Sublethal effects of the toxic alga Heterosigma akashiwo on the southeastern oyster (Crassostrea virginica)

Over the last three years, several blooms of Heterosigma akashiwo (Raphidophyceae) were documented in South Carolina (SC) brackish waters, including areas containing extensive oyster (Crassostrea virginica) beds. This study examined the sublethal effects of H. akashiwo on C. virginica, based on cellular biomarker responses after exposure to laboratory cultures of H. akashiwo isolated from SC waters, and to water collected from two SC H. akashiwo blooms. Exposure to laboratory cultures or blooms of H. akashiwo significantly increased oyster hepatopancreas lysosomal destabilization rates, but had little effect on gill p-glycoprotein (p-gp) expression. Lysosomal destabilization in oysters continued to increase even after a 7-day recovery period in clean seawater, suggesting that H. akashiwo toxin or other cellular byproducts continued to damage the hepatopancreas. These results suggest that even short-term exposures of oysters to high cell densities of H. akashiwo could have long-term adverse physiological effects, and imply that oyster health may be compromised in areas where repetitive H. akashiwo blooms occur.     

Structuring of the cyanobacterial community by pelagic fish in subtropical reservoirs: experimental evidence from Australia

1. Subtropical reservoirs of Australia are commonly subject to summer blooms of cyanobacteria. The potential for food web manipulation to control cyanobacterial blooms was investigated in Lake Maroon, south east Queensland using enclosures in which the density of the Australian gudgeon Hypseleotris spp. was manipulated. 2. Zooplankton biomass and community structure were strongly affected by fish density. A size dependent predation effect of Hypseleotris on zooplankton was observed at ambient fish densities, and the community shifted towards a dominance of copepod juveniles and nauplii. Substantial increases in the populations of Ceriodaphnia and calanoid copepods were observed at low fish densities and in the absence of fish. 3. At ambient fish densities total phytoplankton and the proportion of cyanobacteria were maintained at levels similar to those prevailing at day 0. Total phytoplankton and the proportion of cyanobacteria decreased substantially at low fish densities and in the absence of fish. Chlorophytes became dominant in the ‘no fish’ treatment and the grazing‐resistant species Oocystis and Dictyosphaerium were significantly higher than at ambient fish densities. 4. The experiment demonstrated a strong positive relationship between Hypseleotris density and cyanobacteria, and the results suggest that subtropical reservoirs may be suited to food web manipulation as a means of controlling summer cyanobacterial blooms.     

Monitoring for Heterosigma akashiwo using a sandwich hybridization assay

Field testing of a ribosomal RNA (rRNA)-targeted sandwich hybridization assay (SHA) for Heterosigma akashiwo (Raphidophyceae) in Puget Sound, WA, USA, has showed that the lower limit of detection is well below the level at which cells pose a danger to fish. Moreover, the assay has proven to be both rapid and easy-to-use. Isolates of H. akashiwo from Australia, Japan, New Zealand, South Korea, Spain and USA were correctly identified using the SHA, indicating that this diagnostic tool could be deployed globally. Samples containing H. akashiwo can be preserved for subsequent SHA analysis using several methods: fixation with acidic Lugol’s iodine followed by room temperature storage, collection onto Durapore filters followed by storage at −70 °C or, alternatively, the filters are mixed with a lysis solution buffer and the sample lysate stored at −70 °C. Additionally, we sought to determine whether the SHA could successfully detect H. akashiwo in the presence of clay that might some day be used to mitigate the impacts of natural H. akashiwo blooms. Results from preliminary laboratory trials indicate that clay at the maximum proposed dosage rate does not interfere with the assay. Thus, it may be possible to use the SHA as a simple means of following the fate of H. akashiwo cells during larger-scale clay mitigation trials.     

