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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.

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 10,000 cells ml. 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 − 2 s − 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.

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.

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.

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.

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 M 2 amplitude with lunar nodal cycle. Potential methodology for the medium term prediction of blooms is suggested in Imari Bay using environmental parameters.

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.

We exposed Pacific oysters Crassostrea gigas to the harmful algal species, Chattonella marina and C. antiqua, at various initial algal cell densities for up to 24 h. None of the oysters died, even after exposure to high cell densities of both Chattonella species. Oysters filtered Chattonella from the aquarium water within 24 h, depending on initial algal cell densities. The oyster clearance rates peaked at a specific cell density for each algal species (about 100 cells ml -1 for C. marina and about 10 cells ml -1 for C. antiqua). However, because of differences in cell sizes between the algal species, the algal biovolume per unit water volume varied widely at the same cell density. The rela- tionships between clearance rates and biovolumes of exposed algae were similar between the 2 algal species; clearance rates by the oysters peaked at about 1.0 × 10 6 μm 3 mm -3 . The filtration activity of the oysters was fundamentally affected by their exposure to algal biovolume and not algal cell density. No histological lesions were found after the oysters were exposed to either Chattonella species. Furthermore, immunohistological observations of the digestive glands of the oysters and analysis of their phytopigment content revealed that Chattonella cells that were cleared from the water during exposure entered the cytoplasm of the oyster digestive glands by phagocytosis.

An unarmored dinoflagellate Cochlodinium polykrikoides has formed red tides responsible for fish mass mortalities especially in coastal areas of western Japan and southern Korea almost every summer to autumn. In laboratory culture, the optimum temperature for growth of the species is ca. 27 °C. Since the species cannot survive in water of temperatures of less than 10 °C, it was considered to over-winter in some certain regions as a motile form or resting cyst, and expand its distribution after the temperature increases to a level tolerable for growth. To determine the over-wintering regions and migration pattern of C. polykrikoides, occurrences of the motile cells were surveyed in the coastal and offshore areas of western Kyushu, Japan and south coast of the Korean Peninsula from April 2006 to August 2008. Cells of C. polykrikoides were found at 14 sites during the investigated period. Motile cells occurred throughout the year in Usuka Bay, Hirado of West Japan. From offshore regions of the Goto Islands and off Shin-Nagasaki Fishing Port, motile cells of C. polykrikoides were first detected from late May, and continuously occurred until February in Nama Bay of the Kami-Goto Islands. This first appearance was before red tides of C. polykrikoides reported at coastal areas in western Kyushu. In Korea, this species was first observed in May and disappeared after October in 2007. These occurrence patterns imply that Usuka Bay in Hirado is one of the over-wintering regions in western Kyushu, and also this species is possibly transported into the northern part of the East China Sea by the Tsushima Warm Current every year.

Concomitant with the bloom of a diatom, Coscinodiscus sp., there was a sudden production of mucilage in the central area of Ariake Sound during the autumn of 2007. To obtain basic information about this marine mucilage and identify the causative organism, we have examined the morphological characteristics of Coscinodiscus sp. and the production of transparent exopolymer particles (TEPs) by this species. Light and electron microscopy studies revealed that the morphological characteristics of the species under investigation were consistent with previous descriptions of Coscinodiscus granii and, hence, we identified this species as C. granii. Alcian blue staining revealed that the TEPs produced by C. granii were composed of acidic polysaccharides. The properties of these polysaccharides were examined using two fluorescein isothiocyanate (FITC)-labeled lectins: FITC-concanavalin A (Con A), which is specific for d-mannose and/or d-glucose, and Ulex europaeus agglutinin-1 (UEA-1)-FITC, which is specific for l-fucose. The Con A label scarcely reacted with the surface of the marine mucilage produced by C. granii, while UEA-1-FITC showed a strong reaction. These results indicate that the TEPs produced by C. granii contain more l-fucose than d-mannose and/or d-glucose. Such TEPs with high l-fucose content are known to be highly viscous. We conclude, therefore, that the polysaccharide composition of the TEPs produced by C. granii may determine the viscosity of the marine mucilage.

To elucidate the ichthyotoxic mechanism of the harmful dinoflagellate Cochlodinium polykrikoides, a bioassay using damselfish was conducted. After exposure to a live-cell suspension of C. polykrikoides, all the fish were died within 90 min. In the presence of catalase and superoxide dismutase (SOD), no significant reduction in the toxicity of C. polykrikoides on the fish was observed. Furthermore, no significant levels of reactive oxygen species (ROS) were detected in five strains of C. polykrikoides isolated in Japan. Our results support the idea that certain toxic substances, rather than ROS, are mainly responsible for the fish-killing activity of C. polykrikoides.

We studied the ecological relationships between the bloom-forming dinoflagellate Heterocapsa circularisquama and its infectious viruses in field surveys conducted in western Japan. The occurrence of H. circularisquama blooms in Imari Bay during 2002 and in Ago Bay during 2002 and 2004 was accompanied by specific increase in abundance of viruses lytic to H. circularisquama. Using northern dot-blot analysis, approximately 96% of the clonal virus isolates collected in the field surveys positively reacted with a molecular probe specific for HcRNAV (H. circularisquama RNA virus); hence, viral impacts on H. circularisquama population observed in these field surveys are considered largely due to HcRNAV and/or its closely related viruses. The dynamics of type UA viruses and type CY viruses having complementary host ranges to H. circularisquama clones were different in each survey and considered to reflect fluctuations in abundance of their suitable host cells in situ. The dynamics of H. circularisquama and its viruses in Ago Bay from 2002 to 2004 suggests the concentration of HcRNAV in the sediment prior to the host's blooming season is a significant factor in determining the size and length of the H. circularisquama blooms. These results support the hypothesis that HcRNAV infection is one of the significant factors affecting the population dynamics of H. circularisquama in both quantity (biomass) and quality (clonal composition).

S.costatumの増殖速度・増殖量を最大にする栄養物質要求量は, 窒素が16mg・l-1, リンが0.5mg・l-1, ケイ素が5～10mg・l-1, ビタミンB12が5～10ng・l-1であった。AGP試験の結果は接種藻の調整法によって著しく異なった。接種と共に持ち込まれる栄養物質を飢餓培養処理で祇減することにより, 接種量を比較的多くしても, 栄養物質濃度を正確に反映したAGPを得ることができた。飢餓培養が不十分な場合は過大値を与え, 本藻の場合は, 栄養物質を要求量の1/100に減じた, B12欠如の培地で2回飢餓培養することで適正なAGPが得られた。本試験法を用いて測定した大阪湾海水のAGPは約105cells・ml-1で, 現場海水はスケレトネマ赤潮になり得るポテンシャルを持ち, その増殖を制限している栄養物質は窒素とリンであると推察された。