Recurrent vernal presence of the toxic Alexandrium tamarense/Alexandrium fundyense (Dinoflagellata) species complex in Narragansett Bay,USA

The vernal occurrence of toxic dinoflagellates in the Alexandrium tamarense/Alexandrium fundyense species complex in an enclosed embayment of Narragansett Bay (Wickford Cove, Rhode Island) was documented during 2005 and 2009–2012. This is the first report of regular appearance of the Alexandrium fundyense/Alexandrium tamarense species complex in Narragansett Bay. Thecal plate analysis of clonal isolates using SEM revealed cells morphologically consistent with both Alexandrium tamarense Lebour (Balech) and Alexandrium fundyense Balech. Additionally, molecular analyses confirmed that the partial sequences for 18S through the D1–D2 region of 28S were consistent with the identity of the two Alexandrium species. Toxin analyses revealed the presence of a suite of toxins (C1/2, B1 (GTX-5), STX, GTX-2/3. Neo, and GTX-1/4) in both Alexandrium tamarense (6.31 fmol cell⁻¹ STX equiv.) and Alexandrium fundyense (9.56 fmol cell⁻¹ STX equiv.) isolated from Wickford Cove; the toxicity of a Narragansett Bay Alexandrium peruvianum isolate (1.79 fmol cell⁻¹ STX equiv.) was also determined. Combined Alexandrium tamarense/Alexandrium fundyense abundance in Wickford Cove reached a peak abundance of 1280 cells L⁻¹ (May of 2010), with the combined abundance routinely exceeding levels leading to shellfishing closures in other systems. The toxic Alexandrium tamarense/Alexandrium fundyense species complex appears to be a regular component of the lower Narragansett Bay phytoplankton community, either newly emergent or previously overlooked by extant monitoring programs.     

Detection and quantification of cultured marine Alexandrium species by real-time PCR

The occurrence of harmful algal blooms (HABs) throughout the world has increased and poses a large threat to human health, fishery resources and tourism industries. The genus Alexandrium includes a number of toxic species associated with HABs. Therefore, it is very important to rapidly detect and monitor the harmful algae, such as Alexandrium genus. In this study, a standard curve of plasmid containing 18S rDNA-28S rDNA region from Alexandrium catenella was constructed and 5.8S rDNA sequence served as the primer of the real-time PCR. Cultured A. catenella, Alexandrium affine, Alexandrium lusitanicum and Alexandrium minutum samples were analyzed by real-time PCR using the same set of primers simultaneously. Using microscopy cells counts, 5.8S rDNA copies per cell and total DNA per cell were estimated. This assay method is promising for rapid detection of large number of Alexandrium samples.     

Zooplankton community grazing impact on a bloom of Alexandrium fundyense in the Gulf of Maine

Shipboard grazing experiments were conducted in the Gulf of Maine and on Georges Bank during of June 2006 to estimate zooplankton community grazing impact on a natural bloom of the toxic dinoflagellate Alexandrium fundyense. Surface seawater samples containing natural populations of grazers and A. fundyense from 23 stations were incubated at ambient temperatures. Concentrations of A. fundyense after incubations were compared to those at the start of each experiment to determine net increases due to population growth, or decreases presumed to be primarily due to grazing losses. Abundances of both microzooplankton (tintinnids, oligotrich ciliates, rotifers, copepod nauplii and heterotrophic dinoflagellates) and mesozooplankton (copepod nauplii, copepodites and adult copepods, rotifers, marine cladocerans, and meroplankton) grazers in experimental aliquots were also determined. The total zooplankton community had minimal grazing impact on natural populations of A. fundyense at most stations. At 70% of the stations where grazing experiments were performed, there were no significant differences in initial and final concentrations of A. fundyense. This indicated that growth of, and grazing on A. fundyense were in approximate balance. At 2 stations, which had the highest A. fundyense abundances of the cruise (>10⁴ cells l⁻¹), % of the A. fundyense population grazed per day was significantly negative, indicating that net population growth of A. fundyense exceeded grazing losses. At 5 stations, which had low concentrations of A. fundyense (10²–10³ cells l⁻¹), % of the A. fundyense population grazed per day was significantly positive, indicating that losses of A. fundyense due to grazing exceeded net population growth. For stations with significant differences between Initial and Grazed concentrations of A. fundyense, grazing had the greatest impact at lower concentrations of A. fundyense, and grazing impact by the larger mesozooplankton was inversely related to zooplankton abundance. There was no relationship between microzooplankton abundance and grazing impact on A. fundyense. Grazing exceeded growth only where A. fundyense abundance was low, and growth exceeded grazing only where A. fundyense abundance was high. The inverse relationship between grazing impact and A. fundyense abundance implies that grazing may be capable of retarding bloom development at low concentrations typical of the early stages of a bloom, but at higher concentrations once a bloom becomes established, either grazing maintains a balance with A. fundyense growth, or growth exceeds grazing losses at highest concentrations.     