Broad Salinity Tolerance as a Refuge from Predation in the Harmful Raphidophyte Alga Heterosigma akashiwo (Raphidophyceae)

The ability of harmful algal species to form dense, nearly monospecific blooms remains an ecological and evolutionary puzzle. We hypothesized that predation interacts with estuarine salinity gradients to promote blooms of Heterosigma akashiwo (Y. Hada) Y. Hada ex Y. Hara et M. Chihara, a cosmopolitan toxic raphidophyte. Specifically, H. akashiwo's broad salinity tolerance appears to provide a refuge from predation that enhances the net growth of H. akashiwo populations through several mechanisms. (1) Contrasting salinity tolerance of predators and prey. Estuarine H. akashiwo isolates from the west coast of North America grew rapidly at salinities as low as six, and distributed throughout experimental salinity gradients to salinities as low as three. In contrast, survival of most protistan predator species was restricted to salinities >15. (2) H. akashiwo physiological and behavioral plasticity. Acclimation to low salinity enhanced H. akashiwo's ability to accumulate and grow in low salinity waters. In addition, the presence of a ciliate predator altered H. akashiwo swimming behavior, promoting accumulation in low‐salinity surface layers inhospitable to the ciliate. (3) Negative effects of low salinity on predation processes. Ciliate predation rates decreased sharply at salinities <25 and, for one species, H. akashiwo toxicity increased at low salinities. Taken together, these behaviors and responses imply that blooms can readily initiate in low salinity waters where H. akashiwo would experience decreased predation pressure while maintaining near‐maximal growth rates. The salinity structure of a typical estuary would provide this HAB species a unique refuge from predation. Broad salinity tolerance in raphidophytes may have evolved in part as a response to selective pressures associated with predation.     

Detection of Heterosigma akashiwo (Hada) using specific RNA probes: Variability of RNA content with environmental conditions

The raphidophyte Heterosigma akashiwo, which forms toxic blooms, causes major economical losses to the fish industry because of the fish kills involved. It is therefore important to be able to detect not only H. akashiwo but other toxic phytoplankton species as well, rapidly and accurately to reduce losses by fish kills. With this purpose, DNA sequences from H. akashiwo 18S and 28S rRNA gene regions were studied in silico to design species-specific probes to be used in a microarray format. Three strains of H. akashiwo (AC 265, AC 266 and GUMACC 120) were grown at optimal conditions and transferred into new environmental conditions changing either the light intensity, salinity, temperature or nutrient concentrations, to check if any of these environmental conditions induced changes in the cellular RNA concentration. The aim of this experiment was the calibration of several species-specific probes for the quantification of H. akashiwo. Differences on RNA content were not significant (p < 0.05) in any of the treatments, therefore the calibration curves were validated. The designed probes are reliable for the detection and quantification of H. akashiwo cells in natural waters.     

INTERSTRAIN VARIABILITY IN PHYSIOLOGY AND GENETICS OF HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE) FROM THE WEST COAST OF NORTH AMERICA1

High levels of intraspecific variability are often associated with HAB species, and this variability is likely an important factor in their competitive success. Heterosigma akashiwo (Hada) Hada ex Y. Hara et M. Chihara is an ichthyotoxic raphidophyte capable of forming dense surface‐water blooms in temperate coastal regions throughout the world. We isolated four strains of H. akashiwo from fish‐killing northern Puget Sound blooms in 2006 and 2007. By assessing numerous aspects of biochemistry, physiology, and toxicity, we were able to describe distinct ecotypes that may be related to isolation location, source population, or bloom timing. Contrasting elements among strains were cell size, maximum growth and photosynthesis rates, tolerance of low salinities, amino acid use, and toxicity to the ciliate grazer Strombidinopsis acuminatum (Fauré‐Fremiet). In addition, the rDNA sequences and chloroplast genome of each isolate were examined, and while all rDNA sequences were identical, the chloroplast genome identified differences among the strains that tracked differences in ecotype. H. akashiwo strain 07A, which was isolated from an unusual spring bloom, had a significantly higher maximum potential photosynthesis rate (28.7 pg C · cell^?1 · h^?1) and consistently exhibited the highest growth rates. Strains 06A and 06B were not genetically distinct from one another and were able to grow on the amino acids glutamine and alanine, while the other two strains could not. Strain 07B, which is genetically distinct from the other three strains, exhibited the only nontoxic effect. Thus, molecular tools may support identification, tracking, and prediction of strains and/or ecotypes using distinctive chloroplast gene signatures.     

