Toxic Diatom Pseudo-nitzschia Research Articles

Long-term warming and acidification interaction drives plastic acclimation in the diatom Pseudo-nitzschia multiseries.

Ocean warming (OW) and acidification (OA) are expected to interactively impact key phytoplankton groups such as diatoms, but the underlying mechanisms, particularly under long-term acclimation, remain poorly understood. In this study, we investigated the responses of the toxic diatom Pseudo-nitzschia multiseries to combined changes in temperature (20°C and 30°C) and CO2 concentration (pCO2 400μatm and 1000μatm) using a multi-omics approach over an acclimation period of at least 251 generations. Physiological data suggest that elevated temperature, either alone or in combination with CO2, reduced the net photosynthesis and nitrate uptake rate, thus inhibiting P. multiseries growth. Conversely, elevated CO2 alone stimulated P. multiseries growth. Comparative genome analysis revealed the phenotypic plasticity in response to temperature and pCO2 variations, even after more than 251 generations acclimation period. Temperature was identified as the dominant environmental factor, showing stronger effects than CO2. Transcriptomic profiles indicated that genes involved in stress- and intracellular homeostasis such as Hsps, ubiquitination process and antioxidant defense were mostly down-regulated under long-term warming acclimation. This study demonstrates that P.multiseries responds similarly to both short-term and long-term experimental selection, suggesting that short-term experiments can be used to predict long-term responses.

Genomic diversity in time and space in the toxic diatom Pseudo-nitzschia multistriata

Genomic diversity in time and space in the toxic diatom Pseudo-nitzschia multistriata

Molecular forecasting of domoic acid during a pervasive toxic diatom bloom

In 2015, the largest recorded harmful algal bloom (HAB) occurred in the Northeast Pacific, causing nearly 100 million dollars in damages to fisheries and killing many protected marine mammals. Dominated by the toxic diatom Pseudo-nitzschia australis, this bloom produced high levels of the neurotoxin domoic acid (DA). Through molecular and transcriptional characterization of 52 near-weekly phytoplankton net-tow samples collected at a bloom hotspot in Monterey Bay, California, we identified active transcription of known DA biosynthesis (dab) genes from the three identified toxigenic species, including P. australis as the primary origin of toxicity. Elevated expression of silicon transporters (sit1) during the bloom supports the previously hypothesized role of dissolved silica (Si) exhaustion in contributing to bloom physiology and toxicity. We find that coexpression of the dabA and sit1 genes serves as a robust predictor of DA one week in advance, potentially enabling the forecasting of DA-producing HABs. We additionally present evidence that low levels of iron could have colimited the diatom population along with low Si. Iron limitation represents an overlooked driver of both toxin production and ecological success of the low-iron-adapted Pseudo-nitzschia genus during the 2015 bloom, and increasing pervasiveness of iron limitation may fuel the escalating magnitude and frequency of toxic Pseudo-nitzschia blooms globally. Our results advance understanding of bloom physiology underlying toxin production, bloom prediction, and the impact of global change on toxic blooms.

Influence of a small submarine canyon on biogenic matter export flux in the lower St. Lawrence Estuary, eastern Canada

