Prorocentrum minimum blooms: potential impacts on dissolved oxygen and Chesapeake Bay oyster settlement and growth

The cosmopolitan dinoflagellate Prorocentrum minimum is a recurrent bloom forming species in the Chesapeake Bay and its tributaries, generally observed at its highest levels in late spring and summer. Laboratory studies were conducted to assess potential bloom impacts on diel oxygen concentrations in shallow littoral zones as well as settlement success and post-set growth of the eastern oyster Crassostrea virginica. Using light–dark and dark cultures and periodic diel sub-sampling, bloom levels of P. minimum produced supersaturated oxygen levels at the end of each day while darkened cultures were typified by rapid decreases in dissolved oxygen (DO) (1.1–1.3 mg L⁻¹ h⁻¹) to hypoxic and anoxic levels within 4 days. These data suggest shallow, poorly flushed systems and the biota in them will experience rapid and large diel variations in oxygen, implying recurrent P. minimum blooms need be considered as short-term oxygen stressors for Bay oyster spat and other living resources. Direct effects of P. minimum impacts on oysters were not as expected or previously reported. In one experiment, pre-bloom isolates of P. minimum were grown and then exposed to polyvinyl chloride (PVC) settlement plates to see whether dinoflagellate preconditioning of the hard substrate might affect oyster sets. No differences were noted between set on the PVC with P. minimum exposure to set recorded with filtered seawater, Instant Ocean^®, or Isochrysis. In the second oyster experiment, spat on PVC plates were exposed to field collected P. minimum blooms and a commercial mixture of several other food types including Isochrysis. Oyster growth was significantly higher in P. minimum exposures than noted in the commercial mix. These results, compared to results with other isolates from the same region, indicate substantial positive impact from some of the P. minimum blooms of the area while others separated in space, time, or nutrient status could severely curtail oyster success through toxin production induced by nutrient limitation.     

A fish kill of massive proportion in Kuwait Bay, Arabian Gulf, 2001: the roles of bacterial disease, harmful algae, and eutrophication

In August and September 2001, Kuwait Bay, a semi-enclosed embayment of the Arabian Gulf, experienced a massive fish kill involving over >2500 metric tons of wild mullet (Liza klunzingeri), due to the bacterium Streptococcus agalactiae. In the Bay, this event was preceded by a small fish kill (100–1000 dead fish per day) of gilthead sea bream (Sparus auratus) in aquaculture net pens associated with a bloom of the dinoflagellate Ceratium furca. Sea bream were found to be culture positive for S. agalactiae, but did not show any visible signs of disease. Unusually warm temperatures (up to 35 °C) and calm conditions prevailed during this period. As the wild fish kill progressed, various harmful algae were observed, including Gymnodinium catenatum, Gyrodinium impudicum, and Pyrodinium bahamense var. compressum. Cell numbers of G. catenatum and G. impudicum exceeded 10⁶ l⁻¹ in some locations. All fish tested below the limits of detection for paralytic shellfish poisoning (PSP) and brevetoxins. Clams (Circe callipyga) were positive for PSP but at levels below regulatory limits. Nutrient concentrations, both inorganic and organic, were highly variable with time and from site to site, reflecting inputs from sewage outfalls, the aquaculture operations, a high biomass of decomposing fish, and other sources. It is hypothesized that many factors contributed to the initial outbreak of the bacterial disease, including unusual warm and calm conditions. The same factors, as well as enriched nutrient conditions, also apparently were conducive to the subsequent HAB outbreaks. The detection of PSP, while below regulatory limits, warrants further monitoring to protect human health.     

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.     

