Microalgal cascade and carbon flow through food webs of Lake Nasser

Abstract

Lake Nasser represents one of the most significant artificial reservoirs in the world The carbon flow in the food webs of Lake Nasser khors were studied through a comprehensive study included fatty acids biomarkers (FAS) and stable isotopes signature (SIA) of zooplankton, shrimp and commercial fish species, moreover gut contents analysis for fish species. The work aimed to characterize the contribution of the different organic matter sources (phytoplankton, epilithic, epiphytes, or macrophytes) fueling the food web and to detect the most important microalgal group cascaded to the higher trophic level. The variation in the phytoplankton community based on biovolume and carbon content in Khor Ramla and Khor Abu-Simbel of Lake Nasser during the least and highest water level of 2014, were studied to detect the dominant group and to know if this group consumed by fish species or not. In spring, the maximum biovolume of Khor Abu-Simbel was 234.57 mm3/l and its mean total biovolume (81.63 mm3/l) was higher than that of Khor Ramla (1.94 mm3/l). While during autumn, the mean total biovolume in Khor Ramla (3.65 mm3/l) and Abu-Simbel (3.09 mm3/l) were slightly different. Cyanobacteria were dominant in both the studied khors along the study period with obvious blooming at all stations of Khor Abu-Simbel during spring. Other divisions Chlorophyta, Dinophyta, and Bacillariophyta were considerably present. A total of 180 species were identified in the four groups. Approximately 52 species were belonging to cyanobacteria, while 94 and 32 species representing Chlorophyta and Bacillariophyta. For CCA analysis revealed that there is a difference in the response of phytoplankton taxa towards the physico-chemical variables in the two khors. In Khor Abu-Simbel, Microcystis aeuriginosa, the species caused the blooming, was strongly positively associated with NO2, BOD, EC, NO3, and pH. Seasonal and spatial variations in phytoplankton succession and distribution in Khor Abu-Simbel and Khor Ramla related to the flood cycle have been proved. A combination of chemical, physical, and biological factors rather than a single factor act in harmony to regulate the composition of the phytoplankton community in Lake Nasser. For the attached diatom species, a total of 114 epipelic and epiphytic species were identified. Gut contents analysis (GCA) offers a microscopical overview of the taxa ingested immediately by fish. The fatty acids analysis revealed that diatoms were the most food sources assimilated, cascaded to higher trophic levels and reached apex of the food web. Followed by bacteria and low contribution of other microalgal groups and macrophytes. Diatom was preferred by consumers because of its higher nutritional value and its contents of polyunsaturated fatty acids (EPA and DHA) in a highly significant amount that can’t be synthesized by consumers. The stable isotope signature revealed that phytoplankton, with carbon isotope signature (δ13C) of -22‰ - 25.99‰, is the only basal food resource of diatoms for grazing food chain and also for bacteria in the detrital food chain through the dead cyanobacteria. The results from this study intensely recommend that using a multi-method approach (GCA, SIA and FAS biomarkers) is a more effective technique to study aquatic food webs. This work is the first integral study to investigate the food web in Lake Nasser khors.