A novel Heterosigma akashiwo (Raphidophyceae) bloom extending from a South Carolina bay to offshore waters

In April 2003, a novel Heterosigma akashiwo bloom was observed that extended from Bulls Bay, South Carolina USA, to approximately 8 km offshore. The bloom was associated with a fish kill of approximately 10⁴ fish. The bloom coincided with salinities anomalously low for the region and optimal for H. akashiwo growth. The low salinities were related to the rediversion of freshwater a month earlier from the Cooper River into the Santee River, which partially feeds into Bulls Bay. H. akashiwo identification was confirmed using a species-specific real-time PCR assay modified for the direct amplification of target DNA from the bloom sample. Because this H. akashiwo bloom was associated with a fish kill, and exposure to bloom waters caused sublethal toxic effects on oysters, the resolution of the cause and potential recurrence of the bloom are of importance to fishery management.     

Assessment of Microzooplankton Grazing on <Emphasis Type="Italic">Heterosigma akashiwo</Emphasis> Using a Species- Specific Approach Combining Quantitative Real-Time PCR (QPCR) and Dilution Methods

Delaware’s Inland Bays (DIB) are subject to numerous mixed blooms of harmful raphidophytes each year, and Heterosigma akashiwo is one of the consistently occurring species. Often, Chattonella subsalsa, C. cf. verruculosa, and Fibrocapsa japonica co-occur with H. akashiwo, indicating a dynamic consortium of raphidophyte species. In this study, microzooplankton grazing pressure was assessed as a top–down control mechanism on H. akashiwo populations in mixed communities. Quantitative real-time polymerase chain reaction (QPCR) with species-specific primers and probes were used in conjunction with the dilution method to assess grazing pressure on H. akashiwo and other raphidophytes. As a comparison, we measured changes in chlorophyll a (chl a) to determine whole community growth and mortality caused by grazing. We detected grazing on H. akashiwo using QPCR in samples where chl a analyses indicated little or no grazing on the total phytoplankton community. Overall, specific microzooplankton grazing pressure on H. akashiwo ranged from 0.88 to 1.88 day⁻¹ at various sites. Experiments conducted on larger sympatric raphidophytes (C. subsalsa, C. cf. verruculosa and F. japonica) demonstrated no significant microzooplankton grazing on these species. Grazing pressure on H. akashiwo may provide a competitive advantage to other raphidophytes such as Chattonella spp. that are too large to be consumed at high rates by microzooplankton and help to shape the dynamics of this harmful algal bloom consortium. Our results show that QPCR can be used in conjunction with the dilution method for evaluation of microzooplankton grazing pressure on specific phytoplankton species within a mixed community. An erratum to this article can be found at     

Influence of UV‐B Irradiation on the Interspecific Growth Interaction between Heterosigma akashiwo and Prorocentrum donghaiense

