RESEARCH ARTICLE Prolonged high biomass diatom blooms induced formation of hypoxic-anoxic zones in the inner part of Johor Strait Monaliza Mohd-Din 1 & Mohd Firdaus Abdul-Wahab 1,2 & Shaza Eva Mohamad 3 & Haryati Jamaluddin 1 & Shafinaz Shahir 1 & Zaharah Ibrahim 1 & Kieng Soon Hii 4 & Suh Nih Tan 5 & Chui Pin Leaw 4 & Haifeng Gu 6 & Po Teen Lim 4 Received: 17 December 2019 /Accepted: 17 July 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The Johor Strait has experienced rapid development of various human activities and served as the main marine aquaculture area for the two countries that bordered the strait. Several fish kill incidents in 2014 and 2015 have been confirmed, attributed to the algal blooms of ichthyotoxic dinoflagellates; however, the cause of fish kill events after 2016 was not clarified and the causative organisms remained unknown. To clarify the potential cause of fish kills along the Johor Strait, a 1-year field investigation was conducted with monthly sampling between May 2018 and April 2019. Monthly vertical profiles of physical water parameters (temperature, salinity, and dissolved oxygen levels) were measured in situ at different depths (subsurface, 1 m, 5 m, and 8 m) depending on the ambient depth of the water column at the sampling stations. The spatial-temporal variability of macronutrients and chlorophyll a content was analyzed. Our results showed that high chlorophyll a concentration (up to 48.8 μg/L) and high biomass blooms of Skeletonema, Chaetoceros, Rhizosolenia, and Thalassiosira were observed seasonally at the inner part of the strait. A hypoxic to anoxic dead zone, with the dissolved oxygen levels ranging from 0.19 to 1.7 mg/L, was identified in the inner Johor Strait, covering an estimated area of 10.3 km 2 . The occurrence of high biomass diatom blooms and formation of the hypoxic-anoxic zone along the inner part Johor Strait were likely the culprits of some fish kill incidents after 2016. Keywords Fish kill . Harmful algal bloom . Nutrients . Oxygen depletion Introduction The Johor Strait (JS), also known as the Tebrau Strait, is a narrow strait separating the southern part of the Peninsular Malaysia from the island state of Singapore. A cul-de-sac causeway was constructed a hundred years ago connecting the two countries and has divided the strait into two channels, the eastern and western Johor Straits (EJS and WJS), respec- tively, with limited water exchanges between the two sides of the strait (Koh et al. 1991; Tan et al. 2016). The WJS has a long residence time of approximately a year and is markedly eutrophic as compared with the EJS, which has a residence time of about a month and with lower nutrient concentrations throughout the year (Schmoker et al. 2016). Over the years, the strait has been developed intensively and has been utilized for various human activities including port activities, land reclamation, and urban development (Sakari et al. 2010). The strait also served as a major coastal aquaculture area for both countries. The Johor Strait is receiv- ing inflows from two main rivers: the Skudai River on the western side and the Tebrau River on the eastern side Responsible Editor: Vitor Manuel Oliveira Vasconcelos * Po Teen Lim ptlim@um.edu.my 1 Department of Biosciences, Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia 2 Taiwan-Malaysia Innovation Center for Clean Water and Sustainable Energy (WISE Centre), Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia 3 Department of Environmental and Green Technology (EGT), Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia 4 Bachok Marine Research Station, Institute of Ocean and Earth Sciences, University of Malaya, 16310 Bachok, Kelantan, Malaysia 5 Institute of Oceanography and Environment, University Malaysia Terengganu, 21030 Kuala Nerus, Terengganu, Malaysia 6 Third Institute of Oceanography, Ministry of Natural Resources, Xiamen City, China Environmental Science and Pollution Research https://doi.org/10.1007/s11356-020-10184-6