Phytoplankton Flora Research Articles

Phytoplankton flora of two rivers in Southeast Brazil: Paraibuna and Pomba Rivers, Minas Gerais

Phytoplankton flora of two rivers in Southeast Brazil - Paraibuna and Pomba Rivers, Minas Gerais). This work is a survey of the phytoplankton communities of the Paraibuna and Pomba Rivers. It aims is to contribute to the existing albeit scarce knowledge of the phytoplankton of Minas Gerais and of lotic phytoplankton in general. The results are based on 32 samples collected from Paraibuna and Pomba Rivers, in Minas Gerais State, during the dry (July and August, 2001) and rainy seasons (February 2002). Forty-eight taxa were identified, 20 from Paraibuna River and 28 from Pomba River, thirty-eight of which are new records for Minas Gerais State. The phytoplankton community of Paraibuna River was strongly influenced by a reservoir close to its source, thus the occurrence of lentic species of cyanobacteria and desmids. However, the phytoplankton community of Pomba River was typical of lotic environments, with great number of diatoms and desmids.

낙동강하류의 식물플랑크톤상과 군집구조

본 연구는 낙동강 하류 20개 지점에 대하여 주위 환경 변화에 대응하기 위하여 식물플랑크톤상과 군집구조에 대한 조사를 실시하였다. 식물플랑크톤상과 군집은 2004년 1월부터 12월까지 매월 1회 조사하였다. 조사기간 동안 식물플랑크톤은 총 94속 196 분류군이 동정되었는데, 여름에 124분류군으로 가장 많은 종이 출현한 반면 겨울에 69분류군으로 가장 적은 종이 출현하였다. 지점별 총 출현종수는 낙동강 하구인 지점 20에서 92분류군으로 가장 많았고 지점 8에서 55분류군으로 가장 적게 출현하였다. 분류군별 출현비율은 모든 계절에서 규조류(diatoms)가 40% 이상으로 가장 높았고 녹조류(green algae)가 20%, 다음으로 남조류(blue-green algae) 등의 순으로 조사되었다. 조사기간 동안 생태적 주요종은 60 분류군이 조사되었는데, 그 중 광분포종은 46분류군, 오염지표 종 25분류군, 적조원인종 21분류군, 우점종 8분류군, 출현빈번종 7분류군이었다. 지점간 유사도지수에 의한 군집분석의 결과, 20개 지점은 4개의 그룹으로 분류되어졌다. This study was focused on the investigation of the phytoplankton flora and community structure for changing of water environment at the rivers in the lower part of Nakdong river, The samples for this study were collected every month from Jan. 2004 To Dec. 2004. the 196 species belonged to 94 genera were identified during this study periods. The number of species was recorded the maximum 124 taxa in summer and the minimum 69 taxa in winter, We observed the maximum taxa (92 taxa) at the site No. 20 and the minimum taxa (55 taxa) at the site No. 8, respectively. Among the phytoplankton communities, diatoms were dominated over 40% of total phytoplankton groups. Subdominant groups were green algae (over 20% of total phytplankton groups). During this study periods, the 60 species were investigated ecologically significant species (46 cosmopolitic species, 25 pollution indicator, 21 redtide causative species, 8 dominant species, and 7 frequently occurred species). According to the cluster analysis by similarity index, the study sites could be divided into 4 groups.

NUTRIENT LIMITATION OF BENTHIC ALGAE IN LAKE MICHIGAN: THE ROLE OF SILICA1

In the Laurentian Great Lakes, phytoplankton growth and biomass are secondarily limited by silica (Si), as a result of phosphorus (P) enrichment. Even modest levels of P enrichment can induce secondary Silimitation, which, in turn, promotes a shift from the native diatom phytoplankton flora to chlorophyte and cyanobacteria species. However, very little is known about the nutritional status of benthic populations and their response to nutrient enrichment. Two experiments were performed in the littoral zone of Lake Michigan where nutrients were delivered to in situ benthic algal (episammic and epilithic) assemblages using nutrient‐diffusing substrata. In order to test the hypothesis that benthic algae in Lake Michigan are Si limited, a 2 × 3 factorial experiment was used to deliver all combinations of Si, N, and P to resident assemblages growing on artificial substrata composed of natural (Si rich) versus calcium carbonate (Si poor) sand. A second experiment utilized a serial enrichment to evaluate the role of Si in mediating changes in taxonomic composition. These findings indicate that benthic algae in Lake Michigan exhibit signs of secondary Si limitation, and that their response to enrichment is similar to the phytoplankton. Moreover, natural sand substrata may provide a source of Si to resident benthic algae.