Enhanced solar ultraviolet‐B (UV‐B) irradiation resulted from the stratospheric ozone depletion has adverse impacts on the primary productivity of marine algae. The effects of UV‐B enhancement on marine algae include reduction in photosynthesis and biomass, alteration in species competition, impaired chloroplast function, and damage to DNA. Harmful algal blooms (HAB) are an ubiquitous natural phenomenon caused by the excessive growth of phytoplankton. Many studies have examined the influence of supplemental UV‐B irradiation on different algae, but the effects of UV‐B irradiation on the interspecific growth interaction of HAB species has received little attention. In this study, Heterosigma akashiwo and Prorocentrum donghaiense were chosen to investigate the effect of UV‐B irradiation on the growth interaction between two HAB species by a co‐culture method. In a mixed culture experiment, H. akashiwo inhibited the growth of P. donghaiense in an inoculum dependent manner. In the absence of UV‐B irradiation, H. akashiwo dominated the competition with P. donghaiense under different inoculation proportions. Surprisingly, supplemental UV‐B irradiation alters the growth interaction between the two HAB species. P. donghaiense dominated the competition at the inoculation proportion of H: P = 1: 4. Supplemental UV‐B irradiation also weakens the competition dominance of H. akashiwo at the inoculation proportion of H: P = 1: 1 or H: P = 4: 1. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of two species of macroalgae—Ulva pertusa and Gracilaria lemaneiformis—on growth of Heterosigma akashiwo (Raphidophyceae)

The effects of fresh thalli, culture filtrate, water-soluble extract and dry powder of two species of macroalgae, Ulva pertusa (Chlorophyta) and Gracilaria lemaneiformis (Rhodophyta), on the growth of a bloom-forming microalga, Heterosigma akashiwo, were studied in co-culture under controlled laboratory conditions. Both fresh thalli and culture filtrate of U. pertusa and G. lemaneiformis, particularly in the form of fresh thalli, significantly inhibited microalgal growth; indeed, the microalga was completely killed during the course of the experiment. A clear concentration-dependent relationship was observed between the initial concentration of fresh thalli (either U. pertusa or G. lemaneiformis) and its inhibitory effect on H. akashiwo. Simultaneous nutrient assays showed that nitrate and phosphate were almost exhausted in G. lemaneiformis fresh thalli co-culture but remained well above nutrient limitation for microalgal growth in U. pertusa co-culture, in which the microalgal cells were completely killed. However, daily f/2 medium repletion would obviously alleviate the growth inhibition in G. lemaneiformis co-culture. Since the present study was carried out under controlled conditions, fluctuations in environmental factors (i.e., light, temperature, carbon limitation, bacterial presence and pH) were limited during the experiment. We thus concluded that allelopathy was the most likely explanation for microalgal growth inhibition in U. pertusa co-culture, while the combined roles of allelopathy and nutrient limitation were responsible for growth inhibition in G. lemaneiformis co-culture. Similarly, macroalgal water-soluble extracts and dry powders affected the co-cultured H. akashiwo greatly, with more obvious effects observed in water-soluble extract co-cultures. A dose-dependent relationship was also observed over the course of the experiment. It can be concluded that macroalgal thalli contain some bioactive compounds. The results of the present study suggest that U. pertusa and G. lemaneiformis, especially in the form of fresh thalli, effectively inhibit the growth of H. akashiwo and could thus be potential candidates for use in the control and mitigation of H. akashiwo blooms.     

Alga-lytic activity of Pseudomonas fluorescens against the red tide causing marine alga Heterosigma akashiwo (Raphidophyceae)

A bacterial strain, HAK-13, exhibited strongest activity against Heterosigma akashiwo and was capable of controlling this bloom forming phytoplankton. Based on 16S rDNA sequences and biochemical and morphological characteristics, the strain HAK-13 was determined to be Pseudomonas fluorescens on the basis of 99.9% similarity with reference strains in the DNA databases. The growth of H. akashiwo was strongly suppressed by HAK-13 in all growth phases, with the strongest alga-lytic activity noted against harmful bloom-forming species in the exponential stage (6–22 days). Host range tests showed that HAK-13 also significantly inhibited the growth of Alexandrium tamarense and Cochlodinium polykrikoides but could not destroy Gymnodinium catenatum. P. fluorescens HAK-13 indirectly attacked H. akashiwo by alga-lytic substances that might be located at the compartment of cytoplasmic membrane of the bacterium at a level of 45.86 units/mg of specific activity. The results indicated that P. fluorescens HAK-13 caused cell lysis and death of H. akashiwo, A. tamarense, and C. polykrikoides dramatically and Prorocentrum dentatum slightly. Therefore, P. fluorescens HAK-13 has potential for use as a selective biocontrol of harmful algal blooms.     