Characterization of the affinity for nitrogen, uptake kinetics, and environmental relationships for Prorocentrum minimum in natural blooms and laboratory cultures

During the late spring and early summer of 1998, an extensive bloom of the dinoflagellate Prorocentrum minimum (>93% of phytoplankton cell density) developed in several tributaries of the Chesapeake Bay, USA. In January 1999, a bloom of mixed dinoflagellates (Heterocapsa rotundata, H. triquetra and P. minimum, with P. minimum forming 21% of total phytoplankton cells and 39% of the total biovolume) developed in the mesohaline Neuse Estuary, North Carolina, USA. During these blooms, experiments were carried out to characterize the nitrogen uptake kinetics of these assemblages with ¹⁵N isotopic techniques. Four nitrogenous substrates (NO₃⁻, NH₄⁺, urea, and a mixed amino acids substrate) were used to determine uptake rate and substrate preference. Rates of nitrogen uptake were also measured in P. minimum cultures grown on varying growth nitrogen substrates. The calculated kinetic parameters determined for the P. minimum-dominated field assemblages and the cultures indicated a preference for NH₄⁺. NH₄⁺ was also the primary nitrogen source supporting the blooms. In addition, a high affinity for urea was also found, and urea contributed significantly to the Neuse Estuary bloom. Furthermore, results showed that the regulation of uptake for each of the substrates was different: strong positive relationships between affinity and temperature were found for NH₄⁺ and amino acids, while a negative response was found for NO₃⁻, and very little response to temperature was noted for urea. These differences suggest that a diversity of nitrogen uptake mechanisms may aid the development and maintenance of P. minimum blooms.     

Selective Control of the Prorocentrum minimum Harmful Algal Blooms by a Novel Algal-Lytic Bacterium Pseudoalteromonas haloplanktis AFMB-008041

In this study, we examined the algal-lytic activities and biological control mechanisms of Pseudoalteromonas haloplanktis AFMB-08041, which was isolated from surface seawater obtained at Masan Bay in Korea. In addition, we assessed whether AFMB-08041 could be used as a biocontrol agent to regulate harmful dinoflagellate Prorocentrum minimum. From these experiments, we found that the inoculation of AFMB-08041 at a final density of 2.5 × 10⁴ cfu ml⁻¹ caused P. minimum cells to degrade (>90%) within 5 days. The algal cells were lysed through an indirect attack by the AFMB-08041 bacterial strain. Our results also suggest that the algal-lytic compounds produced by AFMB-08041 may have β-glucosidase activity. However, P. haloplanktis AFMB-08041 was not able to suppress the growth of other alga such as Alexandrium tamarense, Akashiwo sanguinea, Cochlodinium polykrikoides, Gymnodinium catenatum, and Heterosigma akashiwo. Moreover, we observed that the growth of Prorocentrum dentatum, which has a very similar morphological structure to P. minimum, was also effectively suppressed by P. haloplanktis AFMB-08041. Therefore, the effect of AFMB-08041 on P. minimum degradation appears to be species specific. When testing in an indoor mesocosms, P. haloplanktis AFMB-08041 reduced the amount of viable P. minimum cells by 94.5% within 5 days after inoculation. The combined results of this study clearly demonstrate that this bacterium is capable of regulating the harmful algal blooms of P. minimum. In addition, these results will enable us to develop a new strategy for the anthropogenic control of harmful algal bloom-forming species in nature.     

Prorocentrum minimum(clone Exuv) is nutritionally insufficient, but not toxic to the copepod Acartia tonsa

This study tested whether the dinoflagellate Prorocentrum minimum is nutritionally insufficient or toxic to the copepod Acartia tonsa. Experiments were carried out with adult female A. tonsa and the P. minimum clone Exuv, both isolated from Long Island Sound. Initially, the functional and numerical responses of A. tonsa feeding on exponentially growing P. minimum cells were characterized. These experiments revealed that A. tonsa readily ingested P. minimum cells, up to the equivalent of 200% of body carbon day⁻¹, but egg production was relatively low, with a maximum egg production rate of 22% of body carbon day⁻¹. Hence, the egg production efficiency (egg carbon produced versus cell carbon ingested) was low (10%). In a separate experiment, ingestion and egg production rates were measured as a function of food concentration with cells in different growth stages (early-exponential, late-exponential/early-stationary, and late-stationary growth phase) to simulate conditions during a bloom. There was no indication that cells in the stationary phase resulted in lower ingestion or egg production rates relative to actively growing cells. Egg hatching success remained high (>80%) and independent of the cell growth phase. In a third experiment specifically designed to test the hypothesis that P. minimum is toxic, ingestion, egg production and egg hatching success were measured when females were fed mixtures of P. minimum and the diatom Thalassiosira weissflogii, but in which total food concentration was held constant and the proportion of P. minimum in the mixed diet varied. A. tonsa readily ingested P. minimum when it was offered in the mixed diet, with no detrimental effects on egg production or egg hatching observed. Supplementing P. minimum with T. weissflogii increased both the egg production rate and the egg production efficiency. It is concluded that P. minimum is nutritionally insufficient, but not toxic to A. tonsa. Finally, it is estimated that in the field grazing by A. tonsa is approximately equivalent to 30% of the maximum daily growth rate of P. minimum. Hence, copepod grazing cannot be ignored in field and modeling studies of the population dynamics of P. minimum.     

