The major goal of the current study was to evaluate the water quality at a part of the Damietta branch, Nile River of Egypt via water quality index (WQI) and biological indices. Selected 8 physicochemical parameters were measured in surface water to calculate WQI from 10 sampling sites monthly from January to December 2014. At the same time, bottom sediment collected to record benthos macro-invertebrates biodiversity in the Damietta region. WQI is considered a simple tool successfully applied for assessment the surface water quality via measurement of selected physicochemical parameters to produce a numerical value of WQI. The calculated WQI manifested that the highest value was 66.70% at St.9 in the eastern bank, while the lowest was 56.44 % at St.3 in the western bank. One-way analysis of variance (ANOVA) was applied for spatial and temporal variation. Quantitative estimation was carried out by collecting benthos samples in bottom sediment by modified grab samples. Biological assessment is considered a useful tool for evaluating the ecological status of any aquatic ecosystem using macro-invertebrates as a biological indicator. Species of Chironomus sp., Melanoides tuberculata and Corbicula consobrina were the dominating among the whole species according to 36.1%, 14.9%, and 10.4% of total species respectively. They are potential bio-indicators for the polluted ecosystem. Low species diversity and occurrence of pollution-tolerant species indicate that the quality of the ecosystem deteriorates. The biodiversity was recorded high in the eastern sites and decreased westwards. The maximum persistence of benthos in the eastern bank was in spring and summer. This is in agreement with the WQI of surface water results. Diversity indices were applied beside descriptive Statistics for species diversity showed that the annual average abundance was 1730 ind/m2, with a minimum 431 ind/m2, maximum 3569 ind/m2. An integrated Assessment using WQI and biological impact revealed that the water quality status varied between good to medium. The obtained results showed that the surface water of Eastern banks is better than that of Western banks based on distribution communities recorded. A future regular assessment and monitoring are recommended which will allow prediction and protecting this important ecosystem.

Keywords: Water quality index, Biological Assessment, Nile River, Damietta Branch, Macro-invertebrates, Biodiversity index