Bottom-up controls on a mixed-species HAB assemblage: A comparison of sympatric Chattonella subsalsa and Heterosigma akashiwo (Raphidophyceae) isolates from the Delaware Inland Bays, USA

Recent novel mixed blooms of several species of toxic raphidophytes have caused fish kills and raised health concerns in the highly eutrophic Inland Bays of Delaware, USA. The factors that control their growth and dominance are not clear, including how these multi-species HAB events can persist without competitive exclusion occurring. We compared and contrasted the relative environmental niches of sympatric Chattonella subsalsa and Heterosigma akashiwo isolates from the bays using classic Monod-type experiments. C. subsalsa grew over a temperature range from 10 to 30 °C and a salinity range of 5–30 psu, with optimal growth occurring from 20 to 30 °C and 15 to 25 psu. H. akashiwo had similar upper temperature and salinity tolerances but also lower limits, with growth occurring from 4 to 30 °C and 5 to 30 psu and optimal growth between 16 and 30 °C and 10 and 30 psu. These culture results were confirmed by field observations of bloom occurrences in the Inland Bays. Maximum nutrient-saturated growth rates (μ_max) for C. subsalsa were 0.6 d⁻¹ and half-saturation concentrations for growth (K_s) were 9 μM for nitrate, 1.5 μM for ammonium, and 0.8 μM for phosphate. μ_max of H. akashiwo (0.7 d⁻¹) was slightly higher than C. subsalsa, but K_s values were nearly an order of magnitude lower at 0.3 μM for nitrate, 0.3 μM for ammonium, and 0.2 μM for phosphate. H. akashiwo is able to grow on urea but C. subsalsa cannot, while both can use glutamic acid. Cell yield experiments at environmentally relevant levels suggested an apparent preference by C. subsalsa for ammonium as a nitrogen source, while H. akashiwo produced more biomass on nitrate. Light intensity affected both species similarly, with the same growth responses for each over a range from 100 to 600 μmol photons m⁻² s⁻¹. Factors not examined here may allow C. subsalsa to persist during multi-species blooms in the bays, despite being competitively inferior to H. akashiwo under most conditions of nutrient availability, temperature, and salinity.     

Extracellular organics from specific cultures of Heterosigma akashiwo (Raphidophyceae) irreversibly alter respiratory activity in mammalian cells

Raphidophyte blooms have been well documented in several coastal areas around the world. Centring raphidophyte-bloom research has been a focus evolving around issues of ichthyotoxicity, allelopathy and anti-predatory activity. However, the details of these phenomena such as the identity of the compounds and the mechanisms underlying these processes are poorly understood. One such raphidophyte, Heterosigma akashiwo (Hada) Hara et Chihara, has historically received much attention with regard to its ichthyotoxic and allelopathic properties. In this study, we collected extracellular organic compounds from cultures of nine H. akashiwo isolates and tested those exudates on two mammalian cell lines: rat osteoblastic sarcoma (UMR-106) and human embryonic kidney (HEK-293). A tetrazolium colourimetric assay was used to determine the activity of mitochondrial dehydrogenases. Exposure of the mammalian cell lines to exudates collected from cultures of H. akashiwo (strain 764) significantly increased activity in a concentration- and time-dependent manner. Exudate concentrations of as little as 0.3 mg ml⁻¹ elicited a stimulatory response in the mammalian cells. This is comparable to the range of exudate concentrations that were originally in the algal cultures (>0.1 mg ml⁻¹). Significant increases in activity were observed 12–24 h following continuous or 1 h (transient) exposure to the exudate. Production of the stimulatory bioactive exudate was not altered by nutrient-stressed H. akashiwo cultures (reduced iron, phosphate or nitrate). Collectively, these bioactive compound(s) consistently increased cellular activity 3–15-fold. Interestingly, of the nine isolates tested, four of them produced the stimulatory exudate, whereas four others did not produce the stimulatory compound(s) and isolate 560R produced a compound(s) that was inhibitory in nature. Thus, we have shown that cultures of H. akashiwo produce organic compounds that can alter the metabolic activity of mammalian cells. Future isolation and characterization of these bioactive compounds may determine them to have ecological relevance, potentially involved in the ichthyotoxic, allelopathic and/or anti-predatory behaviour of this alga.     

