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

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

Ingestion of the dinoflagellate, Pfiesteria piscicida, by the calanoid copepod, Acartia tonsa

The dinoflagellate, Pfiesteria piscicida, can form harmful algal blooms in estuarine environments. The dominant copepod species usually found in these waters is Acartia tonsa. We tested the ability of A. tonsa to graze the non-toxic zoospore stage of P. piscicida and thus serve as a potential biological control of blooms of this algal species. A. tonsa grazed the non-toxic zoospore stages of both a non-inducible P. piscicida strain (FDEPMDR23) and a potentially toxic strain (Tox-B101156) at approximately equal rates. Ingestion of P. piscicida increased with cell concentration and exhibited a saturated feeding response. Both the maximum number of cells ingested (I_max) and the slope of the ingestion curve (α) of A. tonsa feeding on P. piscicida were comparable to these ingestion parameters for A. tonsa fed similar-sized phytoplankton and protozoan species. When these laboratory ingestion rates were combined with abundance estimates of A. tonsa from the Pocomoke Estuary and Chesapeake Bay, we found that significant grazing control of the non-toxic zoospore stage of P. piscicida by A. tonsa would only occur at high copepod abundances (>10 copepods L⁻¹). We conclude that under most in situ conditions the potential biological control of blooms of P. piscicida is exerted by microzooplankton grazers. However, in the less saline portions of estuaries where maximum concentrations of copepods often occur with low abundances of microzooplankton, copepod grazing coefficients can be similar to the growth rates of P. piscicida.     

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

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

An autecological study of the potentially toxic dinoflagellate Alexandrium affine isolated from Vietnamese waters

The potentially toxic dinoflagellate species Alexandrium affine isolated from Ha Long Bay (Tonkin Gulf), Vietnam was cultured and maintained for morphological, physiological and toxicological studies. Classical morphological examinations including plate pattern were in good agreement with the international nomenclature of the species. The fine structure of A. affine, including morphology of its developmental stages during vegetative and sexual reproduction was found to be typical of other species in the genus. Two general trends in growth of A. Affine from Vietnamese waters were apparent: (1) growth rates were low at low salinities (10 and 15 psu) in all experimental temperatures (21–27 °C); (2) growth rates were high at salinities 25, 30, and 35 psu in all temperatures. There were no significant differences in growth rates at different salinities at low temperature (21 °C), and the most significant difference in growth rate was between high temperature–high salinity and high temperature–low salinity. The optimum temperature and salinity for growth were 24 °C and 30 psu. Maximum division rates per day (0.5–0.7) were at salinities 30 and 35 psu and at temperatures 24 and 27 °C. But the best conditions for division rate were 21 and 24 °C at salinities 30 and 35 psu. Toxicity analyses indicated A. affine to be both toxic and non-toxic at certain times. In the former case, toxicity was very low, 2.28 fmol  per cell; the toxicity component of A. affine was compared with that of A. leei and the mussel Perna viridis including neoSTX, STX, and GTX₁–GTX₄.     

Using quantitative real-time PCR to study competition and community dynamics among Delaware Inland Bays harmful algae in field and laboratory studies

The Delaware Inland Bays (DIB) have experienced harmful algal blooms of dinoflagellates and raphidophytes in recent years. We used quantitative polymerase chain reaction (QPCR) techniques to investigate the community dynamics of three DIB dinoflagellates (Karlodinium veneficum, Gyrodinium instriatum, and Prorocentrum minimum) and one raphidophyte (Heterosigma akashiwo) at a single site in the DIB (IR-32) in summer 2006 relative to salinity, temperature and nutrient concentrations. We also carried out complementary laboratory culture studies. New primers and probes were developed and validated for the 18S rRNA genes in the three dinoflagellates. K. veneficum, H. akashiwo, and G. instriatum were present in almost all samples throughout the summer of 2006. In contrast, P. minimum was undetectable in late June through September, when temperatures ranged from 20 to 30 °C (average 25.7 °C). Dissolved nutrients ranged from 0.1 to 2.8 μM PO₄³⁻ (median = 0.3 μM), 0.7–30.2 μM NO_x (median = 12.9 μM), and 0–19.4 μM NH₄⁺ (median = 0.7 μM). Dissolved N:P ratios covered a wide range from 2.6 to 177, with a median of 40. There was considerable variability in occurrence of the four species versus nutrients, but in general P. minimum and H. akashiwo were most abundant at higher (>40) N:P ratios and dissolved nitrogen concentrations, while K. veneficum and G. instriatum were most abundant at low dissolved N:P ratios (<20) and dissolved nitrogen concentrations < 10 μM. The semi-continuous laboratory competition experiment used mixed cultures of K. veneficum, P. minimum, and H. akashiwo grown at dissolved N:P ratios of 5, 16, and 25. At an N:P of 16 and 25 P. minimum was the dominant alga at the end of the experiment, even at a temperature that was much higher than that at which this alga was found to bloom in the field (27 °C). P. minimum and H. akashiwo had highest densities in the N:P of 25. K. veneficum grew equally well at all three N:P ratios, and was co-dominant at times at an N:P of 5. H. akashiwo had the lowest densities of the three algae in the laboratory experiment. Laboratory and field results showed both interesting similarities and significant differences in the influences of important environmental factors on competition between these harmful algal species, suggesting the need for more work to fully understand HAB dynamics in the DIB.     

