The relationship between phytoplankton and fish in nutrient-rich shallow Lake Qarun, Egypt

The present study focused on the determination of the baseline data and correlations between biological and physicochemical variables, including the assessment of trophic conditions in Lake Qarun. The concentrations of nutrients were high, with the maxima usually in the east subarea (total nitrogen 6.40 mg dm^-3, mineral nitrogen 2.34 mg dm^-3, orthophosphates 0.22 mg dm^-3). A total of 134 phytoplankton species were recorded. Bacillariophyceae and Dinophyceae co-dominated spatially and seasonally in phytoplankton assemblages. The highest phytoplankton density (935 × 10⁴ cells dm^-3) and chlorophyll a content (69.3 μg dm^-3) were recorded in the east subarea of the lake, whereas the largest total and dominant fish (Mugil cephalus and Solea spp.) were in the west. Tilapia zillii and Engraulis encrasicolus were most abundant in the east and in the middle part, respectively. When phytoplankton density decreased from the east toward the west subarea, the Secchi disk depth increased. The TLI-based assessment indicated hypereutrophic waters at most sites of Lake Qarun. Statistically significant positive or negative correlations were found between the dominant fish species: T. zillii and Solea spp., and the phytoplankton density, Dinophyceae density, concentrations of TP, chlorophyll a, ammonium, nitrite and nitrate. Such correlations may be helpful to better understand how to enhance the sustainable fish production.