Potential impacts of brown tide, Aureococcus anophagefferens, on juvenile hard clams, Mercenaria mercenaria, in the Coastal Bays of Maryland, USA

The rate of growth of juvenile hard clams, Mercenaria mercenaria, was studied in the Coastal Bays of Maryland during an outbreak of the brown tide, Aureococcus anophagefferens. Brown tide dominated the plankton community during the month of June 2002, with cell densities at several sites reaching category 3 (>200,000 cells ml⁻¹) levels. Temperatures during the bloom were 18.6–27.5 °C. Nutrient conditions preceding and during the bloom were conducive for the proliferation of A. anophagefferens: while inorganic nitrogen and phosphorus were <1 μg at N or P l⁻¹, urea was elevated during bloom development. Organic nitrogen, phosphorus and carbon were in the range of levels observed in previous brown tide blooms and increased following the collapse of the bloom. Growth rates of juvenile clams were significantly lower during the period of the brown tide bloom than following its collapse. Growth rates of M. mercenaria were found to be negatively impacted at brown tide densities as low as 20,000 cells ml⁻¹, or category 1 levels. The low growth rates of M. mercenaria could not be explained by temperature, as the lowest growth rates were found when water temperatures were at levels previously found to be optimal for growth.     

HPLC pigment records provide evidence of past blooms of Aureococcus anophagefferens in the Coastal Bays of Maryland and Virginia, USA

Concentrations of the accessory phytoplankton pigment 19′-butanoyloxyfucoxanthin (but-fuco), derived from archived high performance liquid chromatography (HPLC) data from the Atlantic coastal bays of Maryland and Virginia (1993–1995 and 1999–2002), were used to determine the presence of Aureococcus anophagefferens at 18 stations. Paired data of direct cell counts of A. anophagefferens and but-fuco concentration data from 2000 to 2002 were linearly regressed (R² = 0.78). This regression was used to estimate historical cell densities from 1994 to 1995 and to improve the spatial resolution from 1999 to 2002. Although the HPLC method used did not permit quantification of but-fuco before 1994, the records indicate that qualitatively A. anophagefferens was present in 1993. Quantitative measurements grouped into bloom index categories showed that annually, peak densities occurred in May to July. Severe Category 3 blooms (>200,000 cells ml⁻¹) were found in 1995, 2001, and 2002. Spatially, concentrations of but-fuco were higher in the northern extent of the study region than in the lower Chincoteague Bay, and along the western shore of Chincoteague Bay than on the eastern side. On an interannual basis, it appeared that A. anophagefferens became more geographically widespread and blooms more intense throughout the study period.     

Assessment of brown tide blooms, caused by Aureococcus anophagefferens, and contributing factors in New Jersey coastal bays: 2000–2002

A 3 year study (2000–2002) in Barnegat Bay-Little Egg Harbor (BB/LEH), New Jersey (USA), was conducted by the New Jersey Department of Environmental Protection, Division of Science Research and Technology (DSRT) in cooperation with several partners to assess brown tide blooms in coastal waters in NJ. Water samples were collected by boat and helicopter at coastal stations from 2000 to 2002 along with field measurements. Aureococcus anophagefferens were enumerated and associated environmental factors were analyzed. A. anophagefferens abundances were classified using the Brown Tide Bloom Index and mapped, along with salinity and temperature parameters, to their geo-referenced location using the ArcView GIS. The highest A. anophagefferens abundances (>10⁶ cells ml⁻¹), including category 3 blooms (≥200,000 cells ml⁻¹) and category 2 blooms (≥35,000 to ≤200,000 cells ml⁻¹), recurred during each of the 3 years of sampling and covered significant geographic areas of the estuary, especially in Little Egg Harbor. While category 3 blooms were generally associated with warmer water temperatures (>16 °C) and higher salinity (>25–26 ppt), these factors were not sufficient alone to explain the timing or distribution of A. anophagefferens blooms. There was no significant relationship between brown tide abundances and dissolved organic nitrogen measured in 2002 but this was consistent with other studies. Extended drought conditions, with corresponding low freshwater inputs and elevated bay water salinities, occurring during this time were conducive to blooms. A. anophagefferens abundances were well above the reported levels that have been reported to cause negative impacts on shellfish. It was shown that over 50% of the submerged aquatic vegetation (SAV) habitat located in Barnegat Bay/Little Egg Harbor was categorized as having a high frequency of category 2 or 3 blooms for all 3 years.     

