WaterSubject

The abundance and preferences of individual invertebrate populations (including zooplankton) closely associated with the substrates provided by aquatic plant structures and open-water areas of Lake Nasser were quantified in this study in order to gain understanding of the importance of submerged macrophyte for invertebrate diversity, and their relation to water properties. The following water parameters were measured: temperature, pH, dissolved oxygen (DO), total dissolved salts, electrical conductivity, turbidity, total suspended solids, carbonate, bicarbonate, nitrate, nitrite, phosphate, sulphate, silica, potassium, total hardness, calcium and magnesium.

Five macrophyte species were recorded: Myriopyllum spicatum, Najas horrida, Potamogeton schweinfurthii, Potamogeton pectinatus and Vallisneria spiralis. In total 67 invertebrate species were recorded, comprising 39 Rotifera, 12 Cladocera, 4 Copepoda, 4 Insecta, 2 Protozoa, 2 Ostracoda and one species of Turbellaria, Tardigrada, Annelida and Nematoda. Thirty-seven species were exclusively epiphytic, 11 species were collectively planktonic and 19 species were found in both habitats. The greatest abundance of epiphytic invertebrates occurred in association with N. horrida–P. schweinfurthii community.

The results indicated that total suspended solids (TSS), TH and NO₂ are the most influential water variables on the distribution of the aquatic macrophyte samples and their invertebrate communities. Also, the study indicates that water variables have a higher impact on the aquatic macrophytes than on the associated invertebrate populations. P, NO₃, K, Na, Mg, Cl and DO were the most influential water variables that dictate the distribution of invertebrate groups recorded in the open-water zone. Water temperature, electric conductivity, pH, NO₂, SO₄^??, SiO₃^?, CO₃^?? and turbidity have a lesser influence of the distribution of the invertebrates recorded in this zone.

Lake Nasser is a subtropical monomictic lake. It is mesotrophic lake. The only source of lake water is the River Nile. The outflow is the continuation of the Nile towards the north. The Nile flood comes once a year in late August originating from the Ethiopian highlands continuing to late November.

Submerged macrophyte communities play a crucial role for animals and lower plants in aquatic ecosystems by providing habitat complexity, shelter, breeding area as well as being substrata for periphyton and sites of abundant food production for many aquatic animals. Therefore, they influence the diversity, abundance and distribution pattern of aquatic invertebrates and vertebrates.

Rotifers, cladocerans and insect larvae are the major secondary producers of both pelagic and vegetation areas on a world-wide scale. Some species were commonly found in both areas, whereas others are found in, or in the near vicinity of, stands of vegetation. However, surprisingly few studies have reported the influences of submerged macrophytes on diversity of invertebrates in freshwater ecosystems. In addition to the prevailing environmental conditions, there are many other factors that may affect epiphytic and plant-associated invertebrate distribution. These are submerged plant attributes (e.g. morphology, surface texture, tissue nutrient contents, defensive chemicals), invertebrate predominant life form, mobility, predation and seasonality Depending on these factors, epiphytic invertebrates may switch their habitats between submerged plants and lake water. Therefore, it was important to measure invertebrates on macrophytes and separately in open lake water to gain a better understanding the impacts of submerged macrophytes on diversity of invertebrates.

reported that the Convention on Biological Diversity set conservation of biodiversity on the world agenda and therefore, gaps in knowledge need to be addressed for actions to be effective and sustainable. Gaps include: species diversity, micro-organisms and their ecological role, ecological and geographical status of species, human capacity to assess and forecast bio-ecological degradation.

This paper aims to fill part of an important gap in biodiversity knowledge by reporting their interacting ecological roles of macrophytes and invertebrates in the aquatic ecosystem of Lake Nasser, a major African sub-tropical reservoir. Also, the paper aims to compare invertebrates associated with macrophyte communities in littoral zone with those in the open water of Lake Nasser for better understanding of the influences of submerged macrophytes on diversity of invertebrates and to determine the effects of various water variables on abundance and distribution of invertebrates in one of the world’s largest man-made lake.

The authors are grateful to the High Dam Lake and Aswan Reservoir Authority, Egyptian Ministry of Irrigation and Water Resources for funding this study’s field trip to Lake Nasser. I would like to express my thanks to Dr. Kevin Murphy, University of Glasgow and Dr. Sidinei Magela Thomaz, Universidade Estadual de Maringá for revising the MS and special thanks to the two referees for their critical reviews and helpful suggestions which greatly improved the manuscript.