Abstract. Submarine canyons enhance shelf–slope sediment exchange and influence hydrodynamic processes, with consequences for biogeochemical cycles. This work documents variations in the vertical export of biogenic matter on the northern shore of the lower St. Lawrence Estuary (LSLE, Quebec, eastern Canada), which is characterized by the presence of an active submarine canyon system. A total of three moorings were deployed from November 2020 to September 2021. One nearshore mooring (PDMc) was deployed in the main axis of the Pointe-des-Monts (PDM) canyon system and was equipped with an acoustic Doppler current profiler (ADCP), and two moorings equipped with sediment traps were deployed in the distal PDM canyon system (PDM-154, PDM-224) and offshore Baie-Comeau (BC-133). The ADCP data revealed the occurrence of a minor sediment remobilization event (December 2020) and a small turbidity current (February 2021) in the canyon. Concurrent elevated fluxes of total particulate matter, particulate organic carbon, particulate nitrogen, and chloropigments showed that these events left a signature in sediment traps PDM-154 and PDM-224 located > 2.6 km further offshore by enhancing lateral dispersion of resuspended sediments. The composition of diatom and dinoflagellate assemblages was similar in the canyon system and offshore BC, but the diatom bloom occurred 2 weeks earlier (in mid-April) at the PDM site. A bloom of the potentially toxic diatom Pseudo-nitzschia seriata was also observed during the second half of September 2021 at the BC site. Annual diatom and dinoflagellate fluxes were almost 2 times lower at the PDM site than at the BC site, possibly due to differences in riverine input and the structure of the water column, as well as increased sediment input and resuspension at the PDM site, leading to limited light availability. This study notably helps identify the relationship between near-bed canyon processes and biogenic matter export in the water column, thereby directly influencing the ecosystem offshore PDM. The study period further covered an anomalously nearly ice-free winter, and thus, in the context of climate change, it provides valuable insight into future trends of biogenic matter export in the LSLE.

Using qPCR and high-resolution sensor data to model a multi-species Pseudo-nitzschia (Bacillariophyceae) bloom in southeastern Australia

Harmful algal blooms, including those caused by the toxic diatom Pseudo-nitzschia, can have significant impacts on human health, ecosystem functioning and ultimately food security. In the current study we characterized a bloom of species of Pseudo-nitzschia that occurred in a south-eastern Australian oyster-growing estuary in 2019. Using light microscopy, combined with molecular (ITS/5.8S and LSU D1-D3 rDNA regions) and toxicological evidence, we observed the bloom to consist of multiple species of Pseudo-nitzschia including P. cf. cuspidata, P. hasleana, P. fraudulenta and P. multiseries, with P. cf. cuspidata being the only species that produced domoic acid (3.1 pg DA per cell). As several species of Pseudo-nitzschia co-occurred, only one of which produced DA, we developed a rapid, sensitive and efficient quantitative real-time polymerase chain reaction (qPCR) assay to detect only species belonging to the P. pseudodelicatissima complex Clade I, to which P. cf. cuspidata belongs, and this indicated that P. cuspidata or closely related strains may have dominated the Pseudo-nitzschia community at this time. Finally, using high resolution water temperature and salinity sensor data, we modeled the relationship between light microscopy determined abundance of P. delicatissima group and environmental variables (temperature, salinity, rainfall) at two sites within the estuary. A total of eight General Linear Models (GLMs) explaining between 9 and 54% of the deviance suggested that the temperature (increasing) and/or salinity (decreasing) data were generally more predictive of high cell concentrations than the rainfall data at both sites, and that overall, cell concentrations were more predictive at the more oceanic site than the more upstream site, using this method. We conclude that the combination of rapid molecular methods such as qPCR and real-time sensor data modeling, can provide a more rapid and effective early warning of harmful algal blooms of species of Pseudo-nitzschia, resulting in more beneficial regulatory and management outcomes.

Ecological time series and integrative taxonomy unveil seasonality and diversity of the toxic diatom Pseudo-nitzschia H. Peragallo in the northern Adriatic Sea