The role of dissolved organic matter (DOM) quality in the growth enhancement of Alexandrium fundyense (Dinophyceae) in laboratory culture1

Several algal species responsible for harmful algal blooms (HABs), such as Alexandrium fundyense, are mixotrophic under certain environmental conditions. The ability to switch between photosynthetic and heterotrophic modes of growth may play a role in the development of HABs in coastal regions. We examined the influence of humic dissolved organic matter (HDOM) derived from terrestrial (plant/soil) and microbial sources on the growth of A. fundyense. We found that a terrestrially derived HDOM, Suwannee River humic acid (SRHA), did enhance A. fundyense growth; however, a microbially derived HDOM, Pony Lake fulvic acid (PLFA) did not enhance growth. A. fundyense grows in association with bacteria in culture and we observed that bacterial cell densities were much lower in A. fundyense cultures than in bacteria‐only cultures, consistent with bacterial grazing by A. fundyense in culture. In bacteria‐only cultures with added algal exudates (EX), the addition of PLFA and SRHA resulted in a slight increase in bacterial cell density compared to cultures without HDOM added. Changes over time in the chemical quality of the HDOM in the A. fundyense cultures reflected contributions of microbially derived material with similar characteristics as the PLFA. Overall, these results suggest that the chemical differences between SRHA and PLFA are responsible for the greater effect of SRHA on A. fundyense growth, and that the differential effect is not a result of an effect on the growth of associated bacteria.     

Testing for resistance of pelagic marine copepods to a toxic dinoflagellate

With few exceptions, the evolutionary consequences of harmful algae to grazers in aquatic systems remain unexplored. To examine both the ecological and evolutionary consequences of harmful algae on marine zooplankton, we used a two-fold approach. In the first approach, we examined the life history responses of two geographically separate Acartia hudsonica (Copepoda Calanoida) populations reared on diets containing the toxic dinoflagellate Alexandrium fundyense . One copepod population was from a region, Casco Bay, Maine, USA, that has experienced recurrent blooms of highly toxic Alexandrium spp. for decades; whereas the other population from Great Bay, New Jersey, USA, has never been exposed to toxic Alexandrium blooms. The life history experiment demonstrated that when the copepod population from New Jersey was reared on a diet containing toxic A. fundyense it exhibited lower somatic growth, size at maturity, egg production and survival than the same population reared on a diet without toxic A. fundyense . In contrast, toxic A. fundyense did not affect the life-history traits of the Maine population. Fitness, finite population growth rate (), was significantly reduced in the New Jersey population, but not in the Maine population. These results are consistent with the hypothesis of local adaptation (resistance) of the historically exposed copepod population to the toxic dinoflagellate. In the second approach, we further tested the resistance hypothesis with a laboratory genetic selection experiment with the naïve New Jersey copepod population exposed to a diet containing toxic A. fundyense. This experiment demonstrated that the ingestion and egg production of adult females of naïve copepods fed A. fundyense improved after three generations of being reared on a diet containing the toxic dinoflagellate. The results of the present study have important implications for understanding how grazer populations may respond to the introduction of toxic algae to their environment, and suggest that grazer resistance may be a feedback mechanism that may lead to bloom control.Co-ordinating editor: Hurst     

