Marine heat waves (MHWs) alter the marine environment, thus influencing phytoplankton communities. Extremely high sea temperature and harmful algal blooms dominated by Karenia mikimotoi occurred in Imari Bay, Japan in the summer of 2025. The aim of this study was to understand the mechanism of the occurrence of the bloom related to the high sea temperature using field surveys and optimum interpolation sea surface temperature products. Before the MHW, diatoms bloomed, with cell densities reaching tens of thousands per milliliter in relatively cool conditions of the western area of Imari Bay. Sea surface temperature rapidly increased to 31.8 degrees C because of continuous sunny conditions, and the anomalously high temperature was identified as a MHW by a statistical analysis using 30 years of the sea surface temperature dataset. Diatoms sharply declined during the short-term MHW, and subsequently the bloom of K. mikimotoi developed after the MHW. This study suggests that short-term MHWs lasting approximately 10 days have the potential to substantially alter the phytoplankton community in coastal regions.

In our previous study, we found that the peaks of chlorophyll fluorescence spectra of K. mikimotoi and Chattonella spp. shift toward higher wavelengths, defined the degree of peak shifts as the fluorescence spectral shift index (FSI), and developed a method for detecting these harmful algal species and an in situ device, the harmful algal indication sensor (HAI sensor). In this study, we further established a method for estimating the cell abundance of harmful algal species based on FSI, and validated the method through laboratory experiments with cultured strains. Furthermore, a series of field observations for K. mikimotoi was conducted in Saiki Bay, Oita, in 2017 and 2018, and in Kujukushima waters, Nagasaki, in 2019 and 2021. Based on the results from observations, the detection method and the cell abundance estimation method were evaluated. High true positive fractions were confirmed in both methods. Significant positive correlations on a logarithmic scale were observed between the estimated cell densities and cell densities counted by microscopy.

Karenia mikimotoi red tides occur every year along the coast of Nagasaki Prefecture. The frequency of occurrence of K. mikimotoi red tide depends on each sea area. K. mikimotoi red tides were observed widely around Nagasaki in 2021. K. mikimotoi red tides were observed every year from late spring in Kujukushima and Sasebo Bay. In Kujukushima and Sasebo Bay, wintering cells were observed by winter investigation. The wintering cells caused the red tides in Kujukushima and Sasebo Bay. Red tides of K. mikimotoi in Omotaka, Tomie and Nomozaki were recorded for the first time in 2021. According to a particle tracking model, the red tides were advected from Sasebo Bay to Omotaka, Tomie, and Nomozaki and might increase in case of favorable meteorological conditions.

Harmful algal blooms (HABs) of unarmored dino%agellate Karenia mikimotoi have often been recorded first in Sasebo Bay, Nagasaki, Kyushu in each year since 2000. For better forecasting of HABs in Sasebo Bay, the cell density of HAB species and other dominant microalgae was regularly monitored and compared with temperature and salinity for 7 years (2013.2019). Presence and density of resting cysts were also estimated from the cells germinated from the seabed sediments collected in 2013 and 2018. Blooms of three HAB species, Heterosigma akashiwo, K. mikimotoi and Chattonella spp., were observed in this study. Blooms of H. akashiwo often formed earlier than the other two, in the range of 20.23 degrees C and in the years with long sunshine hours from early to mid May. Blooms of K. mikimotoi occurred every year, proliferating from May or June, and denser blooms were recorded in the years with higher total water temperature from January to April and total air temperature from December to March. Chattonella blooms were recorded from 2013 to 2015, and mean Chattonella cyst density was 6.7 cysts cm(-3) in 2013, while no cyst was detected in 2018.

Mass mortality of cultured young bluefin tuna Thunnus orientalis, with the loss of approximately 2,400 individuals, occurred in Asou bay, Tsushima, Nagasaki, Japan in April 2017. Blood fluke eggs were observed in the gills of dead fish, but fish without severe infection also died. At the time, a noxious red tide of Heterosigma akashiwo was also detected (13-90 cells mL(-1)). However, the density of blood fluke eggs in the fish gills, or concentration of H. akashiwo cells in the area alone, was not high enough to have caused the mortality. Since both blood fluke eggs and H. akashiwo are known to have deleterious effects on the respiratory function of fish, the mass mortality was most likely caused by their synergistic effects.

Blooms of the dinoflagellate Karenia mikimotoi cause mass mortality of aquaculture fishes and have become a worldwide environmental problem in coastal waters, especially in Asia. One area is Imari Bay, Japan, where blooms have recurrently formed in recent years. This study examines the meteorological and oceanographical conditions causing the recent increase in occurrence. The recent frequency of occurrence (2012–2018) is nearly four-fold previous years (1999–2011). In recent years, total nitrogen increased in the eastern area where the bloom initially develops, and The Tsushima warm current (TWC) have intensified during spring. Spring total nitrogen has increased as a result of climate change involving increasing rainfall and weakening monsoon rather than eutrophication because of the asynchrony between increasing and total nitrogen in rivers. Intensity of TWC changed in synchrony with M2 amplitude with lunar nodal cycle. Potential methodology for the medium term prediction of blooms is suggested in Imari Bay using environmental parameters.