Pseudo-nitzschia H. Peragallo (1900) is a globally distributed genus of pennate diatoms that are important components of phytoplankton communities worldwide. Some members of the genus produce the neurotoxin domoic acid, so regular monitoring is in place. However, the identification of toxic members in routine samplings remains problematic. In this study, the diversity and seasonal occurrence of Pseudo-nitzschia species were investigated in the Gulf of Trieste, a shallow gulf in the northern Adriatic Sea. We used time series data from 2005 to 2018 to describe the seasonal and inter-annual occurrence of the genus in the area and its contribution to the phytoplankton community. On average, the genus accounted for about 15 % of total diatom abundance and peaked in spring and autumn, with occasional outbreaks during summer and large inter-annual fluctuations. Increased water temperature and decreased salinity positively affected the presence of some members of the genus, while strong effects could be masked by an unsuitable definition of the species complexes used for monitoring purposes. Therefore, combining morphological (TEM) and molecular analyses by sequencing the ITS, 28S and rbcL markers, eight species were identified from 83 isolated monoclonal strains: P. calliantha, P. fraudulenta, P. delicatissima, P. galaxiae, P. mannii, P. multistriata, P. pungens and P. subfraudulenta. A genetic comparison between the isolated strains and other strains in the Mediterranean was carried out and rbcL was inspected as a potential barcode marker in respect to our results. This is the first study in the Gulf of Trieste on Pseudo-nitzschia time series from a long-term ecological research (LTER) site coupled with molecular data. We show that meaningful ecological conclusions can be drawn by applying integrative methodology, as opposed to the approach that only considers species complexes. The results of this work will provide guidance for further monitoring efforts as well as research activities, including population genetics and genomics, associated with seasonal distribution and toxicity profiles.

Experimental evidence for shaping and bloom inducing effects of decapod larvae of Xantho poressa (Olivi, 1792) on marine phytoplankton

To study zooplankton–phytoplankton relationships in the diatom-dominated plankton communities of the northern Adriatic we performed feeding experiments with diatoms and zoea I larvae of the brachyuran Xantho poressa. We found that zoea I of X. poressa feed on diatoms of different forms (centric, pennate, colony forming, single celled, with or without setae) and size classes. In a laboratory setup, we presented the zoeas with a mix of diatom species similar to communities observed during blooms regularly found in the northern Adriatic. We report that the grazing activity resulted in a decrease of the relative abundance of the toxic diatom Pseudo-nitzschia calliantha. For the colonial, bloom-forming diatom Skeletonema marinoi our results show a chain length reduction in the presence of zoea I. Of particular interest is the observation that the presence of larvae also resulted in an increased growth rate and abundance of S. marinoi, which resembles bloom induction by grazer presence.

A high resolution melting method for the molecular identification of the potentially toxic diatom Pseudo-nitzschia spp. in the Mediterranean Sea

The aim of this study was to develop and validate a high resolution melting (HRM) method for the rapid, accurate identification of the various harmful diatom Pseudo-nitzschia species in marine environments. Pseudo-nitzschia has a worldwide distribution and some species are toxic, producing the potent domoic acid toxin, which poses a threat to both human and animal health. Hence, it is important to identify toxic Pseudo-nitzschia species. A pair of primers targeting the LSU rDNA of the genus Pseudo-nitzschia was designed for the development of the assay and its specificity was validated using 22 control DNAs of the P. calliantha, P. delicatissima/P. arenysensis complex and P. pungens. The post-PCR HRM assay was applied to numerous unidentified Pseudo-nitzschia strains isolated from the northwestern Adriatic Sea (Mediterranean Sea), and it was able to detect and discriminate three distinct Pseudo-nitzschia taxa from unidentified samples. Moreover, the species-specific identification of Pseudo-nitzschia isolates by the HRM assay was consistent with phylogenetic analyses. The HRM assay was specific, robust and rapid when applied to high numbers of cultured samples in order to taxonomically identify Pseudo-nitzschia isolates recovered from environmental samples.

Temporal and spatial variability of the potentially toxic Pseudo-nitzschia spp. in a eutrophic estuary (Sea of Marmara)