PATTERNS IN HOST RANGE FOR TWO STRAINS OF AMOEBOPHRYA (DINOPHYTA) INFECTING THECATE DINOFLAGELLATES: AMOEBOPHYRA SPP. EX ALEXANDRIUM AFFINE AND EX GONYAULAX POLYGRAMMA1

The endoparasitic dinoflagellate Amoebophrya ceratii (Koeppen) Cachon uses a number of its free‐living relatives as hosts and may represent a species complex composed of several host‐specific parasites. Two thecate host–parasite systems [Amoebophrya spp. ex Alexandrium affine (Inoue and Fukuyo) Balech and ex Gonyaulax polygramma Stein], were used to test the hypothesis that two strains of Amoebophrya have a high degree of host specificity. To test this hypothesis, a series of cross‐infection experiments were conducted, with 10 thecate and three athecate dinoflagellate species as potential hosts. Surprisingly, the two strains of Amoebophrya lacked host specificity and had wider host ranges than previously recognized. Among the host species tested, Amoebophrya sp. ex Alexandrium affine was capable of infecting only species of genus Alexandrium (Alexandrium affine, Alexandrium catenella, and Alexandrium tamarense), while the parasite from Gonyaulax polygramma infected species covering five genera (Alexandrium, Gonyaulax, Prorocentrum, Heterocapsa, and Scripsiella). In the context of previous reports, these results suggest that host specificity of Amoebophrya strains varies from extremely species‐specific to rather unspecific, with specificity being stronger for strains isolated from athecate hosts. Information on host specificity of Amoebophrya strains provided here will be helpful in assessing the possibility of using these parasites as biological control agents for harmful algal blooms, as well as in defining species of Amoebophrya in the future.     

Morphology,toxicity, and phylogeny of Alexandrium (Dinophyceae) species along the coast of China

The toxigenic genus Alexandrium includes ∼30 species, but information about its biogeography at a regional scale is limited. In this study, we explored the diversity of Alexandrium along the coast of China by incubating resting cysts collected from 7 sites. A total of 231 strains of Alexandrium belonging to 7 morphospecies were found. Among them, Alexandrium andersonii, Alexandrium fraterculum, Alexandrium leei, Alexandrium pseudogonyaulax, and Alexandrium tamutum were recorded from the China Sea for the first time. Partial large subunit (LSU) and/or internal transcribed spacer region (ITS1, ITS2, and 5.8S rDNA) sequences revealed two ribotypes of Alexandrium andersonii, Alexandrium leei, and Alexandrium tamarense: Atama complex Group I and IV. Atama complex Group I was exclusively distributed in the Yellow Sea and the Bohai Sea, whereas Group IV was restricted to the East China Sea and South China Sea. Atama complex Group I produced mainly N-sulfocarbamoyl toxins (C1/C2, 61–79% of total toxins) and gonyautoxins (GTX1/4, 17–37%). Alexandrium ostenfeldii strain ASBH01 produced NEO and STX exclusively (65% and 35%, respectively). Our results support the premise that Atama complex Group I is endemic to the Asian Pacific and includes cold water species, whereas Atama complex Group IV tends to inhabit warmer waters.     

