en Ecology Ecology پژوهشي Orginal research papers <span lang="EN" style="font-size:11.0pt"><span style="font-family:&quot;Calibri&quot;,sans-serif">Understanding the influence of abiotic factors on the </span></span><span lang="TR" style="font-size:11.0pt"><span style="font-family:&quot;Calibri&quot;,sans-serif">phytoplankton<b> </b></span></span><span lang="EN" style="font-size:11.0pt"><span style="font-family:&quot;Calibri&quot;,sans-serif">communities is </span></span><span lang="TR" style="font-size:11.0pt"><span style="font-family:&quot;Calibri&quot;,sans-serif">essential </span></span><span lang="EN" style="font-size:11.0pt"><span style="font-family:&quot;Calibri&quot;,sans-serif">for the effective management of aquatic ecosystems. </span></span><span lang="TR" style="font-size:11.0pt"><span style="font-family:&quot;Calibri&quot;,sans-serif">This study was conducted in the Khersan sub-asin, encompassing the Marbareh, Bashar, and Khersan rivers. Seasonal sampling was carried out at five selected stations to assess physicochemical variables and phytoplankton community composition with the aim of evaluating the effects of abiotic parameters on these communities. A total of 55 phytoplankton taxa belonging to four phyla were identified, with Bacillariophyta representing 74.6%. of the total taxa. Non-metric multidimensional scaling (nMDS) revealed that phytoplankton community structure was primarily driven by seasonal variation, with samples clustering distinctly by season, whereas spatial differences among sampling stations had a comparatively minor influence. PERMANOVA confirmed a significant seasonal effect on phytoplankton composition (<i>p</i><0.001).&nbsp; SIMPER analysis indicated that phytoplankton assemblages were more stable during the colder seasons, exhibiting higher within-season similarity in autumn (67.8%) and winter (51.8%) than in the more variable spring (34.7%) and summer (31.7%) communities. Diversity indices did not differ significantly among sampling stations, whereas species richness varied significantly across seasons (<i>p</i><0.001). DistLM marginal tests identified six environmental variables that individually had a significant relationship with phytoplankton community structure (<i>p</i><0.05), with pH showing the strongest individual effect and explaining 17.4% of the variation in the resemblance matrix. The optimal model, selected using the Akaike Information Criterion (AIC), included seven variables water temperature, dissolved oxygen, NH₃, NO₂^-, pH, total organic matter, and discharge, which together explained 71.0% of the total variation. Overall, the findings indicate that phytoplankton community structure in the Khersan sub-basin is jointly regulated by hydrological conditions, seasonal dynamics, and trophic status. The predominance of diatoms throughout most of the study period suggests a relatively stable ecosystem with no evidence of severe eutrophication. These findings provide a valuable baseline for future ecological monitoring, conservation planning, and sustainable management of the Khersan sub-basin.</span></span> Phytoplankton, Community structure, Environmental variables, Multivariate analysis, Khersan River, Iran 749 770 http://jifro.ir/browse.php?a_code=A-10-120-9&slc_lang=en&sid=1 H. Fazli hn_fazli@yahoo.com 100319475328460059322 100319475328460059322 Yes Caspian Sea Ecology Research Center, Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research, Education and Extension Organization (AREEO), Sari, Iran R. Rahmati rahmati764@gmail.com 100319475328460059323 100319475328460059323 No Caspian Sea Ecology Research Center, Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research, Education and Extension Organization (AREEO), Sari, Iran H. Nasrollahzadeh Saravi hn_saravi@gmail.com 100319475328460059324 100319475328460059324 No Caspian Sea Ecology Research Center, Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research, Education and Extension Organization (AREEO), Sari, Iran B. Rahnama rahnama.behzad@gmail.com 100319475328460059325 100319475328460059325 No Caspian Sea Ecology Research Center, Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research, Education and Extension Organization (AREEO), Sari, Iran N. Alishah n.alisha69@gmail.com 100319475328460059326 100319475328460059326 No Caspian Sea Ecology Research Center, Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research, Education and Extension Organization (AREEO), Sari, Iran T. Eskandari taher950@yahoo.com 100319475328460059327 100319475328460059327 No Caspian Sea Ecology Research Center, Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research, Education and Extension Organization (AREEO), Sari, Iran