The toxigenic marine dinoflagellate Alexandrium tamarense as the probable cause of mortality of caged salmon in Nova Scotia

The toxins associated with paralytic shellfish poisoning (PSP) are potent neurotoxins produced by natural populations of the marine dinoflagellate Alexandrium tamarense. In early June 2000, a massive bloom (>7×10⁵ cells l⁻¹) of this dinoflagellate coincided with an unusually high mortality of farmed salmon in sea cages in southeastern Nova Scotia. Conditions in the water column in the harbour were characterised by the establishment of a sharp pycnocline after salinity stratification due to abundant freshwater runoff. In situ fluorescence revealed a high sub-surface (2–4 m depth) chlorophyll peak related to the plankton bloom. The intense bloom was virtually monospecific and toxicity was clearly related to the concentration of Alexandrium cells in plankton size fractions. Cultured clonal isolates of A. tamarense from the aquaculture sites were very toxic on a per cell basis and yielded a diversity of PSP toxin profiles, some of which were similar to those from plankton concentrates from the natural bloom population. The toxin profile of plankton concentrates from the 21–56 μm size fraction was complex, dominated by the N-sulfocarbamoyl derivative C2, with levels of other PSP toxins GTX4, NEO, GTX5 (=B1), GTX3, GTX1, STX, C1, and GTX2, in decreasing order of relative abundance. Although no PSP toxin was found systemically in the fish tissues (liver, digestive tract) from this salmon kill event, the detection of Alexandrium cells and low levels of PSP toxins in salmon gills provide evidence that the enhanced mortalities were caused by direct exposure to toxic Alexandrium cells and/or to soluble toxins released during the bloom.     

Use of a real-time remote monitoring network (RTRM) and shipborne sampling to characterize a dinoflagellate bloom in the Neuse Estuary, North Carolina, USA

The spatial-temporal distribution of a dinoflagellate bloom dominated or co-dominated by Prorocentrum minimum was examined during autumn through early spring in a warm temperate, eutrophic estuary. The developing bloom was first detected from a web-based alert provided by a network of real-time remote monitoring (RTRM) platforms indicating elevated dissolved oxygen and pH levels in upper reaches of the estuary. RTRM data were used to augment shipboard sampling, allowing for an in-depth characterization of bloom initiation, development, movement, and dissipation. Prolonged drought conditions leading to elevated salinities, and relatively high nutrient concentrations from upstream inputs and other sources, likely pre-disposed the upper estuary for bloom development. Over a 7-month period (October 2001–April 2002), the bloom moved toward the northern shore of the mesohaline estuary, intensified under favorable conditions, and finally dissipated after a major storm. Bloom location and transport were influenced by prevailing wind structure and periods of elevated rainfall. Chlorophyll a within bloom areas averaged 106 ± 13 μg L⁻¹ (mean ± 1 S.E.; maximum, 803 μg L⁻¹), in comparison to 20 ± 1 μg L⁻¹ outside the bloom. There were significant positive relationships between dinoflagellate abundance and TN and TP. Ammonium, NO₃⁻, and SRP concentrations did not decrease within the main bloom, suggesting that upstream inputs and other sources provided nutrient-replete conditions. In addition, PAM fluorometric measurements (09:00–13:00 h) of maximal PSII quantum yield (F_v/F_m) were consistently 0.6–0.8 within the bloom until late March, providing little evidence of photo-physiological stress as would have been expected under nutrient-limiting conditions. Nitrogen uptake kinetics were estimated for P. minimum during the period when that species was dominant (October–December 2001), based on literature values for N uptake by an earlier P. minimum bloom (winter 1999) in the Neuse Estuary. The analysis suggests that NH₄⁺ was the major N species that supported the bloom. Considering the chlorophyll a concentrations during October and December and the estimated N uptake rates, phytoplankton biomass was estimated to have doubled once per day. Bloom displacement (January–February) coincided with higher diversity of heterotrophic dinoflagellate species as P. minimum abundance decreased. This research shows the value of RTRM in bloom detection and tracking, and advances understanding of dinoflagellate bloom dynamics in eutrophic estuaries.     