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

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

Growth of dinoflagellates, Ceratium furca and Ceratium fusus in Sagami Bay, Japan: The role of nutrients

In order to study the influence of nutrients on the growth characteristics of the dominant dinoflagellates, Ceratium furca and Ceratium fusus, in the temperate coastal area of Sagami Bay, Japan, we conducted field monitoring from January 2000 to December 2005 and performed laboratory culture experiments. In the field study, population densities of C. furca and C. fusus were high, even in low nutrient concentrations (N: 1.58 μM, P: 0.17 μM). Both species were more abundant in the surface and sub-surface layers than in the bottom layers during the stratification periods. In the laboratory study, the specific growth rates of C. furca and C. fusus increased gradually along with increasing nutrients up to the T₅ (N: 5 μM, P: 0.5 μM) and T₁₀ (N: 10 μM, P: 1 μM) concentration levels, after which the growth rate plateaued at the T₅₀ (N: 50 μM, P: 5 μM) concentration level. In contrast, the nutrient uptake rates of both species continuously increased, indicating “luxury consumption”, i.e., excessive cellular storage not related to growth rate. The half-saturation constants (K_s) of C. furca for nitrate (0.49 μM) and phosphate (0.05 μM) were slightly higher than C. fusus (0.32 and 0.03 μM, respectively). We offer two reasons why the two Ceratium population densities were maintained at high levels in low nutrient conditions. First, these two species have a competitive advantage over other algal species because of low K_s values and specific characteristics for nutrient uptake such as luxury consumption. Their ability to obtain nutrients through alternative methods, such as phagotrophy, might contribute to bloom formation and population persistence. Second, the cell densities of both Ceratium species increased along with nitrate concentrations in the media even when phosphorus was held constant. In particular, the growth of C. furca was directly supported by various nitrogen sources such as nitrate, ammonium, and urea, although the highest growth rates were observed only in the nitrate-enriched cultures. Our field and laboratory results revealed that the growth rates of the two Ceratium species increased readily in high N:P nutrient conditions (i.e., conditions of P limitation) indicating an advantage over other algal species in phosphorus-limited environments such as Sagami Bay.     

A mechanistic model for understanding invasions: using the environment as a predictor of population success

Aim We set out to develop a temperature‐ and salinity‐dependent mechanistic population model for copepods that can be used to understand the role of environmental parameters in population growth or decline. Models are an important tool for understanding the dynamics of invasive species; our model can be used to determine an organism’s niche and explore the potential for invasion of a new habitat. Location Strait of Georgia, British Columbia, Canada. Methods We developed a birth rate model to determine the environmental niche for an estuarine copepod. We conducted laboratory experiments to estimate demographic parameters over a range of temperatures and salinities for Eurytemora affinis collected from the Nanaimo Estuary, British Columbia (BC). The parameterized model was then used to explore what environmental conditions resulted in population growth vs. decline. We then re‐parameterized our model using previously published data for E. affinis collected in the Seine Estuary, France (SE), and compared the dynamics of the two populations. Results We established regions in temperature–salinity space where E. affinis populations from BC would likely grow vs. decline. In general, the population from BC exhibited positive and higher intrinsic growth rates at higher temperatures and salinities. The population from SE exhibited positive and higher growth rates with increasing temperature and decreasing salinity. These different relationships with environmental parameters resulted in predictions of complex interactions among temperature, salinity and growth rates if the two subspecies inhabited the same estuary. Main conclusions We developed a new mechanistic model that describes population dynamics in terms of temperature and salinity. This model may prove especially useful in predicting the potential for invasion by copepods transported to Pacific north‐west estuaries via ballast water, or in any system where an ecosystem is subject to invasion by a species that shares demographic characteristics with an established (sub)species.     