Spatial and temporal variability and bloom formation of the potentially toxic diatom Pseudo-nitzschia spp. was investigated weekly to monthly from October 2009 to October 2010 in a eutrophic estuary, the Golden Horn. Pseudo-nitzschia spp. were detected in 195 of 512 samples (38%) collected throughout the year. Two species, P. calliantha and P. pungens, were identified based on the SEM examination. Blooms of Pseudo-nitzschia occurred in the lower and middle estuary in January and May. The bloom in January mainly comprised P. calliantha. In the bloom in early May, P. calliantha made up 72% of the Pseudo-nitzschia cells and P. pungens 28%. However, the contribution of P. pungens increased to 83% in late May. The Pseudo-nitzschia blooms occurred at low temperature (9–15°C) and moderate salinity (17–18), and for P. calliantha a significant negative correlation was found with temperature and a significant positive correlation with salinity. The percentage of Pseudo-nitzschia cells decreased gradually from lower to upper estuary (59–14%), correlating with a decrease in Secchi depth (5.5–0.5 m). Principal components analyses (PCA) were used to explore the spatial and temporal variability of environmental factors in relation to Pseudo-nitzschia abundances, and showed that NH4, pH, Secchi depth and DO values were the most important factors reflecting spatial differences, while temperature, salinity, Chl-a and Si:N were more important factors showing temporal differences. High abundances of P. pungens correlated mainly with pH, Secchi depth and DO values, whereas P. calliantha also correlated with NO3 + NO2. Low light availability due to high concentrations of suspended material and very variable environmental conditions (e.g. pH, DO and NH4) may have limited growth of Pseudo-nitzschia in the upper estuary. Regular monitoring of Pseudo-nitzschia is important for improving the understanding of the influence of environmental parameters on bloom dynamics in the study area.

Induction of domoic acid production in the toxic diatom Pseudo-nitzschia seriata by calanoid copepods

The toxic diatom Pseudo-nitzschia seriata was exposed directly and indirectly (separated by a membrane) to copepods, Calanus hyperboreus and C. finmarchicus, to evaluate the effects of the copepods on domoic acid production and chain formation in P. seriata. The toxicity of P. seriata increased in the presence of the copepods. This response was chemically mediated without physical contact between the organisms suggesting that it was induced by potential waterborne cues from the copepods or changes in water chemistry. Domoic acid production may be related to defense against grazing in P. seriata although it was not shown in the present study. To evaluate if the induction of domoic acid production was mediated by the chemical cues from damaged P. seriata cells, live P. seriata cells were exposed to a P. seriata cell homogenate, but no effect was observed. Chain formation in P. seriata was affected only when in direct contact with the copepods. This study suggests that the presence of zooplankton may be one of the factors affecting the toxicity of Pseudo-nitzschia blooms in the field.

Interactive effects of irradiance and temperature on growth and domoic acid production of the toxic diatom Pseudo-nitzschia australis (Bacillariophyceae)

The influence of temperature and light on the growth, physiology and domoic acid (DA) production of the pennate toxic diatom Pseudo-nitzschia australis isolated from the English Channel was studied in semi-continuous culture. The effects of eight irradiances (35–400μmolphotonsm2s−1) and a temperature gradient (3.5–25.5°C) were investigated. The highest growth rates (0.47–0.83d−1) were observed between 18.6 and 13.5°C, with an optimum irradiance for growth at 100μmolphotonsm2s−1. A decrease in the optimum growth temperature was observed with an increase in irradiance. Cell chlorophyll a content decreased at both low and high extreme temperatures and high irradiances. Stable and high values of Fv/Fm ratios below 21°C highlight the great acclimation capacity of P. australis, which may explain its wide biogeographic distribution. The strain studied produced DA without nutrient limitation and during exponential growth although at lower levels than in other studies (0.15–2.0pgDAcell−1). Results underline the importance of light–temperature interactions for growth and DA production. Toxin production increased with increasing temperature and light, and DA production increased exponentially with the growth rate. These results show that DA is not only produced under nutrient stress or by unhealthy cells in P. australis. This study underlines the need to take the ability of P. australis to produce DA during the exponential growth into consideration to understand the link between toxin production and bloom dynamics. These results will not only help explore the processes involved, but also help parameterize future models of growth and DA production especially for P. australis.