Effects of a Prorocentrum minimum bloom on light availability for and potential impacts on submersed aquatic vegetation in upper Chesapeake Bay

Extraordinary spring blooms of the dinoflagellate Prorocentrum minimum have been a recurring feature of upper Chesapeake Bay for many years. Though not thought to be toxic in Chesapeake Bay, these blooms produce extraordinarily high concentrations of chlorophyll, thereby increasing light attenuation. A particularly large event occurred in the spring of 2000. Here, we assess the impact of the spring 2000 P. minimum bloom on habitat quality for submerged aquatic vegetation (SAV) in the mesohaline region of Chesapeake Bay and its tributaries. We determined the light absorption and scattering spectrum of P. minimum on a per cell basis by analyzing inherent optical properties of natural samples from the Rhode River, Maryland, which were overwhelmingly dominated by P. minimum. Using these per cell properties, we constructed a model of light penetration incorporating observed cell counts of P. minimum to predict the impact of the bloom on other tributaries and main stem locations that experienced the bloom. Model estimates of diffuse attenuation coefficients agreed well with the limited measurements that were available. Impacts of the mahogany tide on diffuse attenuation coefficient ranged from negligible (10–30% increase above the seasonal median in the Patapsco and Magothy rivers), to a greater than six-fold increase (Potomac River). Attenuation coefficients in tributaries to the north and south of the bloom region either decreased or were unchanged relative to seasonal medians. Segments with SAV losses in 2000 were mostly the same as those that experienced the P. minimum bloom. Segments north and south of the bloom area mostly had SAV increases in 2000. Though all of the segments that experienced a decline in SAV area after the spring 2000 bloom showed an increase in 2002, the 2000 setback interrupted what otherwise has been a slow recovery in mid-Bay SAV, demonstrating the adverse impact of P. minimum blooms on SAV populations in Chesapeake Bay.     

Effects of light on nitrogen and carbon uptake during a Prorocentrum minimum bloom

During a 4-week period in late spring 1998 an extensive Prorocentrum minimum (Pavillard) Schiller bloom developed in several tributaries of the Chesapeake Bay. Experiments were carried out in one of these tributaries using ¹³C and ¹⁵N isotopic techniques to characterize C and N uptake as a function of irradiance during the course of this bloom. Uptake rates of N substrates (NO₃⁻, NH₄⁺, urea, and an amino acid mixture) and C substrates (bicarbonate and urea) were measured. For each N substrate, short-term uptake rates (0.5 h) were not substantially different over the irradiance range measured, suggesting that N uptake of this dinoflagellate was not strongly light-dependent over this time scale. Dark uptake rates of all N substrates ranged between 35 and 113% of light uptake rates. Over the duration of the P. minimum bloom, however, total ambient N uptake rates increased with increasing natural irradiance. Uptake of bicarbonate showed typical light-dependent photosynthetic characteristics and the measured photosynthetic parameters suggested that at least on the short time scale (0.5 h), P. minimum cells were adapted to high light. Rates of C uptake from the substrate urea were minimal, <1% of total C uptake from photosynthesis, but doubled over the course of the bloom, and like N uptake, were not strongly light-dependent on the short time scale (0.5 h). Significant N dark uptake by P. minimum was likely to have been important by providing N sources over the daily scale to sustain the bloom.     