Identification of unique microbiomes associated with harmful algal blooms caused by Alexandrium fundyense and Dinophysis acuminata

Biotic interactions dominate plankton communities, yet the microbial consortia associated with harmful algal blooms (HABs) have not been well-described. Here, high-throughput amplicon sequencing of ribosomal genes was used to quantify the dynamics of bacterial (16S) and phytoplankton assemblages (18S) associated with blooms and cultures of two harmful algae, Alexandrium fundyense and Dinophysis acuminata. Experiments were performed to assess changes in natural bacterial and phytoplankton communities in response to the filtrate from cultures of these two harmful algae. Analysis of prokaryotic sequences from ecosystems, experiments, and cultures revealed statistically unique bacterial associations with each HAB. The dinoflagellate, Alexandrium, was strongly associated with multiple genera of Flavobacteria including Owenweeksia spp., Maribacter spp., and individuals within the NS5 marine group. While Flavobacteria also dominated Dinophysis-associated communities, the relative abundance of Alteromonadales bacteria strongly co-varied with Dinophysis abundances during blooms and Ulvibacter spp. (Flavobacteriales) and Arenicella spp. (Gammaproteobacteria) were associated with cells in culture. Eukaryotic sequencing facilitated the discovery of the endosymbiotic, parasitic dinoflagellate, Amoebophrya spp., that had not been regionally described but represented up to 17% of sequences during Alexandrium blooms. The presence of Alexandrium in field samples and in experiments significantly altered the relative abundances of bacterial and phytoplankton by both suppressing and promoting different taxa, while this effect was weaker in Dinophysis. Experiments specifically revealed a negative feedback loop during blooms whereby Alexandrium filtrate promoted the abundance of the parasite, Amoebophrya spp. Collectively, this study demonstrates that HABs formed by Alexandrium and Dinophysis harbor unique prokaryotic and eukaryotic microbiomes that are likely to, in turn, influence the dynamics of these HABs.     

A multi-phylum study of grazer-induced paralytic shellfish toxin production in the dinoflagellate Alexandrium fundyense: A new perspective on control of algal toxicity

The present study surveyed grazer-induced stimulation of paralytic shellfish toxin (PST) production by the marine dinoflagellate Alexandrium fundyense. The survey included species, known to graze upon A. fundyense, from five phyla: the protists, Polykrikos kofoidii (Dinoflagellata) and Tiarina fusus (Ciliophora), the bivalve molluscs Mytilus edulis and Mya arenaria (Mollusca), the ascidians, Molgula manhattensis and Botrylloides violaceus (Chordata), and the copepod, Eurytemora herdmani (Arthropoda). Direct (grazers in contact with cells of A. fundyense) and indirect (grazers not in contact with cells of A. fundyense) induction assays were carried out with protists and copepods. Only indirect assays were carried out with molluscs and ascidians. Indirect assays also tested whether induction of PST production occurred via kairomones or feeding-related cues. All metazoan grazers induced PST production. By contrast, neither of the two species of protistan grazer induced PST production. Direct and indirect inductions of PST production were evident for the copepod, with direct induction being significantly higher than indirect induction. Effects upon PST production by phylum, species (nested within phylum), and interactions of phylum by diet, and species by diet, were evident. When induction of PST production occurred, a kairomone effect was apparent, except for M. edulis. Similarly, feeding-related cues were evident, except for E. herdmani. An asymptotic relationship between the magnitude of indirect induction of PST production and total cell ingestion by the grazers suggests a saturation response of grazer-induced PST production.     

Alexandrium diversaporum sp. nov., a new non-saxitoxin producing species: Phylogeny,morphology and sxtA genes

Species of the PST producing planktonic marine dinoflagellate genus Alexandrium have been intensively scrutinised, and it is therefore surprising that new taxa can still be found. Here we report a new species, Alexandrium diversaporum nov. sp., isolated from spherical cysts found at two sites in Tasmania, Australia. This species differs in its morphology from all previously reported Alexandrium species, possessing a unique combination of morphological features: the presence of 2 size classes of thecal pores on the cell surface, a medium cell size, the size and shape of the 6″, 1′, 2⁗ and Sp plates, the lack of a ventral pore, a lack of anterior and posterior connecting pores, and a lack of chain formation. We determined the relationship of the two strains to other species of Alexandrium based on an alignment of concatenated SSU-ITS1, 5.8S, ITS2 and partial LSU ribosomal RNA sequences, and found A. diversaporum to be a sister group to Alexandrium leei with high support. A. leei shares several morphological features, including the relative size and shapes of the 6″, 1′, 2⁗ and Sp plates and the fact that some strains of A. leei have two size classes of thecal pores. We examined A. diversaporum strains for saxitoxin production and found them to be non-toxic. The species lacked sequences for the domain A4 of sxtA, as has been previously found for non-saxitoxin producing species of Alexandrium.     

