en Ecology Ecology پژوهشي Orginal research papers <span style="font-size:11pt"><span style="text-justify:inter-ideograph"><span style="line-height:120%"><span style="font-family:Calibri,sans-serif"><span style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times="">Harmful algal blooms caused by dinoflagellates have significant adverse effects on environmental and public health. This study aimed to investigate the effect of water physicochemical parameters on the annual cycle of epiphytic dinoflagellates in the northern Chabahar Bay coastal waters of the Oman Sea (Iran). The macroalgal samples with associated epiphytes were collected seasonally from 6 coastal sites in spring, summer, atumn 2019 and winter 2020. The water physicochemical parameters were measured, and the data were analyzed using a one-way ANOVA and the principal component analysis (PCA).</span></span></span> <span style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times="">Twelve potentially toxic </span></span></span><span style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times="">dinoflagellate </span></span></span><span style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times="">species from five genera were identified during the four sampling seasons</span></span></span><span style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times="">. <i>Amphidinium</i></span></span></span><i><span style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times=""> carterae</span></span></span></i><span style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times=""> with an average of 11.22% and <i>A. operculatum</i> with an average of 10.77% of the total abundance of epiphytic dinoflagellates were the dominant species, and <i>Gambierdiscus australes</i> showed an average of 6.48%.</span></span></span><span style="font-size:12.0pt"><span style="line-height:120%"><span new="" roman="" style="font-family:" times="">Based on the PCA, the abundance of certain species was found to be influenced by different environmental factors. The PCA revealed that NO₂, NO₃ and SiO₄ values had the greatest impact at sites with high abundances of <i>A. operculatum</i>, <i>Prorocentrum concavum</i>, <i>P. emarginatum</i>, <i>P. rhathymum</i> and <i>G. balechii</i>. Furthermore, PO₄ concentration had the greatest impact at the sites with high abundances of <i>A. carterae</i>, <i>P. lima</i>, <i>Ostreopsis lenticularis</i>, <i>O. heptagona</i>, <i>G. balechii</i>, <i>G. toxicus</i>, <i>G. australes</i> and <i>Coolia monotis</i>. The results obtained highlighted a significant impact of dissolved oxygen, pH, salinity, temperature and nutrients on the epiphytic dinoflagellate species abundances in the study area.</span></span></span><span style="font-size:12.0pt"><span style="background:#ffc000"><span style="line-height:120%"><span style="font-family:"Times New Roman",serif"></span></span></span></span></span></span></span></span> Epiphytic Dinoflagellates, Microphytobenthos, Annual Cycle, HABs, Harmful Algal Blooms, Red tide 1184 1206 http://jifro.ir/browse.php?a_code=A-10-4753-1&slc_lang=en&sid=1 M. Nazari m.nazari.ec@gmail.com 100319475328460055349 100319475328460055349 No Department of Marine Biology, Faculty of Marine Sciences, Chabahar Maritime University, Chabahar, Iran G. Attaran Fariman gilan.attaran@gmail.com 100319475328460055350 100319475328460055350 Yes Department of Marine Biology, Faculty of Marine Sciences, Chabahar Maritime University, Chabahar, Iran Y.B. Okolodkov yuriokolodkov@yahoo.com 100319475328460055351 100319475328460055351 No Instituto de Ciencias Marinas y Pesquerías (ICIMAP-UV), Universidad Veracruzana, Calle Mar Mediterráneo Núm. 314, Fracc. Costa Verde, Boca del Río 9429, Mexico