Toxicity of Karlodinium micrum (Dinophyceae) associated with a fish kill in a South Carolina brackish retention pond

A dinoflagellate bloom was found associated with a fish kill event in a South Carolina brackish water retention pond. A multi-analytical approach was used to confirm the identity of the bloom dinoflagellate and evaluate its potential toxicity. Karlodinium micrum was confirmed through light microscopy, pigment profile comparisons, species-specific PCR, and gene sequence data. Necropsy findings on several fish were suggestive of an acute kill event. Toxicity of filtrate from bloom samples was tested by a hemolytic assay using rainbow trout (Oncorhynchus mykis) erythrocytes and an ichthyotoxicity assay using larval zebrafish (Danio rerio). Hemolytic activity was measurably high (>80% hemolysis) in both whole filtrate and fractionated filtrate (from the 80% MeOH C₁₈ column elution). This fraction also demonstrated high ichthyotoxic activity as exposed fish experienced rapid death. These results implicate toxic K. micrum as a causative factor in fish death in a non-aquaculture brackish pond associated with a housing development, and extend recent findings linking this species to fish kills in aquaculture ponds.     

Occurrences of Prorocentrum minimum (Pavillard) in México

Prorocentrum minimum is a planktonic dinoflagellate known to produce red tides that can be harmful. To recognize localities and understand occurrences of Prorocentrum minimum blooms in Mexico, published data of plankton from 1942 to present, as well as unpublished data from the authors, were reviewed. Studies and reports covered marine and coastal waters of México during different periods. Presence of P. minimum were reported in the Pacific coast, Gulf of California, Gulf of México, and the Caribbean, but blooms have been only reported since 1990. Thirteen bloom events were recorded. Six occurred in shrimp ponds and seven near aquaculture regions or coastal areas where intensive agriculture is practiced. Most of the blooms can be associated with damage to the surrounding marine biota either in aquaculture ponds or open waters. Direct toxicity has not been fully evaluated, but data suggest that low oxygen may not easily explain all of the damage. Interestingly, for yet unknown reasons, cells belonging to the triangular morphotype have seldom been reported in México.     

Potential toxicity of the freshwater Chrysochromulina species C. parva (Prymnesiophyceae)

A mass development of Chrysochromulina parva Lackey — 614 000 cells per ml, associated with fish mortality, is reported from a small Danish lake. None of the analyses performed showed any reason for the fish kill. Even oxygen conditions were favourable. Thus toxins from the Chrysochromulina might be responsible for the fish kill. If true, this would be the first record of toxicity in a freshwater Chrysochromulina.     

The effects of Heterosigma akashiwo on juvenile Oncorhynchus tshawytscha and its implications for fish culture

The effects of natural blooms of Heterosigma akashiwo on freshwater-and saltwater-acclimated juvenile chinook salmon were assessed. Rates of fish mortality in the blooms were independent of acclimation of fish to seawater and the ambient oxygen levels, but were dependent on concentration of algae and ambient water temperatures. No pathological abnormality to gills or other internal organs in the fish were evident. Aeration or oxygenation of fish cages did not enhance or inhibit fish survival in a H. akashiwo bloom. Cause of death was considered to be due to a labile ichthyotoxic agent.     

Impacts and potential effects due to Prorocentrum minimum blooms in Chesapeake Bay

The dinoflagellate Prorocentrum minimum (P. minimum) can be found in all seasons and over a broad range of habitat conditions in the Chesapeake Bay and its tributaries. Blooms (>3000 cells ml⁻¹), locally referred to as ‘mahagony tides’, were restricted to salinities of 4.5–12.8 psu, water temperatures of 12–28 °C, and occurred most frequently in April and May. P. minimum blooms have been detected at routine water quality monitoring stations located in the main channel of the Bay and tidal tributaries. Nearshore investigations of bloom events, however, have accounted for the majority of events recorded in excess of 10⁵ cells ml⁻¹. Mahogany tides were associated with widespread harmful impacts including anoxic/hypoxic events, finfish kills, aquaculture shellfish kills and submerged aquatic vegetation losses. We summarize the state of knowledge regarding physical and chemical factors related to P. minimum blooms, their abundance, distribution and frequency, and ecological effects in Chesapeake Bay.     