Effect of salinity and pH on growth and agar yield of Gracilaria tenuistipitata var. liui in laboratory and outdoor cultivation

Acclimation responses of the red alga Gracilaria tenuistipitata var. liui collected on the northwest coast of Philippines were determined in laboratory setups and outdoor cultivation tanks in Haifa, Israel. Growth under laboratory conditions was influenced by all three variables studied, namely, temperature (20 or 30 ^°C), salinity (20, 30 or39‰) and seawater pH (6.5, 7.0, 8.0 or ≥ 9.0). In 250 mL flasks lacking pH control growth was influenced by temperature only at 20 ‰, whereas at 39 ‰, growth rates were similar at 20 or 30 ^°C. In 500 mL cylinders in which pH was controlled, growth rates were significantly different at a pH of 6.5 and 7.0 for all salinities, with maximal rates occurring in 39 ‰. At pH 8.0, and above, growth rates between salinities were similar and reduced to approximately 50% at a pH of 9.0 compared to rates at a pH of 6.5. Photosynthesis responses generally resembled growth responses both, in 250 mL and 500 mL cultures. In 40-L outdoor tanks, weekly growth and agar yields were apparently enhanced by increasing light intensities (up to full sunlight) and nutrient concentrations (up to 0.2 mM PO₃ ^2- and 2.0 mM NH₄ ⁺), and rates averaged four times higher than rates determined in the smaller flask cultures. This study shows broad salinity tolerance of G. tenuistipitata var. liui and its ability to sustain growth rates that are among the highest measured for Gracilaria spp. in outdoor cultures. This revised version was published online in August 2006 with corrections to the Cover Date.     

Environmental influences on akinete germination and development in Nodularia spumigena (Cyanobacteriaceae), isolated from the Gippsland Lakes, Victoria, Australia

This study was carried out to investigate the genesis of N. spumigena blooms by specifically studying the effects of environmental variables (salinity, nitrogen, phosphorus and light) on the germination of N. spumigena akinetes. Optimal conditions for maximum germination and germling growth were determined by exposing akinetes to a range of salinities and nutrient (nitrogen and phosphorus) concentrations under two different irradiances. At pre-determined time periods, treatments were sampled and the percent germination and length of germlings assessed. The results indicated that akinete germination and germling growth were optimal at salinities from 5 to 25 and significantly reduced outside this range. A positive correlation in germination was observed with increasing nutrient (phosphorus and nitrate) concentration. Similarly, germling growth increased with increasing concentrations of both nutrients. Irradiance significantly influenced both germination and growth during salinity experiments, whereas in nutrient addition experiments, irradiance had no effect on germination; however, growth was significantly influenced during phosphorus addition experiments. Consequently, salinity and light appeared to be most critical in the germination process for N. spumigena akinetes, with phosphorus most important for germling growth. The study showed that N. spumigena may be able to germinate under environmental conditions outside its optimal range, but the growth of the germling is significantly reduced, which in turn suggests that its ability to form a bloom outside its optimal environmental conditions would also be greatly reduced.     

Distribution Patterns of Isomorphic Cold-Water Dinoflagellates (Scrippsiella/Woloszynskia Complex) Causing ‘red tides’ in the Baltic Sea

During the latest years medium-sized (15–30 μm), single-celled dinoflagellates have been reported to form blooms in the northern Baltic Proper and the Gulf of Finland in winter and spring. Recent studies (Kremp et al., 2003. Proceedings of the 7th International conference of Modern and Fossil Dinoflagellates, September 21–25, Nagasaki, Japan, 66 pp.) indicate that those blooms are caused by two isomorphic species – Scrippsiella hangoei (Schiller) Larsen, and a new species, tentatively belonging to the genus Woloszynskia. Until now there has been no report on how widely distributed these phytoplankton species are in the Baltic Sea. In this study, the occurrence of Scrippsiella/Woloszynskia complex in the entire Baltic Sea was investigated, by using monitoring data from 1997 to 2003. The species occurred in a salinity range from 2 to 8 PSU. Highest concentrations were observed at salinity 4.5–6.5 PSU. Maximum cell densities of Scrippsiella/Woloszynskia complex in the water column were mainly obtained in April or in the beginning of May by the water temperature <3 °C prior to stratification was formed. In the central Gulf of Finland, the second maximum was found in 1999 and 2002 by the temperature >6 °C. Bloom formations in the Baltic Proper and in the Gulf of Finland may not only be explained by optimum temperature and salinity, but also with other factors e.g. high nutrient concentrations and good seeding conditions from the sediments.     

