Primary production in open waters is driven by phyto-plankton and many species have been described in the SMR since systematic research began earlier in the twentieth century (e.g., Gayral, 1954). The trend in availability of phytoplanktons related to the seasonal changes is comparable to that in European lakes with early and late summer peaks. However, high turbidity of some lakes can strongly depress phytoplankton growth despite an excess of nutrients. Green algae tend to be the most diverse group in all lakes except enriched lakes, where blue-greens (cyanophytes) can dominate, making the open water highly turbid and green. For diatoms, Cyclotella species such as C. ocellata are common in the hard water upland lakes but in the lowland sites, more salt tolerant taxa are found, including Thalassiosira weissfloggi and Cyclotella meneghiniana. In the Nile system, Aulaco-seira granulata is usually the common plantkonic diatom. Phytoplankton was scarce in acidic Megene Chitane but some Cyclotella pseudostelligera and several acidophilous diatom species were recorded.

Being at the base of the food chain, the phytoplank-ton sustains the zooplankton as well as fisheries. Rotifers seem to dominate where organic pollution is high, such as in Lake Manzala (Egypt), but elsewhere crustaceans such as Daphnia magna, Ceriodaphnia dubia, Moina micrura, Dunhevidea crassa, Chydorus sphaericus, Alona esteparica, Alona quadrangula, Leidigia ciliata, Dipatomus cyaneus admotus, Meta-diaptomus chevreuxi, Hemidiaptomus maroccanus, Mixodiaptomus laciniatus atlantis, and Diacyclops bicuspidatus are common. One characteristic of some of the SMR zooplankton species is their ability to withstand high salt and desiccation, and estivation

Table 1 Summary examples of mean values for some water chemistry attributes in selected North African lakes

Sidi Bou Ghaba Azigza Zerga Chitane Korba Manzala Qarun pH 8.3 8.1 8.1 5.5 8.1 8.2 8.7

Merjas Sidi bou Ghaba and Zerga are on the western coast of Morocco, Aguelmane Azigza in the Middle Atlas Mountains, Megene Chitane and Lac de Korba are on the north and east coast of Tunisia, respectively, and Lake Manzala and Birket Qarun are in Egypt. Note that the upland lake M. Chitane alone has acidic water and that both Zerga and Korba are linked to the sea by channels. Ca - calcium, Na - sodium, TDS - total dissolved salts.

in pond habitats (dayas types 1-4) is usual. Crustaceans showing summer dormancy are the Notostracha (Triops concriformis mauritanicas, Lepidurus apus), Anostraca (Chirocephalus diaphanous), Conchostra-cha (Cyzicus sp.), Cladocera (Daphnia, Alona, Chy-dorus, and Leydigia), and copepods (Diaptomidae Neolovenula alluaudi and Eudiaptomus cyaneus, Cyclopidae Metacyclops minutus, and Harpacticoida).

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