An extensive Cochlodinium bloom along the western coast of Palawan, Philippines

A massive fish kill and water discoloration were reported off the western coast of Puerto Princesa, Palawan, Philippines in March 2005. Phytoplankton analysis revealed a near monospecific bloom of the dinoflagellate, Cochlodinium polykrikoides, with cell concentrations ranging from 2.5 × 10⁵ to 3.2 × 10⁶ cells per liter. Ground truth data were supplemented by processed satellite images from MODIS Aqua Level 2 data (1 km resolution) from January to April 2005, which revealed high surface chlorophyll-a levels (up to 50 mg/m³) offshore of west and southwest Palawan as early as February 2005. The bloom extended 310 km in length and 80 km in width at its peak in March off the central coast (Puerto Princesa). By April, the bloom declined in intensity, but was still apparent along the northern coast (El Nido). Fluctuations in chlorophyll levels off the western coast of Sabah, Malaysia and Brunei during this time period suggested that the bloom was not limited to the coast of Palawan. Satellite imagery from Sabah in late January revealed a plume of chl-a that is believed to be the source of the C. polykrikoides bloom in Palawan. This plume drifted offshore, advected northward via the basin-wide counterclockwise gyre, and reached nutrient-rich, upwelled waters near Palawan (due to a positive wind stress curl) where the dinoflagellate bloomed and persisted for 2 months from March to April 2005.     

Restoration of Botshol (The Netherlands) by reduction of external nutrient load: The effects on physico-chemical conditions,plankton and sessile diatoms

The nature reserve Botshol (Utrecht, The Netherlands), consisting of two shallow lakes, ditches and reedland, originated from excavation of peat by man in the 17th century. Up to 1960 Botshol was a clear-water Charophyte lake system. Since the sixties water quality deteriorated and phytoplankton concentrations increased, while the number and dispersion of Chara species decreased. Several restoration measures were attempted to restablish a Charophyte-dominated ecosystem. This paper reports the promising results of this restoration experiment and mentions some complications that arose in restoring the reserve to a less fertile state. The restoration measures have resulted in a sixfold reduction of the external phosphorus load, from 0.6 to 0.1 g m^–2.y^–1, and in a significant reduction of phosphorus levels at all locations. Moreover, the light climate improved and the phyto- and zooplankton compositions changed considerably Unexpectedly, a bloom ofPrymnesium parvum and a fish kill were observed during the last three months of 1990. Despite this fish kill the restoration of the lake is successful so far.     

An unusual bloom of the dinoflagellate Akashiwo sanguinea off the central Oregon, USA, coast in autumn 2009

An algal bloom caused by the dinoflagellate Akashiwo sanguinea was observed in October–November 2009 along the central Oregon coast (44.6°N), off Newport, Oregon, U.S.A. In this paper, the conditions are described which led to the development and demise of this bloom. The bloom was observed for 1 month from 5-October until 4-November with the peak of abundance on 19-October (347,615 cells L⁻¹). The A. sanguinea bloom followed September blooms of the diatoms Pseudo-nitzschia spp, Chaetoceros debilis, and the dinoflagellate Prorocentrum gracile. The bloom occurred when nitrate and silicate concentrations were <2 μM and <8 μM, respectively, and when the water column was stratified. This A. sanguinea dinoflagellate bloom event was closely related to the anomalous upwelling conditions in 2009: upwelling ceased early, at the end of August, whereas a normal upwelling continues into early October. This relaxation extended to near the end of September as a prolonged downwelling event, but then active upwelling reappeared in October and November. The explanation for the occurrence of the A. sanguinea bloom in October may be related to a combination of a prior diatom bloom, a stratified water column with low nutrient concentration in September, and an active upwelling event in October. As for the ultimate source of the cells, the hypothesis is that the seed stock for the A sanguinea bloom off Oregon was southward transport of cells from the Washington coast where a massive bloom of A. sanguinea was first observed in September 2009.     