DESCRIPTION OF VIRIDILOBUS MARINUS (GEN. ET SP. NOV.), A NEW RAPHIDOPHYTE FROM DELAWARE'S INLAND BAYS

Delaware's Inland Bays (DIB), USA, are subject to blooms of potentially harmful raphidophytes, including Heterosigma akashiwo. In 2004, a dense bloom was observed in a low salinity tributary of the DIB. Light microscopy initially suggested that the species was H. akashiwo; however, the cells were smaller than anticipated. 18S rDNA sequences of isolated cultures differed substantially from all raphidophyte sequences in GenBank. Phylogenetic analysis placed it approximately equidistant from Chattonella and Heterosigma with only ~96% sequence homology with either group. Here, we describe this marine raphidophyte as a novel genus and species, Viridilobus marinus (gen. et sp. nov.). We also compared this species with H. akashiwo, because both species are superficially similar with respect to morphology and their ecological niches overlap. V. marinus cells are ovoid to spherical (11.4 × 9.4 μm), and the average number of chloroplasts (4 per cell) is lower than in H. akashiwo (15 per cell). Pigment analysis of V. marinus revealed the presence of fucoxanthin, violaxanthin, and zeaxanthin, which are characteristic of marine raphidophytes within the family Chattonellaceae of the Raphidophyceae. TEM and confocal microscopy, however, revealed diagnostic microscopic and ultrastructural characteristics that distinguish it from other raphidophytes. Chloroplasts were in close association with the nucleus and thylakoids were arranged either parallel or perpendicular to the cell surface. Putative mucocysts were identified, but trichocysts were not observed. These features, along with DNA sequence data, distinguish this species from all other raphidophyte genera within the family Chattonellaceae of the Raphidophyceae.     

The association of algicidal bacteria and raphidophyte blooms in South Carolina brackish detention ponds

Over the past 5 years, raphidophyte blooms have been frequently observed along the South Carolina coastal zone. During the 2002, 2003, and 2004 sampling seasons, we investigated temporal fluctuations of algicidal bacteria abundance against raphidophycean flagellates (Heterosigma akashiwo, Chattonella subsalsa, and Fibrocapsa japonica) using the microplate most probable number (MPN) method in three Kiawah Island brackish stormwater detention ponds (K1, K2, and K75). Local axenic isolates of H. akashiwo, C. subsalsa, and F. japonica were obtained and their susceptibility to algicidal bacteria tested. A total of 195 algicidal bacterial strains were isolated from raphidophyte blooms in the study ponds, and 6 of them were identified at the genus level, and the taxonomic specificity of their algicidal activity was tested against local (pond) and nonlocal isolates of raphidophytes (3 species, 10 total strains). In the ponds, a consistent association was found between raphidophyte bloom development and an increase in bacteria algicidal to the bloom species. In 12 of 15 cases, bloom decline followed the increase in algicidal bacteria to maximum abundances. Although variability was found in the taxonomic specificity of the algicidal bacteria effect (i.e. the number of raphidophyte species affected by a particular bacteria strain) and raphidophyte susceptibility (i.e. the number bacteria strains affecting a particular raphidophyte species), a toxic effect was always found when strains of a raphidophyte species were exposed to algicidal bacteria isolated from a bloom caused by that same species. The results suggest that algicidal bacteria may be an important limiting factor in raphidophyte bloom sustenance and can promote bloom decline in brackish lagoonal eutrophic estuaries.     

