The aim of this work is to examine the effects of land use and seasonal change on the physical-chemical variables and phytoplankton composition in the fresh water bodies. Two water bodies were investigated during the study, Nagy-Morotva oxbow shallow lake in Hungary and the Tigris River within Mosul city in Iraq. In the Nagy-Morotva oxbow shallow lake, the study aimed to examine whether differences exist among the various zones of an oxbow lake with different land uses based on physical-chemical variables and dominant of phytoplankton species. This oxbow lake consists of two ends bordered by settlements and open water areas used primarily for fishing, with a protected area featuring large aquatic plant coverage and two transition zones towards the open water areas. The lake receives periodic water replenishment only at one end from one of the open water areas, and the water is also used for irrigation during summer. Our research was conducted within a vegetation time of spring, early summer, late summer, and autumn, which revealed significant differences in physical and chemical properties of the water and the characteristic algal species among the various zones of the lake in different sampling times. Oscillatoria sp. was a characterized species in the zones that effected by municipal sewage and irrigation. Our study findings suggest that the ecological classification of smaller water bodies requires regular and extensive studies on the physical- chemical variables of the water and the composition of the phytoplankton, which can be influenced by seasonal changes, vegetation coverage, land use, and water replenishment. Phytoplankton can act as a bio-indicator of water quality, responding quickly and sensitively to changes in the environment. Land use, including agricultural and urban activities, has a significant impact on the dominant phytoplankton species and physical-chemical variables in water bodies, and can lead to pollution and negative effects on the aquatic ecosystem. Our research highlights the importance of ongoing monitoring and management of water bodies to ensure the preservation and maintenance of water quality and aquatic biodiversity. During the investigation in the Tigris River within Mosul city, we investigated the impact of land use on the phytoplankton community and physical-chemical variables in the different zones of the river. Agricultural and urban activities were found to have a significant impact on the water quality of the river, with point and non-point source pollutants affecting the river from upstream through agricultural activities and by urban activities in the middle section of the city, respectively from both banks. Our research was conducted by examining phytoplankton species and physical-chemical variables at 16 sites during each vegetation time, spring, summer, autumn, and winter. The observed a clear connection between pollution caused by different land uses and the effects on the algal community and physical-chemical variables in different zones and seasons of the year. Whereas Oscillatoria sp. was a characterized species in the agricultural zone. Our results indicated a great separation between the four investigated zones (A, B, C, and D) based on changes in the phytoplankton communities and physical-chemical variables. The observed alterations can be attributed to varying land uses: Nutrients released from agricultural regions entered zone A; zone B displayed not only agricultural nutrient influx but also prevalent high levels of CODsMn and TSS; zone C experienced elevated CODsMn and TDS due to diffuse and point-source pollution from urban areas; and the urban influence in zone D resulted in the presence of reduced nitrogen forms (with a notable concentration of nitrate-nitrogen) alongside elevated values of TSS, TDS, and PO4-P.