The ability of the red macroalga, Porphyra purpurea (Rhodophyceae) to inhibit the proliferation of seven common harmful microalgae

Harmful algal blooms (HABs) caused by microalgae have become a serious threat to public health, tourism, fisheries, ecosystems, and economies around the world. Recent studies have indicated that some macroalgae may inhibit the growth of HABs-forming microalgae. The red macroalgae, Porphyra spp. (nori), are the most valuable maricultured seaweed with a global annual production worth of over $US 1.8 billion. Here, we report on the ability of Porphyra purpurea, indigenous to the US east coast, to inhibit the growth of seven common harmful algae, including the brown tide-forming pelagophyte Aureococcus anophagefferens; the raphidophyte Chattonella marina; the toxic diatom Pseudo-nitzschia multiseries; and the dinoflagellates Cochlodinium polykrikoides, Karlodinium veneficum, Karenia brevis, and Prorocentrum minimum. Controlled laboratory experiments with saturating nutrients and normal pH levels demonstrated that fresh thalli of P. purpurea added at environmentally realistic levels (mg L−1) strongly inhibited the growth of the harmful algae in a manner dependent on dose of both the macro- and microalgae. Extract, in equivalent dosages, of both fresh and dried P. purpurea also exhibited the same or more significant effects, which demonstrated that P. purpurea produced stable allelochemicals that contributed toward the observed growth-inhibiting effects. Experi-mentally co-culturing of live P. purpurea thalli and A. anophagefferens with high or low nutrient levels during harmful brown tides consistently yielded significant reductions in cell densities. High levels of macroalgae (fresh thalli and the extract of fresh thalli) did not alter the growth or survival of larval fish (Cyprinodon variegatus; 1 week old). Therefore, our results, together with the well-known nutrient-removal capacity of seaweeds, allow us to conclude that P. purpurea, and likely other species of Porphyra, can be cultured as an ecologically safe, environmentally friendly, and economically viable measure to prevent, control, or mitigate microalgal HABs in coastal ecosystems, adding benefit to integrated multitrophic aquaculture systems.

Response of diatom and silicoflagellate assemblages to climate change in the Santa Barbara basin during the past 178 years and the rise of the toxic diatom Pseudo-nitzschia australis

Response of diatom and silicoflagellate assemblages to climate change in the Santa Barbara basin during the past 178 years and the rise of the toxic diatom Pseudo-nitzschia australis

Toxic Pseudo-nitzschia spp. in the northwestern Adriatic Sea: characterization of species composition by genetic and molecular quantitative analyses

Various genetic aspects of the toxic diatom Pseudo-nitzschia species in the northwestern Adriatic Sea were investigated. For this area, limited or no knowledge is available regarding the genetic diversity and geographical patterns of Pseudo-nitzschia spp., as well as the toxin content. Phylogenetic analyses identified strains belonging to P. delicatissima, P. calliantha, P. pungens and P. mannii. Networks of haplotypes inferred from Adriatic and worldwide strains revealed two main haplotypes in P. delicatissima from the Mediterranean and north Atlantic, with a single panmictic population in P. calliantha, and Adriatic P. pungens strains sharing the most frequent haplotype. The quantitative real-time PCR (qrt-PCR) assay was developed to estimate the number of rDNA copies and their variation among Pseudo-nitzschia species and strains. Qrt-PCR analysis showed that P. delicatissima and P. calliantha had different average rDNA copy numbers per cell (P < 0.001). It is suggested that different rDNA copy numbers among species might be used to discriminate between morphotypes identified using light microscopy. We also discuss how the rDNA copy number variability found among P. pungens strains from different months (P < 0.001) may relate to physiological activities and/or adaptative strategies. Northwestern Adriatic P. delicatissima strains produced domoic acid at low concentrations.

Response of diatoms and silicoflagellates to climate change and warming in the California Current during the past 250 years and the recent rise of the toxic diatom Pseudo-nitzschia australis