Extensive genetic diversity and rapid population differentiation during blooms of Alexandrium fundyense (Dinophyceae) in an isolated salt pond on Cape Cod,MA, USA

In Massachusetts, paralytic shellfish poisoning (PSP) is annually recurrent along the coastline, including within several small embayments on Cape Cod. One such system, the Nauset Marsh System (NMS), supports extensive marshes and a thriving shellfishing industry. Over the last decade, PSP in the NMS has grown significantly worse; however, the origins and dynamics of the toxic Alexandrium fundyense (Balech) populations that bloom within the NMS are not well known. This study examined a collection of 412 strains isolated from the NMS and the Gulf of Maine (GOM) in 2006–2007 to investigate the genetic characteristics of localized blooms and assess connectivity with coastal populations. Comparisons of genetic differentiation showed that A. fundyense blooms in the NMS exhibited extensive clonal diversity and were genetically distinct from populations in the GOM. In both project years, genetic differentiation was observed among temporal samples collected from the NMS, sometimes occurring on the order of approximately 7 days. The underlying reasons for temporal differentiation are unknown, but may be due, in part, to life‐cycle characteristics unique to the populations in shallow embayments, or possibly driven by selection from parasitism and zooplankton grazing; these results highlight the need to investigate the role of selective forces in the genetic dynamics of bloom populations. The small geographic scale and limited connectivity of NMS salt ponds provide a novel system for investigating regulators of blooms, as well as the influence of selective forces on population structure, all of which are otherwise difficult or impossible to study in the adjacent open‐coastal waters or within larger estuaries.     

Differential responses of populations of the copepod Acartia hudsonica to toxic and nutritionally insufficient food algae

Nutritional insufficiency and toxicity are deleterious effects of phytoplankton on grazers. We hypothesize that toxic food is likely to have stronger evolutionary selective effects on grazers than nutritionally insufficient food. We explore this hypothesis in comparative studies of egg production and egg hatching of the copepod Acartia hudsonica challenged with both a toxic and a nutritionally insufficient alga. Experiments lasting 6 days, in which mixtures of different proportions of the suspect and a control alga were offered as food to female copepods, showed that the dinoflagellate Alexandrium fundyense, which bears paralytic shellfish toxins, was toxic to A. hudsonica. In contrast, the diatom Phaeodactylum tricornutum was nutritionally insufficient to A. hudsonica. In another set of experiments, the effects of A. fundyense and P. tricornutum, respectively, as sole foods on egg production and egg hatching success of two geographically separated populations (Maine and Connecticut) of the copepod A. hudsonica were examined in common-environment experiments, after being raised under identical conditions for two generations. The location in Maine regularly experiences toxic blooms of Alexandrium sp. whereas the location in Connecticut does not. During a 6-day period, A. fundyense reduced the egg production rates of the Connecticut copepod population, but not of the Maine population. In contrast, the diatom P. tricornutum reduced the egg production of both populations. These results of this study are consistent with the hypothesis of local adaptation to toxic food, but not to nutritionally insufficient food.     