Harmful algal blooms (HABs) are associated with water quality degradation, which damages fisheries, ecosystems, and public health. Consequently, techniques for detecting the occurrence of HABs have been developed globally. However, conventional techniques are unable to discriminate the causative species/genera of HABs objectively. Therefore, the aim of the present study was to develop a technique for frequently analyzing the causative genera of HAB events using repeat digital photography with a stationary device named “HABcam.” The HABcam was installed at Okigamisenishi monitoring station in the inner western area of Ariake Bay, Japan, and the digital images obtained were analyzed to quantify the sea surface color and to estimate the probability of a HAB occurrence on a daily basis using Bayes' theorem. The estimated probability of a HAB occurrence accurately discriminated a HAB occurrence or non-occurrence in the research area on 19/21 days (=90.5%) for Chattonella spp. and 7/8 days (=87.5%) for Skeletonema spp. These findings indicate that this technique can be used to objectively determine the causative genera during HAB events and to observe HABs with high accuracy and at a high frequency.

An exceptionally extensive bloom of the dinoflagellate Karenia mikimotoi caused mass mortality of aquaculture fishes leading the severe economic loss in 2017 in Imari Bay, Japan, Development of the 2017 bloom completely differed from other recent blooms (in 2012 and 2014). Bloom dynamics and its controlling factors were examined by field survey, numerical modeling and satellite data analysis. The bloom initially developed in the eastern area of Imari Bay as in the 2012 and 2014 blooms. Subsequently, the bloom broadly expanded to the western area and cell density increased to over 10,000 cells ml(-1). Horizontal expansion of the bloom resulted in mass mortality of aquaculture fishes. Horizontal expansion and increase of cell density occurred accompanying a temperature decrease and nutrient increase in the mid and lower layers of the central area. Results of the numerical model indicated two factors related to the passage of a typhoon that promoted the bloom; horizontal bloom expansion induced by the wind-driven current and nutrient supply from the typhoon-induced coastal upwelling. This implies that the effect of the typhoon differentiated the 2017 bloom from previous blooms. The risk of harmful blooms as in the 2017 Karenia bloom in Imari Bay potentially will increase due to the effects of global warming.

Red tides of Karenia mikimotoi blooms have severely impacted marine fish culture in Japanese waters, however, blooms cannot be accurately predicted because they are controlled by a combination of numerous spontaneous environmental parameters, including unknown parameters. Nevertheless, these parameters must eventually control photosynthesis and result in growth, and measuring the photosynthetic activities of this species may be useful for predicting blooms. In this study, K. mikimotoi blooms were predicted in Sasebo Bay, Nagasaki, Japan, in 2014 and 2015 by measuring the maximum quantum yield (Fv/Fm) and relative photochemical electron transport rate (ETR) using pulse-amplitude-modulation (PAM) fluorometers (a cuvette-type and a microscopy-type fluorometer). In 2014, a raphidophycean Heterosigma akashiwo bloom initially dominated at the end of May and was followed by a K. mikimotoi bloom. This change was previously characterized by a decrease in the Fv/Fm of H. akashiwo and an increase in K. mikimotoi. In 2015, a moderate K. mikimotoi bloom was detected at the beginning of the survey and was predicted to increase thereafter based on the population's high Fv/Fm and ETR values. Additionally, these values were highest in the mid-deeper layer, as expected for K. mikimotoi, which prefers lower light intensities in the water column. In both years, blooms of K. mikimotoi were replaced by Prorocentrum dentatum, and these successions were initially recognized by PAM fluorometer measurements. These dinoflagellate blooms eventually vanished due to massive diatom blooms, which were accelerated by nutrient inputs from rivers after heavy precipitation. Furthermore, the activation of these diatom populations was also primarily detected based on their Fv/Fm and ETR increases, especially in the surface layer. Again, this location is expected because diatoms prefer high light intensity. In this study, it was found that various environmental factors influenced the Fv/Fm and ETR values, especially light and nutrient stresses, and these values served as useful proxies for the phytoplankton physiological status. These values could be used to predict the blooming or succession of phytoplankton.

Mass mortality of cultured bluefin tuna Thunnus orientalis, with the loss of approximately 1,800 individuals, occurred in the coastal area of Tsushima Island, Nagasaki in September 2015. A red tide of harmful unarmored dinflagellate Cochlodinium polykrikoides and turbid water were observed simultaneously with the fish mortality. Since tuna farming has not long been established, the impacts of red tide and water environment on cultured bluefin tuna are not fully understood. To understand the cause of bluefin tuna mortality, we examined cell densities of C. polykrikoides and environmental factors at Asou Bay and oSshore from west Tsushima Island. At the time of the mortality of cultured bluefin tuna, C. polykrikoides were 172. 795 cells mL(-1) around the cages. The cell density of C. polykrikoides red tide was lower than that of previous cases of mortality of different fish species. Moreover, mortalities of bluefin tuna were obvious at the cages exposed to turbid water caused by river water in% ow due to heavy rain, 3.8-7.6 FTU. These facts suggest that cultured bluefin tuna is rather sensitive to C. polykrikoides red tide and turbid water.