Diatoms and silicoflagellate assemblages were examined in two year-increments of varved samples spanning the interval from 1748 through 2007 in Santa Barbara Basin (SBB) box core SBBC0806 to determine the timing and impact of possible 20th century warming on several different components of the plankton. Diatoms (Thalassionema nitzschioides = TN) and silicoflagellates (Distephanus speculum s.l. = DS) indicative of cooler waters and a shallow thermocline begin to decline in the 1920s and persistently compose a lower percentage of the assemblage in the SBB by about 1940. Prior to 1940, TN constituted on average ∼30% of the Chaetoceros-free diatom sediment assemblage and DS on average ∼36% of the silicoflagellate assemblage. Between 1940 and 1996, these relative abundances were ∼20% (TN) and ∼8% (DS). These results are consistent with results from planktonic foraminifera and radiolarians that indicate an influence of 20th century warming on marine ecosystems before most scientific observations began. Cooling of surface waters coincident with the one of the strongest La Niña events of the 20th century (and a return to negative PDO conditions) in late 1998 brought about a return to pre-1940 values of these cool water taxa (TN ∼31%, DS ∼25%). However, this recent regional cooling appears to have been accompanied by profound changes in the diatom assemblage. Pseudo-nitzschia australis, and Pseudo-nitzschia multiseries, diatom species associated with domoic acid, a neurotoxin that causes shellfish poisoning and marine mammal deaths, rapidly became dominant in the SBB sediment record at the time of the regional cooling (1999) and increased substantially in numbers as a bloom-forming taxon (relative to Chaetoceros spores) in 2003. Prior to 2003, diatom blooms recorded in the SBB sediment record consisted predominantly of Chaetoceros spores and less commonly of Rhizosolenia-related species (Neocalyptrella robusta and Rhizosolenia setigera). Fecal pellets dominated by valves of P. australis, however, were particularly abundant in both the 2003 and 2006 samples, coincident with recorded incidents of domoic acid increase and widespread shellfish poisoning in the SBB.

Spatial and temporal trends of the toxic diatom Pseudo-nitzschia in the Southeastern Atlantic United States

Data collected by NOAA Phytoplankton Monitoring Network volunteers, from the beginning of the program (2001) through 2010, was used to assess the spatial and temporal trends of Pseudo-nitzschia spp. from North Carolina through northern Florida along the southeastern US coastline. Pseudo-nitzschia spp. was present from North Carolina to Florida, and was most common in North and South Carolina. Across the majority of the Atlantic southeast US, the highest rates of occurrence were observed in late summer, early fall, with most areas experiencing the lowest rate of occurrence in the spring. The Outer Banks of North Carolina, however, experienced a peak of occurrence in late winter to early spring in addition to a late summer, early fall peak. Pseudo-nitzschia was found in temperatures ranging from less than 5°C to 35°C and salinities from 5 to 37. Six unique bloom events were documented during this period of nine years, three of which contained detectable levels of domoic acid. The majority of these bloom events and all of the toxic events occurred in the Outer Banks of North Carolina. Given the extent and intensity of coverage afforded by the NOAA PMN, this program provides the optimal approach to not only assess past trends but to monitor environmental changes and emerging trends in the dynamics of this toxigenic species. Understanding the dynamics of this species allows resource managers to better predict the threats associated with domoic acid.

Blooms of the potentially toxic diatom Pseudo-nitzschia calliantha Lundholm, Moestrup & Hasle in coastal waters of the northern Adriatic Sea (Croatia)

The genus Pseudo-nitzschia contains more than 30 species, though most of them are discernible only on the basis of ultrastructural or genetic differences. We present a taxonomic study of Pseudo-nitzschia calliantha Lundhum, Hasle & Moestrup in the coastal waters of the northern Adriatic Sea together with domoic acid (DA) measurements on natural and breeding populations of shellfish. Sampling carried out monthly at 13 stations during 2007 showed Pseudo-nitzschia species to be present with high abundances during the entire year, forming blooms in coastal waters (reaching abundances of up to 1.2 × 10 6 cells L −1) during the late summer/early autumn, when they contributed up to 70% of the whole phytoplankton community. Ultrastructural examination provided evidence that the observed blooms were formed by the potentially toxic species P. calliantha. After the P. calliantha bloom, DA was detected at 3 stations along the north-eastern Adriatic coast with concentrations ranging from 0.11 to 1.32 μg g −1 in shellfish tissue. The distribution patterns of the species were analyzed statistically in order to explain the relationships between environmental variables and population dynamics. We here report on a mass appearance of the potentially toxic P. calliantha in the coastal waters of the northern Adriatic.