Intraspecific trait variation and trade‐offs within and across populations of a toxic dinoflagellate

Intraspecific trait diversity can promote the success of a species, as complementarity of functional traits within populations may enhance its competitive success and facilitates resilience to changing environmental conditions. Here, we experimentally determined the variation and relationships between traits in 15 strains of the toxic dinoflagellate Alexandrium ostenfeldii derived from two populations. Measured traits included growth rate, cell size, elemental composition, nitrogen uptake kinetics, toxin production and allelochemical potency. Our results demonstrate substantial variation in all analysed traits both within and across populations, particularly in nitrogen affinity, which was even comparable to interspecific variation across phytoplankton species. We found distinct trade‐offs between maximum nitrogen uptake rate and affinity, and between defensive and competitive traits. Furthermore, we identified differences in trait variation between the genetically similar populations. The observed high trait variation may facilitate development and resilience of harmful algal blooms under dynamic environmental conditions.     

A Quantitative Assessment of the Role of the Parasite Amoebophrya in the Termination of Alexandrium fundyense Blooms within a Small Coastal Embayment

Parasitic dinoflagellates of the genus Amoebophrya infect free-living dinoflagellates, some of which can cause harmful algal blooms (HABs). High prevalence of Amoebophrya spp. has been linked to the decline of some HABs in marine systems. The objective of this study was to evaluate the impact of Amoebophrya spp. on the dynamics of dinoflagellate blooms in Salt Pond (MA, USA), particularly the harmful species Alexandrium fundyense. The abundance of Amoebophrya life stages was estimated 3–7 days per week through the full duration of an annual A. fundyense bloom using fluorescence in situ hybridization coupled with tyramide signal amplification (FISH- TSA). More than 20 potential hosts were recorded including Dinophysis spp., Protoperidinium spp. and Gonyaulax spp., but the only dinoflagellate cells infected by Amoebophrya spp. during the sampling period were A. fundyense. Maximum A. fundyense concentration co-occurred with an increase of infected hosts, followed by a massive release of Amoebophrya dinospores in the water column. On average, Amoebophrya spp. infected and killed ∼30% of the A. fundyense population per day in the end phase of the bloom. The decline of the host A. fundyense population coincided with a dramatic life-cycle transition from vegetative division to sexual fusion. This transition occurred after maximum infected host concentrations and before peak infection percentages were observed, suggesting that most A. fundyense escaped parasite infection through sexual fusion. The results of this work highlight the importance of high frequency sampling of both parasite and host populations to accurately assess the impact of parasites on natural plankton assemblages.     

A comparative analysis of Alexandrium catenella/tamarense blooms in Annaba Bay (Algeria) and Thau lagoon (France); phosphorus limitation as a trigger

Environmental conditions ultimately leading to blooms of the toxic dinoflagellate Alexandrium catenella/tamarense were investigated at two Mediterranean sites (Annaba Bay, Algeria and Thau lagoon, France). Three years were examined in details: 1992 (a pre-Alexandrium period), 2002 (a year with the first bloom in Annaba) and 2010 (a year with a major bloom in Annaba). Most conditions were similar, but ammonium concentrations were much higher in Annaba (up to 100 μM) than in Thau (up to 10 μM). First records of A. catenella/tamarense were in 1995 for Thau and 2002 for Annaba, and coincided with soluble reactive phosphorus (SRP) decreasing below a concentration of about 1 μM. No other environmental variable could be related to those blooms. Thus, it is likely that the large reductions in SRP at both sites led to phosphorus limitation of a certain number of phytoplankton species and favored the development of A. catenella/tamarense.     

The growth,toxicity and genetic characterization of seven strains of Alexandrium catenella (Whedon and Kofoid) Balech 1985 (Dinophyceae) isolated during the 2009 summer outbreak in southern Chile