The genus Pseudo- nitzschia (Bacillariophyceae) in continental shelf waters of Argentina (Southwestern Atlantic Ocean, 38–55°S)

The distribution pattern of Pseudo- nitzschia species, associated phytoplankton flora and its relationships with main environmental factors were studied for the first time in continental shelf surface waters of the Argentine Sea (Southwestern Atlantic Ocean, 38–55°S). Both qualitative and quantitative samples, collected during summer and fall 2003, were examined using light and scanning electron microscopy. Results indicated that the genus Pseudo- nitzschia has a wide distribution along the studied area. It was present at low densities, with infrequent peak abundances and appeared most frequently as a minor component of the diatom populations that typically develop on the continental shelf of the Argentine Sea. Moreover, phytoplankton communities were numerically dominated by unidentified phytoflagellates (≤5 μm) throughout almost all samples analyzed. Eight Pseudo- nitzschia species were identified in our study: P. australis, P. fraudulenta, P. heimii, P. lineola, P. pungens, P. cf. subcurvata, P. turgidula and P. turgiduloides. Of these, P. heimii, P. lineola and P. turgiduloides are new records for the Argentine Sea. Their presence in the area is attributable to the influence of southerly cold water masses. Spatial and temporal variations of the environmental parameters recorded in the study area generally determined the distribution of Pseudo- nitzschia species. P. pungens and P. australis were widely distributed and reached high densities, especially in waters with elevated temperatures and salinities (around 15 °C, 33.8 psu) and low nutrients concentrations. On the other hand, P. heimii, P. lineola, P. turgidula and P. turgiduloides showed a more restricted distribution, with lower densities in relatively cold, less saline (8 °C, 32.45 psu) and nutrient-rich waters. From the Pseudo- nitzschia species found throughout this survey, P. australis, P. fraudulenta, P. pungens and P. turgidula are known as domoic acid (DA) producers around the world, but there is little information on the potential toxicity of these species in Argentina.

Mallomonas multiunca var. pocosinensis var. nov. (Synurophyceae) from freshwater localities along coastal sections of North Carolina, U.S.A.

A new variety of Mallomonas multiunca, M. multiunca var. pocosinensis Siver var. nov., is described from very acidic, humic waterbodies situated on the Atlantic Coastal Plain of North Carolina, U.S.A. The new variety is similar to the type in scale structure, but differs in possessing a unique type of bristle. Bristles of M. multiunca var. pocosinensis have a double cleft-like expanded distal end somewhat similar to the plume-like bristles of Mallomonas plumosa. The concave region of the distal hooked end is interrupted by the presence of a long pointed tooth that essentially divides the cleft into two sections. Based on initial findings it appears that this taxon is an important member of the phytoplankton flora in waterbodies along coastal regions of North Carolina.

A spectrum of phytoplankton flora along salinity gradient in the eastern Niger Delta area of Nigeria

The distribution of phytoplankton at different salinities in the eastern Niger Delta of Nigeria was investigated. The paper aims to contribute to the dearth of salinity-based distribution of organisms as well as phytoplankton studies in the region. A total of 64 taxa of phytoplankton were identified and classified into four divisions of Bacillariophyta (49 taxa), Chlorophyta (7 taxa), Cyanophyta (4 taxa) and Dinophyta (4 taxa). Salinity was found to produce floristical gradients. Three assemblages of phytoplankton were recognized which were those found at low narrow salinity range, those occurring at wide salinity zone and those with high narrow salinity range. Phytoplankton found in narrow salinity range were predominantly chlorophytes, while those occurring at high and narrow salinity were dinoflagellates and some diatoms.

Checklist of Phytoplankton of Shendam Reserviour in Plateau State, Nigeria

A qualitative study of phytoplankton flora was carried out in Shendam reservoir, Plateau state of Nigeria. Seventy one species were recorded indicating a fairly low species number when compared to other Nigerian waters. The major groups of phytoplankton found were chlorophyceae, bacilliarophyceae, cyanophyceae and dirophyceae. The study revealed that the reservoir has a high potential for thriving fishery. Journal of Aquatic Sciences Volume 16, April 2001, pp. 61-63 Key words: Phytoplankton, Nigeria

Fatty acid and lipid content of Oreochromis niloticus L. in Ethiopian lakes — dietary effects of phytoplankton