Microsatellite primers in the planktonic diatom Pseudo‐nitzschia multistriata (Bacillariophyceae)

Seven microsatellite loci were characterized for the toxic diatom Pseudo-nitzschia multistriata Takano (Takano) to investigate intraspecific variability and estimate population genetic structure over blooms, seasons, and sexual and vegetative reproduction. Selected microsatellites consisted of di- and trinucleotide repeats in the core region, and showed four to twelve alleles per locus in strains of P. multistriata collected in the Gulf of Naples (Italy). Primer pairs were species-specific since they positively amplified against conspecific strains from Portugal and Spain but failed to generate PCR products from the diatoms Pseudo-nitzschia pseudodelicatissima (Hasle) Hasle and Leptocylindrus minimum Gran. The seven selected microsatellite markers will be useful in studying population dynamics of Pseudo-nitzschia multistriata in space and time.

Temporal and spatial variability in Pseudo-nitzschia spp. in Alabama coastal waters: A “hot spot” linked to submarine groundwater discharge?

The potentially toxic diatom Pseudo-nitzschia is common in the northern Gulf of Mexico. Seven sites along the Alabama Gulf Coast have been monitored weekly to bi-weekly for Pseudo-nitzschia spp., which were detected in 489 of 829 samples (59%) taken between November 2003 and July 2008. Mean population density peaked at 19.6 ± 3.2 °C but bloom densities (>10 6 cells L −1) occurred at 20–32 °C. Mean population density peaked at a salinity of 30.1 ± 3.2, with blooms occurring between salinities of 26 and 32. Peaks in abundance occurred in April–May, with secondary peaks in fall. A cluster analysis of the relative frequency distributions of abundance by site showed that Little Lagoon Pass had a strong dissimilarity compared to other sites, due to a higher frequency of bloom densities and a lower frequency of absences. Salinities at Little Lagoon Pass were higher and less variable than at other sites. Pseudo-nitzschia spp. were absent more frequently from sites at the mouths of Perdido and Mobile Bays, where salinity was lower and more variable. Freshwater transport from Baldwin County, which lies between these bays, has previously been shown to be primarily through submarine groundwater discharge into the Gulf of Mexico. Groundwater in Baldwin County has high nitrate concentrations and discharge is most likely to occur adjacent to Little Lagoon. Blooms of Pseudo-nitzschia spp. at Little Lagoon Pass in spring were highly correlated with discharge from the Styx River, a proxy for groundwater discharge. Little Lagoon Pass may therefore be a hot-spot for blooms of Pseudo-nitzschia spp., because local maxima in discharge result in nutrient availability without significant reductions in salinity.

Fate of domoic acid ingested by the copepod Acartia clausi

Two important issues in the studies of harmful algae include ecological role of the toxic compounds and their fate through the food web. The aims of this study were to determine whether the production of domoic acid is a strategy evolved to avoid predation and the role of copepods in the fate of this toxic compound through the food web. The copepod Acartia clausi was fed with single and mixed cultures of the toxic diatom Pseudo-nitzschia multiseries and the non-toxic diatom Pseudo-nitzschia delicatissima. Ingestion rate as a function of diatom abundance was the same for the toxic and non-toxic Pseudo-nitzschia species, indicating no selective feeding behaviour against P. multiseries. The toxins ingested by the copepods did not affect mortality, feeding behaviour, egg production and egg hatching of the copepods. Copepods assimilated the 4.8% of the total domoic acid ingested. Although the amount of toxins daily detoxificated by the copepods was 63.6%, the copepods accumulated domoic acid in their tissues. We conclude that domoic acid is not toxic for copepods and, probably for this reason, this toxin does not act as feeding deterrent for copepods. However, even though the production of domoic acid has apparently not evolved to deter predation, copepods may play an important role on the fate of this toxic compound through the marine food web.