Characterization, dynamics, and ecological impacts of harmful Cochlodinium polykrikoides blooms on eastern Long Island, NY, USA

We report on the emergence of Cochlodinium polykrikoides blooms in the Peconic Estuary and Shinnecock Bay, NY, USA, during 2002–2006. Blooms occurred during late summer when temperatures and salinities ranged from 20 to 25 °C and 22 to 30 ppt, respectively. Bloom patches achieved cell densities exceeding 10⁵ ml⁻¹ and chlorophyll a levels exceeding 100 μg l⁻¹, while background bloom densities were typically 10³–10⁴ cells ml⁻¹. Light, scanning electron and ultrathin-section transmission electron microscopy suggested that cells isolated from blooms displayed characteristics of C. polykrikoides and provide the first clear documentation of the fine structure for this species. Sequencing of a hypervariable region of the large subunit rDNA confirmed this finding, displaying 100% similarity to other North American C. polykrikoides strains, but a lower similarity to strains from Southeast Asia (88–90%). Bioassay experiments demonstrated that 24 h exposure to bloom waters (>5 × 10⁴ cells ml⁻¹) killed 100% of multiple fish species (1-week-old Cyprinodon variegates, adult Fundulus majalis, adult Menidia menidia) and 80% of adult Fundulus heteroclitus. Microscopic evaluation of the gills of moribund fish revealed epithelial proliferation with focal areas of fusion of gill lamellae, suggesting impairment of gill function (e.g. respiration, nitrogen excretion, ion balance). Lower fish mortality was observed at intermediate C. polykrikoides densities (10³–10⁴ cells ml⁻¹), while fish survived for 48 h at cell densities below 1 × 10³ cells ml⁻¹. The inability of frozen and thawed-, or filtered (0.2 μm)-bloom water to cause fish mortality suggested that the thick polysaccharide layer associated with cell membranes and/or a toxin principle within this layer may be responsible for fish mortality. Juvenile bay scallops (Argopecten irradians) and American oysters (Crassostrea virginica) experienced elevated mortality compared to control treatments during a 9-day exposure to bloom water (5 × 10⁴ cells ml⁻¹). Surviving scallops exposed to bloom water also experienced significantly reduced growth rates. Moribund shellfish displayed hyperplasia, hemorrhaging, squamation, and apoptosis in gill and digestive tissues with gill inflammation specifically associated with areas containing C. polykrikoides cells. In summary, our results indicate C. polykrikoides blooms have become annual events on eastern Long Island and that bloom waters are capable of causing rapid mortality in multiple species of finfish and shellfish.     

Toxic and noxious phytoplankton in Big Glory Bay,Stewart Island,New Zealand

Diurnal vertical profile sampling of the water column, during a fish killing bloom of the raphidophycean alga Heterosigma akashiwo, revealed a phytoplankton population otherwise composed almost entirely of a variety of dinoflagellates. Of these Glenodinium danicum, Dinophysis acuta, Polykrikos schwartzii, Ceratium furca and Gyrodinium spirale were predominant. The distribution of the major species within the phytoplankton were documented and evidence of synchronous vertical migration of H. akashiwo, G. danicum and P. schwartzii was observed. Extracts of shellfish obtained during the bloom and tested by mouse bioassay showed no PSP toxicity but a marginal degree of DSP toxicity. During a subsequent one year phytoplankton monitoring programme another potentially noxious species (Chaetoceros convolutus) appeared and the seasonal reoccurrence of species present during the bloom (e.g. H. akashiwo) was observed. Important year to year differences in the summer phytoplankton (diatom versus flagellate dominated populations) were apparent and analysis of climate data showed that these differences related to different weather conditions prevailing during the two summer periods sampled. The data suggest the fish killing bloom was giving a chance to develop by a prolonged period of warm, calm weather (during which several heavy rainfall events occurred) leading to stable hydrographic conditions (i.e. stratification) and an increase in the retention time of water within the bay.