Abstract— Herbivorous tilapia fish, Oreochrornis niloticus L. (Cichlidae), collected from five Ethiopian lakes were analyzed for lipid and fatty acid contents of the dorsal muscle using spectrophotometry and gas‐liquid chromatography. The results showed remarkable variation between the lakes in both the lipid and total fatty acid content of the fish, 1.7–21 and 1.6–9.3% of dry weight (DW), respectively. Most fish from Lakes Haiq and Chamo contained higher levels of fat (≥ 10% DW) compared to fish from Lakes Ziway, Langeno and Awassa which contained ≤ 5% DW fat. A total of 28 fatty acids were identified. The saturated fatty acids ranged from 5.3–30 mg. gg−1 DW, monounsaturated Fatty acids from 1.3–30 and polyunsaturated fatty acids from 6.8–29 mg. g−1 DW. The most abundant fatty acids were palmitic acid, docosahexaenoic acid (DHA), oleic acid, stearic acid, palmitoleic acid, arachidonic acid and eicosapentaenoic acid (EPA). EPA and DHA alone amounted to 2.6–15 mg. g−1 DW. The ω3/ω6 ratios ranged from 1.3–7.6 and L. Haiq fish showed the highest ratios, 5.1–7.6, indicating that the fat was of high nutritional quality. The reason for such variation is probably the varied diet available to the fish in the various lakes, This study, along with previous reports, confirms that the phytoplankton flora constituting the diet of O. niloticus varies greatly between the study lakes. The variation in phytoplankton is also reflected by the fatty acid pattern of net samples taken from the study lakes.

Vertical migration of Rhizosolenia mats and their significance to NO3− fluxes in the central North Pacific gyre

Journal of Plankton Research Vol.18 no.7 pp.1103-1121. 19% Vertical migration of Rhizosolenia mats and their significance to NO 3 fluxes in the central North Pacific gyre Abstract. Rhizosolenia mat abundance, distribution and chemical composition were studied on two cruises in the central North Pacific gyre in order to determine large-scale distribution patterns and contribution to upward nitrogen (N) flux. These macroscopic diatom mats are composed of multiple species of Rhizosolenia that exploit subsurface nitrate pools by vertically migrating below the euphotic zone. Although numerically dominated by the small-diameter species, R.fallax (73-95% of total num- bers), mat biovolume was dominated by large-diameter (>50 u.m diameter) Rhizosolenia spp. (85-99% of total volume). Integrated mat abundance was substantially higher when mats accumulated at the surface during calm weather (£80 mats nv 2 ) than during windy periods (<23.1 mats m ! ), suggesting that many mats are found below diver-accessible depths. Chemical composition data indicated that nega- tively buoyant mats were physiologically stressed compared to positively buoyant mats; negatively buoyant mats had significantly higher carbon (C):N ratios and carbohydrate per mat, and lower pro- teinxarbohydrate ratios and internal NO,- pools than positively buoyant mats. These ratios suggest that N is a key determinant of buoyancy behavior, and are consistent with vertical migration by mats to exploit deep N pools. The maximum ascent rate of mats was 6.4 m h ' with no relationship to mat size or biovolume. Short-term O 2 evolution revealed no significant photoinhibition; conversion to C fixation yielded assimilation numbers of 4.7 and 7.3 ng C u,g-' chl h 1 in negatively buoyant and positively buoy- ant mats, respectively, although photosynthetic parameters were not statistically different between the two buoyancy classes. Based on photosynthetic rates, ascent rates and estimated N uptake rates, we calculate that a complete migration cycle requires 3.6-5.4 days. When combined with two different estimates of average abundance, we estimate that mats could transport 3.9-40 (imol N nr 2 day 1 into the euphotic zone. Using the wide range of literature values for vertical diffusive transport, this represents < 1 -2000% of the NO, flux into the euphotic zone and the average equivalent of 3-35% of the new NO, consumed in the surface mixed layer. Introduction The phytoplankton flora of the open sea contains very large (up to 10 p-m 3 ), rare, non-motile cells capable of positive buoyancy at rates of several meters per hour. This buoyant flora contains representatives of the diatoms Ethmodiscus (Villareal, 1992) and Rhizosolenia (Villareal, 1988; Moore, 1994), the non-motile dinoflagel- late Pyrocystis (Kahn and Swift, 1978) and the prasinophyte Halosphaera (see Jen- kinson, 1986). Rhizosolenia mats, macroscopic assemblages of buoyant and non-buoyant Rhizosolenia spp. reaching up to 30 cm in size (Carpenter et ai, 1977), can also ascend at rates of meters per hour (Villareal and Carpenter, 1989). Although noted from the warmer waters of all oceans, mats appear to be particu- larly abundant in the central North Pacific gyre (Villareal and Carpenter, 1989). Positive buoyancy by diatoms can only be achieved via density regulation and provides a clear advantage to cells in stratified, stable water columns where turbu- © Oxford University Press Downloaded from http://plankt.oxfordjournals.org/ at University of California, Irvine on June 13, 2014 Tracy A.Villareal, Samantha Woods, J.Keith Moore 2 and Karen Culver- Rymsza 1 Environmental Sciences Program, University of Massachusetts, 100 Morrissey Blvd, Boston, MA 02125-3393 and 'Graduate School of Oceanography, University of Rhode Island, Kingston, RI02881, USA Present address: College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Administration Building, Corvallis, OR 97331-5503, USA