The effects of salinity on the cellular permeability and cytotoxicity of Heterosigma akashiwo

A laboratory study using the fish‐killing raphidophyte Heterosigma akashiwo was conducted to examine its capability to grow at salinities below oceanic, and to test the perceived relationship between reduced salinities and increased cytotoxicity. A nonaxenic strain of H. akashiwo isolated from the U.S. Pacific Northwest was exposed to a combination of three salinity (32, 20, and 10) and five temperature (14.7°C, 18.4°C, 21.4°C, 24.4°C and 27.8°C) conditions. Our results demonstrate that cell permeability and cytotoxicity are strongly correlated in unialgal cultures of H. akashiwo, which both increased as salinity decreased from 32 to 10. Furthermore, over a broad median range of salinities (10 and 20), neither temperature nor specific growth rate was correlated with cytotoxicity. However, in cultures grown at the salinity of 32, both temperature and specific growth rate were inversely proportional to toxicity; this relationship was likely due to the effect of contamination by an unidentified species of Skeletonema in those cultures. The presence of Skeletonema sp. resulted in a cytotoxic response from H. akashiwo that was greater than the response caused by salinity alone. These laboratory results reveal the capability of H. akashiwo to become more toxic not only at reduced salinities but also in competition with another algal species. Changes in cell permeability in response to salinity may be an acclimation mechanism by which H. akashiwo is able to respond rapidly to different salinities. Furthermore, due to its strong positive correlation with cytotoxicity, cellular permeability is potentially associated with the ichthyotoxic pathway of this raphytophyte.     

Triggers of phytoplankton bloom dynamics in permanently eutrophic waters of a South African estuary

The permanently eutrophic Sundays Estuary experiences recurrent harmful algal blooms (HABs) of Heterosigma akashiwo (Raphidophyceae). This study aimed to identify the environmental variables shaping phytoplankton community composition and succession patterns during a typical spring/summer harmful algal bloom (HAB) period. Monitoring of abiotic and phytoplankton variables was undertaken over the period of a month in 2016. Surface water salinity corresponding to mesohaline conditions (9 to 12) was a prerequisite for site selection. During the study, two HABs (>550 µg Chl a l^?1) of H. akashiwo occurred, each lasting for approximately a week in duration. Analyses highlighted nutrient depletion (i.e. nitrate and phosphate concentrations) as the key constraint on bloom duration. When the density of H. akashiwo decreased, the community composition became more diverse with species belonging to Bacillariophyceae and Dinophyceae becoming more abundant; albeit to a lesser degree (<180 µg Chl a l^?1). Dissolved oxygen shifted from super-saturated conditions (>14 mg l^?1) during peak HAB conditions, to instances of bottom water oxygen depletion (2–4 mg l^?1) during the decay phase. These findings highlight the potential severity of transforming a catchment from natural to one that is highly regulated by agricultural practices, while also emphasising the need for management intervention.     

Algicidal effects on Heterosigma akashiwo and Chattonella marina (Raphidophyceae), and toxic effects on natural plankton assemblages by a thiazolidinedione derivative TD49 in a microcosm

The algicidal effects of the thiazolidinedione derivative TD49 on Heterosigma akashiwo and Chattonella marina (Raphidophyceae) were assessed, and the response of the planktonic community and environment to the algicide was evaluated in a microcosm, quantifying 12 L. The abundance of over 80 % of H. akashiwo and C. marina declined in a day significantly in microcosms to which TD49 was added (final concentration 2 μM), and this was correlated with an abrupt decline in the culture pH. The number of protists (i.e., ciliates) other than H. akashiwo and C. marina gradually increased with time in the TD49 treatments, implying that the decline in numbers of H. akashiwo and C. marina cells resulting from TD49 treatment was a major factor in the growth of the other organisms. However, TD49 may be toxic to aquatic zooplankton communities, even though it is a highly selective algicide for harmful algae bloom species. The study indicates that TD49 is an effective agent for the control for H. akashiwo and C. marina blooms in enclosed and eutrophic water bodies.