A massive bloom of the dinoflagellate Karenia mikimotoi appeared in 2014 in Imari Bay, Japan. Bloom dynamics and hydrographical conditions were examined by field survey. The bloom initially developed in the eastern area of Imari Bay, subsequently after rainfall during the neap tides, cell density exceeded over 10,000cellsml. Vertical distribution of K. mikimotoi was primarily controlled by the light intensity and secondarily by the water quality during the daytime. Almost all cell-density maxima occurred in depths with weak daytime light intensities of <300μmolm−2s−1. In some cases of weak light intensity, cell-density maxima occurred in depths with favorable hydrodynamic conditions for the growth. Spatially classified areas were identified by cluster analysis using the growth rate calculated from seawater temperature and salinity. This study quantitatively evaluated the environmental factors of the eastern area, where the bloom initially occurred, during the development of the bloom.

In both 2009 and 2010, massive Chattonella blooms occurred in Tachibana Bay. Observation results show that high cell densities of Chattonella were distributed in the central area of Tachibana Bay with low salinity water. Model results indicate that the low salinity water originated from the Ariake Sea and intruded into Tachibana Bay during the northerly or weak winds. It is suggested that low salinity water was mainly discharged from the northern area of the Ariake Sea. Northerly wind enhanced the horizontal advection of the low salinity water intruding into Tachibana Bay originating from the northern area of the Ariake Sea.

In 2010, a massive bloom of the raphidophycean flagellate Chattonella occurred in the Ariake Sea and Tachibana Bay. Bloom dynamics and hydrographical conditions were examined by field survey. The development and decline of the bloom occurred three times in Tachibana Bay. First and third bloom developments synchronized with precipitation, and the second bloom developed in synchronization with a salinity decrease which occurred in relation to an increase of river discharge from the Chikugo River which takes several days to flow from the Ariake Sea. These results imply that the bloom was transported with the low salinity water from the Ariake Sea to Tachibana Bay. During blooms along the northern coast of Shimabara Peninsula, the predominant phytoplankton species changed from Chattonella to Skeletonema. Low salinity water intrusion induced an interregional difference of the Chattonella and Skeletonema bloom spatially-differentiated by the salinity in the Ariake Sea and Tachibana Bay.

The effects of temperature on growth and production of Lipophilic Toxins (LT) by a monoclonal culture of Dinophysis caudata was studied. The cell density of D. caudata increased significantly with increasing temperature, and was the highest under 27, 30, and 32.5 °C. Temperature affected the average specific growth rate (µ) during the exponential growth phase (EG), which increased from 15 °C to 30 °C, and then decreased at 32.5 °C. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) revealed that this strain of D. caudata produced only pectenotoxin-2 (PTX-2) whose concentration increased significantly with incubation period, except at 32.5 °C. It was significantly different between temperatures ≤18 °C, ≥21 °C, and 32.5 °C. The cellular toxin production (CTP, pg·cell−1·day−1) showed variation with growth phase and temperature, except at 32.5 °C. The average net toxin production (Rtox) was not affected by temperature. During EG, the average specific toxin production rate (µtox) increased significantly with increase in temperature, reaching a peak of 0.66 ± 0.01 day−1 at 30 °C, and then decreased. Over the entire growth span, µtox was significantly correlated to µ, and this correlation was most significant at 27 and 30 °C. During EG, µtox was affected by both temperature and growth. This study shows that temperature affects growth and toxin production of this strain of D. caudata during EG. In addition, a positive correlation was found between toxin production and growth.

2007 年春季および 2008 年春季と秋季の諫早湾における，透明細胞外重合物質粒子（transparent exopolymer particles: TEP）の消長と，それに及ぼす要因について調査を行った。TEP 濃度と水温，塩分，クロロフィル a 濃度には相関がみられた。また，植物プランクトン群集中における珪藻の存在比は，細胞数密度で平均 94% であり，TEP 濃度と相関があった。これらの結果は，春季と秋季の諫早湾における TEP 現存量は珪藻による産生が大きく寄与していることを示す。

Mass mortality of Manila clams occurring occasionally during summer in different years on intertidal fishery grounds along the northern coast of Isahaya Bay in Ariake Sound is ascribed to raphidophyte outbreaks and low dissolved oxygen concentrations during submergence. To prevent mortality, clams enclosed in cages were kept suspended at depths that are free from these factors in the offshore water column. The survival of clams (28-mm mean shell length) was monitored off three fishery grounds for 41 days during July to September, 2008. Different water depths (1.5, 2.5 m) and cage-exchange frequencies (0, 2) were tested. At the same time the survival of clams (25-45 mm shell lengths) on one fishery ground was monitored for 60 days. Following the cage experiment, the clams were transported back to the fishery ground and maintained until March, 2009. The final survival rates in the three programs were 90-97% (cages), 11% (ground), and 84% (back transplantation). On cages with no exchanges for 56 days, clams were covered with fouling organisms, resulting in recovery failure. As the degree of the coverage is temporally variable, a recommended schedule for caging is the entire duration of < 1 month, exchanging in 2 weeks.