First occurrence of Cochlodinium blooms in Sabah, Malaysia

Harmful algal blooms (HABs) resulting in red discoloration of coastal waters in Sepanggar Bay, off Kota Kinabalu, Sabah, East Malaysia, were first observed in January 2005. The species responsible for the bloom, which was identified as Cochlodinium polykrikoides, coincided with fish mortalities in cage-cultures. Determinations of cell density between January 2005 and June 2006 showed two peaks that occurred in March–June 2005 and June 2006. Cell abundance reached a maximum value of 6 × 10⁶ cells L⁻¹ at the fish cage sampling station where the water quality was characterized by high NO₃–N and PO₄–P concentrations. These blooms persisted into August 2005, were not detected during the north–east monsoon season and occurred again in May 2006. Favorable temperature, salinity and nutrient concentrations, which were similar to those associated with other C. polykrikoides blooms in the Asia Pacific region, likely promoted the growth of this species. Identification of C. polykrikoides as the causative organism was based on light and scanning microscopy, and confirmed by partial 18S ribosomal DNA sequences of two strains isolated during the bloom event (GenBank accession numbers DQ915169 and DQ915170).     

Effects of temperature, salinity and irradiance on the growth of the red tide dinoflagellate Gyrodinium instriatum Freudenthal et Lee

The effects of temperature, salinity and irradiance on the growth of the red tide dinoflagellate Gyrodinium instriatum Freudenthal et Lee were examined in the laboratory. Exposed to 45 different combinations of temperature (10–30 °C) and salinity (0–40) under saturating irradiance, G. instriatum exhibited its maximum growth rate of 0.7 divisions/day at a combination of 25 °C and a salinity of 30. Optimum growth rates (>0.5 divisions/day) were observed at temperatures ranging from 20 to 30 °C and at salinities from 10 to 35. The organism could not grow at ≤10 °C. In addition, G. instriatum burst at a salinity of 0 at all temperatures, but grew at a salinity of 5 at temperatures between 20 and 25 °C. It is noteworthy that G. instriatum is a euryhaline organism that can live under extremely low salinity. Factorial analysis revealed that the contributions of temperature and salinity to its growth of the organism were almost equal. The irradiance at the light compensation point (I₀) was 10.6 μmol/(m² s) and the saturated irradiance for growth (I_s) was 70 μmol/(m² s), which was lower than I_s for several other harmful dinoflagellates (90–110 μmol/(m² s)).     

The effects of major environmental factors and nutrient limitation on growth and encystment of planktonic dinoflagellate Akashiwo sanguinea

The bloom-forming dinoflagellate Akashiwo sanguinea is commonly observed in estuarine and coastal waters around the world. Annually recurrent blooms of this species have been observed in the coastal waters of China, particularly in the Sishili Bay, Yantai since 2004. However, limited studies have been conducted on the recurrence mechanism of A. sanguinea other than periodical monitoring of its population dynamics and associated environmental variables. Thus, to further explore the bloom and succession mechanisms of A. sanguinea in the field, we studied the effects of major nutritional components on the growth and encystment of A. sanguinea, as well as the effects of key environmental factors on the growth of A. sanguinea through a series of laboratory trials. Our results indicated that A. sanguinea was able to grow well within the temperature range of 20–25 °C, salinity range of 20 - 30, with the maximum laboratory irradiance of 78.14 μE m⁻² s⁻¹, and was able to survive and grow in low nutrient. However, lower concentrations of nutrients (e.g., nitrate, phosphate) and higher ammonium exerted different degrees of limiting effects on the growth of A. sanguinea, and induced 2.3–21.24% of vegetative cells to form resting cysts simultaneously in laboratory cultures. On the other hand, very limited or no cyst formation was observed in nutrient-replete or extremely low nutrient cultures, indicating the threshold effect of nutritional stress on the encystment of A. sanguinea. The physiological strategy of encystment of A. sanguinea in nutrient-limiting environment facilitates the survival and succession of A. sanguinea species in fluctuating seawaters, and provides seed sources for reoccurring algal blooms under favorable environmental conditions.     

