (RMSE) lower than 1 m, annual variation of 25 m), water temperature profiles (RMSE of 0.22 -1.41 °C, range 13-28 °C) and cyanobacteria biomass (RMSE of 1 -57 µg Chl a L -1 , range 0-206 µg Chl a L -1 ). The model also helped understand the succession of blooms in both years. The model results suggest that the higher growth rate of Microcystis aeruginosa during favourable temperature and light conditions allowed it to outgrow Aphanizomenon ovalisporum. Our results show that simple model configurations not only can be sufficient for theoretical works when few major processes can be identified, but also for operational applications. This approach could be transposed on other hypereutrophic lakes and reservoirs to describe the competition between dominant phytoplankton species, contribute to early warning systems or be used for management scenarios.

The increasing occurrence of cyanobacterial bloom in freshwaters worldwide is of great importance because of public health risks. In addition, they are very likely to have negative impact on ecological and economic aspects. In this study, the seasonal succession of phytoplankton population in Lake Karaoun in Lebanon was monitored from May 2009 to June 2011. The physicochemical parameters of lake water were then monitored for 1 year, from June 2010 until June 2011, to correlate the physicochemical parameters with the phytoplankton population in the lake. Our results showed, for the first time in Lebanon, that the eutrophied Lake Karaoun has been under the invasion of toxic cyanobacterial blooms since May 2009. The cyanobacterial bloom was persistent from late spring (May) until late fall (December) for 2 consecutive years. The high water temperature in the summer season is the main factor that has been affecting the growth of the cyanobacteria. The most frequently encountered bloom‐forming species were Microcystis aeruginosa and Aphanizomenon ovalisporum, which were either present individually or coexistent. The obtained results showed that during the period of cyanobacterial bloom, a deterioration of water quality defined by low levels of dissolved oxygen, total dissolved solids, and electric conductivity was reported. During cyanobacterial bloom period, the concentration of the orthophosphate–P (PO4‐P) was very minimal. The measured high value of chlorophyll‐a concentration during cyanobacterial bloom period (48.6 μg/L) was attributed to high photosynthetic activity. Cyanobacterial blooms can cause a variety of water‐quality problems in Lake Karaoun in addition to human health risk.

Eutrophication and harmful algal blooms have become a worldwide environmental problem. Understanding the mechanisms and processes that control algal blooms is of great concern. The phytoplankton community of Karaoun Reservoir, the largest water body in Lebanon, is poorly studied, as in many freshwater bodies around the Mediterranean Sea. Sampling campaigns were conducted semi-monthly between May 2012 and August 2013 to assess the dynamics of its phytoplankton community in response to changes in physical-chemical and hydrological conditions. Karaoun Reservoir is a monomictic waterbody and strongly stratifies between May and August. Changes in its phytoplankton community were found to be a result of the interplay between water temperature, stratification, irradiance, nutrient availability and water level. Thermal stratification established in spring reduced the growth of diatoms and resulted in their replacement by green algae species when nutrient availability was high and water temperatures lower than 22 °C. At water temperature higher than 25 °C and low nutrient concentrations in summer, blooms of the cyanobacterium Microcystis aeruginosa occurred. Despite different growth conditions in other lakes and reservoir, cyanobacterium Aphanizomenon ovalisporum dominated at temperatures lower than 23 °C in weakly stratified conditions in early autumn and dinoflagellate Ceratium hirundinella dominated in mixed conditions, at low light intensity and a water temperature of 19 °C in late autumn. We believe that the information presented in this paper will increase the knowledge about phytoplankton dynamics in the Mediterranean region and contribute to a safer usage of reservoir waters.

International audienceMany reservoirs have been constructed throughout the world during the 20th century, with many also suffering from eutrophication. The resulting increased phytoplankton biomass in reservoirs impairs their use. Except for Lake Kinneret, the environmental status of lakes and reservoirs in the Middle East is poorly documented. Karaoun reservoir, also known as Qaroun, Qaraoun or Qarun, is the largest water body in Lebanon, having been constructed for irrigation and hydropower production. This present study reviews Karaoun reservoir, including its characteristics, uses, water quality and phytoplankton succession, to assess the environmental status of the reservoir on the basis of the few existing previous publications about the reservoir. Since 2004, which is 39 years after its construction, the reservoir is considered to be hypereutrophic, with low phytoplankton biodiversity and regular blooms of toxic cyanobacteria. The nutrient and trace metal concentrations would not prevent use of the reservoir for a drinking water supply for Beirut, as is currently being planned, although not all the micropollutants in the lake were documented. Karaoun reservoir is compared to other monitored lakes and reservoirs around the Mediterranean Sea. They share annual toxic cyanobacteria blooms of Aphanizomenon ovalisporum and of Microcystis aeruginosa. The phytoplankton composition and succession of Karaoun reservoir is more similar to El Gergal reservoir (Spain) than nearby natural lakes such as Lake Kinneret (Israel) and Lake Trichonis (Greece). Phytoplankton diversity in Karaoun reservoir was the lowest, due to higher nutrient concentrations and a larger decrease in water level in the dry season. Karaoun reservoir represents an interesting example of the potential response of the phytoplankton community in other lakes and reservoirs during the drought periods expected to occur as a result of global climate change. © 2014 Wiley Publishing Asia Pty Ltd