The phytoplankton of Lake Liddell, New South Wales: chlorophyll a concentrations, species seasonal succession, and covariation with nutrients

Samples of the phytoplankton in a freshwater lake, Lake Liddell, New South Wales (Lat: 32° 22′ S, Long. 150° 1′ E) were collected every 4 weeks between October 1987 and November 1988. Chlorophyll a concentrations ranged from 1.8 μg 1−1 to 9.1 μg 1−1 and were positively correlated with the following nutrient parameters: total and nett mass additions of nitrate/nitrite-N and total-N, total additions of Kjeldahl-N, and nett mass addition N-P ratios. There was no correlation between lake nutrient concentrations and chlorophyll a. Factors other than nutrient concentrations appeared to be effecting chlorophyll a concentrations as summer levels were low despite nutrient concentrations being at a maximum. In spring and summer the phytoplankton was dominated by chlorophytes, with dinoflagellates and diatoms most abundant in autumn. During winter cyanobacteria were the most abundant. The relative abundance of chlorophytes was positively correlated with in lake nitrate/nitrite-N concentrations whereas the relative abundance of cyanobacteria was negatively correlated with this parameter. Based on chlorophyll a concentrations and the phytoplankton flora Lake Liddell can be classified as mesotrophic.

Phytoplankton flora in the Warri/Forcados Estuary of southern Nigeria

The phytoplankton flora of the Warri/Forcados estuary varied from typical fresh and brackish water forms to purely distinct marine species. These were made up of members of the Bacillariophyta (about 80 species), Chlorophyta (over 90 species), Cyanophyta (16 species), Dinophyceae (10 species) and the Euglenophyta (8 species). The bulk of the species belonging to the Chlorophyta, Cyanophyta and the Euglenophyta were almost exclusively fresh-water in distribution, being limited to habitats of less than 0.5‰ salinity. Members of the Bacillariophyta were found in all habitats, occurring freely in freshwater, estuarine, and marine ecosystems. In all cases, no true examples of euryhalinity were recorded. The majority of dinoflagellates encountered, as well as a number of marine diatoms, manifested stenohaline peculiarities as they could not tolerate a wide salinity range. The overall influence of the periodic shift in salinity and some physico-chemical parameters along the stretch of the estuary brought about by strong tidal effects and the variable seasonal volume of water discharge or runoff on the pattern and limit of distribution of phytoplankton species is highlighted.

The distribution and ecology of Spiniferomonas (Chrysophyceae) in Connecticut (USA)

Siver, P. A. 1988. The distribution and ecology of Spiniferomonas (Chrysophyceae) in Connecticut (USA). ‐ Nord. J. Bot. 8: 205–212. Copenhagen. ISSN 0107–055X.The genus Spiniferomonas was found to be a common member of the phytoplankton flora in Connecticut lakes, present in over thirty‐seven percent of the 113 samples collected during 1984. Thirteen of the 15 known freshwater Spiniferomonas species were found including S. crucigera, recorded for the first time from North America. Six species were new records for the United States. S. trioralis, S. bilacunosa and S. bourrellyi were the most common taxa. Ecological preferences of some species are presented. This study represents the first major survey of the genus in the United States.