Photosystem II photochemistry and physiological parameters of three fodder shrubs, <Emphasis Type="Italic">Nitraria retusa</Emphasis>, <Emphasis Type="Italic">Atriplex halimus</Emphasis> and <Emphasis Type="Italic">Medicago arborea</Emphasis> under salt stress

Nitraria retusa and Atriplex halimus (xero-halophytes) plants were grown in the range 0–800 mM NaCl while Medicago arborea (glycophyte) in 0–300 mM NaCl. Salt stress caused a marked decrease in osmotic potential and a significant accumulation of Na⁺ and Cl⁻ in leaves of both species. Moderate salinity had a stimulating effect on growth rate, net CO₂ assimilation, transpiration and stomatal conductance for the xero-halophytic species. At higher salinities, these physiological parameters decreased significantly, and their percentages of reduction were higher in A. halimus than in N. retusa whereas, in M. arborea they decreased linearly with salinity. Nitraria retusa PSII photochemistry and carotenoid content were unaffected by salinity, but a reduction in chlorophyll content was observed at 800 mM NaCl. Similar results were found in A. halimus, but with a decrease in the efficiency of PSII (F′v/F′m) occurred at 800 mM. Conversely, in M. arborea plants we observed a significant reduction in pigment concentrations and chlorophyll fluorescence parameters. The marked toxic effect of Na⁺ and/or Cl⁻ observed in M. arborea indicates that salt damage effect could be attributed to ions’ toxicity, and that the reduction in photosynthesis is most probably due to damages in the photosynthetic apparatus rather than factors affecting stomatal closure. For the two halophyte species, it appears that there is occurrence of co-limitation of photosynthesis by stomatal and non-stomatal factors. Our results suggest that both N. retusa and A. halimus show high tolerance to both high salinity and photoinhibition while M. arborea was considered as a slightly salt tolerant species.     

The combined effects of temperature and salinity on hatching success and the survival,growth, and development of the larval stages of Metapenaeus bennettae (Racek & Dall)

During the spawning season of the estuarine prawn Metapenaeus bennettae (Racek & Dall), laboratory and field experiments were conducted to examine the combined effects of temperature and salinity on hatching success of eggs and the survival, growth and development of larvae. Response surface analysis showed that optimal levels of temperature and salinity for maximum hatching success varied depending on conditions during spawning. Similarly, temperature and salinity conditions that produced maximum survival and growth of larvae depended on conditions during rearing prior to experimental temperature/salinity treatments. At the onset of feeding, larvae showed the lowest tolerance to changes in temperature and salinity. Supplementary feeding experiments in the laboratory, and survival rates in field experiments indicated that starvation was a more potent factor than the effects of temperature and salinity in determining survival through the protozoeal larval stages. Late larval stages were relatively indifferent to the effects of temperature and salinity. It is suggested that, during early development, adaptive response to the prevailing physical conditions enhances survival in an estuarine environment.     

Wind-driven river plume dynamics and toxic Alexandrium tamarense blooms in the St. Lawrence estuary (Canada): A modeling study

In the lower St. Lawrence estuary (LSLE, eastern Canada), blooms of the toxic dinoflagellate Alexandrium tamarense are a recurrent phenomenon, resulting in paralytic shellfish poisoning outbreaks every summer. A first coupled physical–biological model of A. tamarense blooms was developed for this system in order to explore the interactions between cyst germination, cellular growth and water circulation and to identify the effect of physical processes on bloom development and transport across the estuary. The simulated summer (1998) was characterized by an A. tamarense red tide with concentrations reaching 2.3 × 10⁶ cells L⁻¹ along the south shore of the LSLE. The biological model was built with previously observed A. tamarense cyst distribution, cyst germination rate and timing, and A. tamarense growth limitation by temperature and salinity. The coupled model successfully reproduced the timing of the A. tamarense bloom in 1998, its coincidence with the combined plumes from the Manicouagan and Aux-Outardes (M-O) rivers on the north shore of the estuary, and the temporal variations in the north-south gradients in cell concentrations. The simulation results reveal that the interaction between cyst germination and the estuarine circulation generates a preferential inoculation of the surface waters of the M-O river plume with newly germinated cells which could partly explain the coincidence of the blooms with the freshwater plume. Furthermore, the results suggest that the spatio-temporal evolution of the bloom is dominated by alternating periods of retention and advection of the M-O plume: east or north-east winds favor the retention of the plume close to the north shore while west or north-west winds result in its advection toward the south shore. The response of the simulated freshwater plume to fluctuating wind forcing controls the delivery of the A. tamarense bloom from the northern part of the estuary to the south shore. In addition, our results suggest that a long residence time of the M-O plume and associated A. tamarense population in the LSLE during the summer 1998 contributed to the development of the red tide. We thus hypothesize that the wind-driven dynamics of the M-O plume could partly determine the success of A. tamarense blooms in the LSLE by influencing the residence time of the blooms and water column stability, which in turn affects A. tamarense vertical migrations and growth.     