The dinoflagellate Alexandrium catenella causes recurrent harmful algal blooms in southern Chile. This species belongs to the “Alexandrium tamarense/catenella/fundyense species complex” (the “tamarensis complex”), defined by morphological attributes. Ribosomal sequences serve to differentiate five evolutionary lineages (clades) in this species complex. These distinctions reflect the geographic distribution and toxicity of the populations rather than their morphological designations. Despite the social and economic impact that harmful blooms produce in Chile, few strains of A. catenella have been isolated. Moreover, physiological and/or genetic studies of the group are scarce. The aim of this work was to examine possible physiological and genetic variability among populations of A. catenella having different geographical origins but isolated from the same toxic event. Seven strains of A. catenella were isolated and established from phytoplankton samples collected in the Aysén and Los Lagos regions of southern Chile during a recent outbreak (February–March 2009). Growth, toxicity, and ITS sequences were compared among these strains. All the strains included in this study were grouped with strains belonging to the previously described “North America” clade. The genetic diversity detected among Chilean strains was 3%, a much higher value than those reported for comparisons among strains from other parts of the world. In addition, a remarkable variability of growth parameters and toxicity was detected among strains. Strain PFB45 showed the highest PSP toxin content, whereas strain PFB41 had the lowest value of this parameter but had the highest maximum cell density. In strains PFB38, PFB42, and PFB37, more than 98% of the total PSP toxin content occurred in the form of gonyautoxins (primarily GTX-4,1 and GTX-3,2). In strains PFB39, PFB36, and PFB45, neoSTX, and STX toxins were detected. These results demonstrated remarkable variability at the genetic and physiological level among strains of A. catenella isolated from the same outbreak. No correlations were found between the phenotypic traits (growth and toxicity) and the genetic affiliation of the strains studied.     

Experimental study on the impact of dinoflagellate Alexandrium species on populations of the rotifer Brachionus plicatilis

To investigate harmful effects of the dinoflagellate Alexandrium species on microzooplankton, the rotifer Brachionus plicatilis was chosen as an assay species, and tested with 10 strains of Alexandrium including one known non-PSP-producer (Alexandrium tamarense, AT-6). HPLC analysis confirmed the PSP-content of the various strains: Alexandrium lusitanicum, Alexandrium minutum and Alexandrium tamarense (ATHK, AT5-1, AT5-3, ATCI02, ATCI03) used in the experiment were PSP-producers. No PSP toxins were detected in the strains Alexandrium sp1, Alexandrium sp2.Exposing rotifer populations to the densities of 2000 cells ml⁻¹ of each of these 10 Alexandrium strains revealed that the (non-PSP) A. tamarense (AT-6) and two other PSP-producing algae: A. lusitanicum, A. minutum, did not appear to adversely impact rotifer populations. Rotifers exposed to these three strains were able to maintain their population numbers, and in some cases, increase them. Although some increases in rotifer population growth following exposures to these three algal species were noted, the rate was less than for the non-exposed control rotifer groups.In contrast, the remaining seven algal strains (A. tamarense ATHK, AT5-1, AT5-3, ATCI02, ATCI03; also Alexandrium sp1 and Alexandrium sp2) all have adverse effects on the rotifers. Dosing rotifers with respective algal cell densities of 2000 cells ml⁻¹ each, for Alexandrium sp1, Alexandrium sp2, and A. tamarense strains ATHK and ATCI03 showed mean lethal time (LT₅₀) on rotifer populations of 21, 28, 29, and 36h, respectively. The remaining three species (A. tamarense strains AT5-1, AT5-3, ATCI02) caused respective mean rotifer LT₅₀s of 56, 56, and 71 h, compared to 160 h for the unexposed “starved control” rotifers. Experiments to determine ingestion rates for the rotifers, based on changes in their Chlorophyll a content, showed that the rotifers could feed on A. lusitanicum, A. minutum and A. tamarense strain AT-6, but could graze to little or no extent upon algal cells of the other seven strains. The effects on rotifers exposed to different cell densities, fractions, and growth phases of A. tamarense algal culture were respectively compared. It was found that only the whole algal cells had lethal effects, with strongest impact being shown by the early exponential growth phase of A. tamarense. The results indicate that some toxic mechanism(s), other than PSP and present in whole algal cells, might be responsible for the adverse effects on the exposed rotifers.