Limnology of oligotrophic dune lakes at Cape Flattery, North Queensland

Physical, chemical and biotic attributes of 16 lakes and ponds in the siliceous dunefields of Cape Flattery, in the humid tropics of Australia, have been investigated. The dune lakes are similar to those of dunefields in south-eastern Queensland, with very low to low conductivity (62-338 �S cm-1), low pH (3.9-6.8), and low to high humic content (gilvin 0.0-31.0 g440 m-1). These lakes are apparently not perched above the local water table. The ionic compositions of all lakes were very similar, with NaCl predominating and with very low concentrations of Mg, Ca, K, and SO4. Bicarbonate was absent or negligible in most lakes. The oligotrophic lakes are characterized by a desmid-diatom limnetic plankton of moderate diversity (12-35 species per lake). Of 144 taxa of phytoplankton recognized, 58% were desmids and 15% diatoms. The zooplankton was of low diversity and dominated by Calamoecia ultima. Twenty-nine species of aquatic macrophytes and 11 species of fish were recorded. Freshwater turtles (possibly Chelodina rugosa Ogilby), and the estuarine crocodile, Crocodylus porosus Schneider, were recorded from some lakes. Principal component analysis of chemical data distinguished three groups of lakes: a series of humic-stained ponds and lakes, a group of clear-water lakes with little or no humic staining, and a former barrier lagoon. Cluster analysis of the phytoplankton flora consistently segregated the clear-water lakes from humic-stained lakes but, in general, concordance of chemical, physiographic and biotic characteristics was poor. Existing classification schemes for Australian dune lakes, based on similar sets of physiographic, chemical and biotic data, do not cater well for the Cape Flattery lakes. A more useful classification may derive from consideration of the hydrological processes influencing their water balance and chemical characteristics.

Clustering and canonical correspondence analysis of phytoplankton and environmental variables in Swedish lakes

Phytoplankton data consisting of 145 species from a limnological study of lakes from relatively undisturbed areas throughout Sweden were analysed in relation to 11 physical and chemical environmental variables. Three multivariate methods were applied: WPGMA clustering and TWINSPAN for classification, and detrended canonical correspondence analysis (DCCA), a recent technique which extracts ordination axes that can be related directly to variation in the environment. Three types of lakes were recognized consistently: acid humic lakes with Gonyostomum semen as the dominant species, very acid impoverished lakes with rather few, stress-tolerant species, and subarctic lakes with low total biomass but with a varied phytoplankton flora. DCCA allowed a straightforward display of the locations of lakes and species along environmental gradients (including the acidification gradient) reflected in phytoplankton composition. It is suggested that such analyses may be a useful tool for the early detection of environmental change.

Shade‐adapted Algae beneath Ice and Snow in the Northern Bothnian Sea

AbstractThe winter‐spring phytoplankton flora in the coastal waters of the northern Bothnian Sea consists mainly of marine cold water species even adapted to low light. These dinoflagellates and diatoms seem to be selectively promoted by the seven months of low light and water temperature. They are able to grow during this period of subsaturated light and have their abundance maxima around ice‐break. Solar radiation measured under ice and snow cover was in the same order of magnitude as the compensation points of some of the species. In summer, these algae species vanish from the water, and surely survive in the form of inactive life stages.

Fluctuations in a late devonian-early Mississippian phytoplankton flora of Ohio, U.S.A

Fossil diversity and abundance fluctuations of Late Devonian (Famennian)—Early Mississippian (Tournaisian) phytoplankton assemblages were determined in a stratigraphic sequence of samples in Barberton test core 3, from northeast Ohio. Both diversity and abundance were relatively high during deposition of the lower Chagrin Shale (Upper Devonian) and both decreased during deposition of the upper Chagrin Shale. The overlying Cleveland Shale (Upper Devonian) is characterized by sharply increased abundance with a corresponding (although less marked) increase in diversity; in contrast, both abundance and diversity were at relatively low levels during deposition of the Lower Mississippian Bedford Shale, the uppermost unit of the sequence. These trends agree with the phytoplankton periodicity model presented by Tappan, which indicates large-scale extinction at the end of the Devonian. Many of the acritarch and prasinophycean algal taxa have restricted stratigraphic occurrences within the section studied, making them useful for local and perhaps worldwide correlation. The stratigraphically restricted taxa provide a basis for the proposal of five biostratigraphic zones within the Upper Famennian (Upper Devonian) and a portion of the Lower Mississippian. In ascending order occur: the Gorgonisphaeridium ohioense Assemblage Zone, the Acriora petala Assemblage Zone, the Muraticavea enteichia Range Zone, the Cymatiosphaera labyrinthica Range Zone, and the Navifusa drosera Assemblage Zone.