Nutrient enrichment promotes survival and dispersal of drifting fragments in an invasive tropical macroalga

The effect of nutrient availability on growth, survival, and photosynthetic performance of drifting fragments of the invasive red alga Hypnea musciformis was studied in Maui (Hawaii), where this species smothers native reef communities and forms localized blooms. H. musciformis does not sexually reproduce in Hawaii and drifting fragments represent the only pathway by which H. musciformis can disperse and invade new areas. Growth rates decreased with age and approached zero when fragments aged 32 days. Increased nutrient availability did not result in increased relative growth rates during this period. In contrast to growth, photosynthetic performance remained unaffected through time and showed no clear relationship with nutrient availability. Increased nutrient availability increased fragment survival and fragments survived for >2 months in the high nutrient treatment (3.0 μmolPO₄ + 30.0 μmolNH₄). This indicates that increased nutrient availability increases the dispersal potential of H. musciformis. Low growth rates of drifting Hypnea fragments increased recruitment success since attachment success of this epiphytic species decreased with increasing fragment size. H. musciformis thus uses resources for survival and maintenance rather than growth, resulting in long competency periods and optimal recruitment, which likely contribute to its success as an invader of Hawaiian reef communities.     

Ecology of phytoplankton communities dominated by Aureococcus anophagefferens: the role of viruses, nutrients, and microzooplankton grazing

We investigated the impact of viruses, nutrient loading, and microzooplankon grazing on phytoplankton communities in two New York estuaries that hosted blooms of the brown tide alga Aureococcus anophagefferens during 2000 and 2002. The absence of a bloom at one location during 2002 allowed for the fortuitous comparison of a bloom and non-bloom year at the same location as well as a comparison of two sites experiencing bloom and non-bloom conditions during the same year. During the study, blooms were found at locations with high levels of dissolved organic nitrogen and lower nitrate concentrations compared to a non-bloom location. Experimental additions of inorganic nitrogen and phosphorus yielded growth rates within the total phytoplankton community which significantly exceeded control treatments in 83% of experiments, while A. anophagefferens experienced significantly increased growth during only 20% of experimental inorganic nutrient additions. Consistent with prior research, these results suggest brown tides are not caused by eutrophication, but instead are more likely to occur when sources of labile DOM are readily available. Microzooplankton grazing rates on the total phytoplankton community during a bloom were lower than grazing rates at a non-bloom site, and grazing rates on A. anophagefferens were lower than grazing rates on the total community on some dates, suggesting that reduced grazing mortality may also promote brown tides. Mean densities of viruses during blooms (3 × 10⁸ ml⁻¹) were elevated compared to most estuarine environments and were twice the levels found at a non-bloom site. Experimental enrichment of the natural viral densities yielded a significant increase in A. anophagefferens growth rates relative to control treatments when background levels of viruses were low (<1.7 × 10⁸ ml⁻¹), suggesting that viruses may promote bloom occurrence by regenerating DOM or altering the composition of microbial communities.     

GROWTH AND TOXICITY OF PRYMNESIUM PARVUM (HAPTOPHYTA) AS A FUNCTION OF SALINITY,LIGHT, AND TEMPERATURE1

The growth rate, stationary cell concentration, and toxicity of Prymnesium parvum N. Carter were measured using a strain isolated from Texas inland waters. We used a multifactor experimental approach with multiple regression analysis to determine the importance of environmental factors, including temperature, light, and salinity to these algal measurements. Exponential growth rate was unimodal in relation to temperature, salinity, and irradiance, with an estimated maximal growth of 0.94 d^?1 occurring at 27°C, 22 practical salinity units (psu), and 275 μmol photons·m^?2·s^?1. Stationary cell concentrations also had unimodal responses to temperature and salinity but increased with irradiance. Maximal cell concentrations were estimated to occur at 26°C and 22 psu. Both maximum growth rate and highest stationary cell concentrations were measured at levels of each factor resembling warm, estuarine conditions that differ from the conditions under which blooms occur in inland waters in the southwestern United States. Acute toxicity to fish was highest at the lowest salinity and temperature levels, conditions not optimal for exponential growth but similar to those under which blooms occur in inland waters. Our results imply that summer blooms could occur in inland waters of the southwestern United States. Generally, they have not, suggesting that factors other than those investigated in this research influence bloom dynamics.