Heavy Metal Tolerance in Algae Isolation from Polluted Lakes Near the Sudbury, Ontario Smelters

Abstract The phytoplankton flora of lakes close to the Sudbury smelters is poor both in species and in numbers. The lakes have abnormally high levels of several toxic heavy metals, including nickel and copper. Algal isolates were obtained from two lakes which contained up to 3 ppm of nickel and 0.7 ppm of copper in solution. These isolates were found to be tolerant to higher levels of nickel, copper and silver than were laboratory strains. Their growth response to metals in the medium is distinctive and different from that of the lab strains. Lab strains of Saenedesmus and Chlorella stop growing at 0.1 ppm Cu and 0.5 ppm Ni. “Adapted” lake strains of Saenedesmus, however, continue to grow up to 1.0 ppm Cu and 3.0 ppm Ni. “Adapted” strains of Chlorella stop growing at 0.4 ppm Cu. Both of the “adapted” lake isolates showed a gradually decreasing growth rate with increasing concentrations of metals, whereas the lab strains showed total inhibition at a low metal concentration. Lab and lake strains both take up copper. The uptake is linear with concentration in the media. The lake isolates continued growth until a Cu concentration of 2400+ ppm was reached (on dry t. basis). This suggests a mechanism of metal tolerance rather than one of exclusion as means of survival. The ecological implications of these adaptations to high levels of toxic metals are discussed, especially the finding that the tolerant algae are adapted to high silver levels, even though silver is not a pollutant in the lakes.

Notes on a New Locality at Ljungsbro in the Lower Ordovician of Östergötland, Southern Sweden

Abstract A new locality in the Lower Ordovician limestone sequence is reported from Ljungsbro in the county of Ostergotland, southern Sweden. An account of the Quaternary deposits and a brief description of the underlying Lower Ordovician limestone are given. A rich and well-preserved macro- and semi-macrofossil fauna is listed and an exquisite phytoplankton flora has been examined.

Some Physico-Chemical Investigations of Stratification in Abbot's Pool, Somerset: The Distribution of Some Dissolved Substances

Chemical investigations of surface water from Abbot's Pool have been carried out since autumn 1962 (Karim 1965; Eaton 1967) and will be discussed by Eaton, Karim & Moss (in prep.). Briefly, this work indicated that the nutrient content of the water was high, reflecting the fertile nature of the catchment area (Moss 1967), and analysis of the phytoplankton (Karim 1965) and epipelic flora (Eaton 1967) demonstrated that many species present were characteristic of eutrophic waters. After the establishment of thermal stratification as a regular occurrence in summer together with extreme oxygen gradients both in winter and spring (Moss 1969; Happey 1970a, 1970b), investigations into the chemical content of the deeper water in the pool were made. Muller (1938), Einsele & Vetter (1938), Pearsall & Mortimer (1939) and Mortimer (1941, 1942) all demonstrated that the exhaustion of dissolved oxygen in the hypolimnion of a lake during thermal stratification is accompanied by reduction processes producing an increase in the concentration of dissolved substances, such as silica, phosphate and ammonia; but in a shallow water mass, such as Abbot's Pool, it was thought that chemical changes might not be as extreme as in larger deeper lakes.

麻痺性貝毒-I

1. A series of paralytic shellfish poisoning occurred from eating “akazara-gai”, a kind of scallop (Chlamys sp.) in Ofunato City, Iwate Prefecture from May 18 to 20, 1961, in which 20 persons including one death developed symptoms. The epidemiological data indicated that the outbreak was quite similar, to that reported in foreign countries. 2. The highest toxicity in “akazara-gai” and also in mussel was detected in the mid-gut gland and the toxicity in the former shellfish was more than doubled the latter. 3. A marked difference in the toxicity was seen in the shellfish collected in a period from May 20 to July 1, 1961, the sample collected on June 1, about 2 weeks after the outbreak, being the highest. 4. Almost no appreciable decrease in toxicity was observed when a crude poison was heated in Sörensen's phosphate buffer of pH 7.0 at 80°C. for 60 minutes; about 50% was destroyed at 100°C. for 30 minutes, and completely destroyed by autoclaving at 120°C. for 20 minutes. 5. A marked change in the phytoplankton flora was noted when the shellfish became poisonous. Especially, the number and species of diatoms decreased extremely, a dinoflagellate, Peridinium sp. becoming predominant. However, the true source of poison remained to be solved.