Interannual variability of Aureococcus anophagefferens in Quantuck Bay, Long Island: natural test of the DON hypothesis

This study examined benthic and pelagic rate processes from the perspective of benthic dissolved organic matter (DOM) and its possible role in Aureococcus anophagefferens population dynamics. Sampling was conducted in Quantuck Bay, Long Island, New York, at three times in the summer of 2000 and two times in the summer of 2001. A. anophagefferens exhibited a large bloom between the May and July 2000 sample periods, but a smaller bloom was captured in the September 2000 sampling. Densities throughout 2001 were significantly lower than during 2000. There were few differences in most parameters measured between years, but the largest difference was the seasonal increase in both particulate (POM) and dissolved organic matter (DOM) during 2000 that was not observed during 2001. In particular, DOP accumulated the most, followed by DON and DOC, which resulted in significant seasonal decreases in the C:N:P ratios of the DOM pools. On the contrary, changes in elemental ratios of POM were not observed. The seasonal accumulation of DON appeared to be driven largely (50%) by the flux of DON from the benthos in 2000, but during 2001, all measured DON fluxes were into the sediment from the water column. This is consistent with the lack of accumulation during this year. There was little evidence for changes in microzooplankton grazing pressure between 2000 and 2001, and therefore the accumulation of DON and DOP during 2000 could have provided a competitive advantage to A. anophagefferens over other picoalgal species (e.g., Synechococcus) resulting in the significant blooms observed in 2000.     

Interaction of nitrogen source and light intensity on the growth and photosynthesis of the brown tide alga Aureococcus anophagefferens

High ratios of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) have been suggested to favor the growth of the brown tide alga Aureococcus anophagefferens. DON could provide a particular advantage in low light levels, as occur when blooms induce self-shading. We examined the effects of varying DON:DIN ratios on the photosynthetic abilities of cultured Aureococcus at two light intensities, 93 and 17 μmol photons m⁻² s⁻¹. Glutamic acid and urea were used as DON sources, and the remainder of the nitrogen was added as nitrate.In experiments examining Aureococcus growth with varying ratios of DON_Glu:DIN_Nitrate at two light intensities in batch culture, higher growth rates and biomass were observed in treatments containing DIN than in those with DON only, which contrasts with the results of previous studies. In semi-continuous growth experiments with varying DON_Urea:DIN_Nitrate ratios, low light cultures with urea had higher growth rates than those without urea. Also, the effective target area for light absorption per cell and photosystem II efficiency were greater for the low light cultures of each nutrient treatment, particularly when DON:DIN mixtures (33 and 67% N_Urea) were used. The same pattern was seen in the maximum photosynthetic rates per cell in the light-saturated (P_m^cell) and in the initial slope (α^cell) of the PE (photosynthesis versus irradiance) curve, and in PON, POC and chlorophyll a cell⁻¹. This indicates that the ability of Aureococcus to acclimate to low light conditions may be enhanced by the presence of both organic and inorganic nitrogen sources. These results suggest that Aureococcus physiology and photosynthesis are different during growth on a mixture of urea-N and nitrate than when either nitrogen source is present alone. Results of this study suggest that Aureococcus may not respond to all DON substrates in the same way, and that mixtures of DON and DIN may provide for higher photosynthetic rates, especially at low light. Our results did not, however, support earlier suggestions that growth on DON alone provides the brown tide alga with a large advantage at low light levels.     

Temporal variations in phytoplankton, particulates, and colored dissolved organic material based on optical properties during a Long Island brown tide compared to an adjacent embayment

Since 1985, the coastal embayments of Long Island, New York, have been plagued with recurrent blooms, aptly called brown tides, of the pelagophyte Aureococcus anophagefferens. The distinct ocean color observed during these blooms suggests that optical methods can be used as a tool to study, detect, and track brown tides. Thus, the goal of our project was to compare the optical properties and pigment composition during bloom and non-bloom conditions and assess temporal variations in the phytoplankton and other constituents in the seawater associated with bloom development. From 17 May to 8 June 2000, we measured a time series of particle size distributions and concentrations as well as size-fractioned algal pigments and optical properties in two Long Island embayments where brown tides are known to occur. During our study, A. anophagefferens represented an insignificant contribution to the algal community in West Neck Bay (WNB), whereas a bloom developed in Quantuck Bay (QB). Initially, temperature and salinity were similar at the two locations; however, bulk optical properties, chlorophyll, and particle concentrations were nearly a factor of 2 greater at QB. Bulk optical properties remained constant at WNB, yet increased exponentially at QB as the bloom developed. The composition of particulates, including phytoplankton, varied little at QB, and the optical properties suggested the dominance of A. anophagefferens (confirmed by microscopy). The largest temporal variations were observed in the colored dissolved organic material (CDOM); the colloidal (0.2–0.7 μm) fraction, exhibiting a strong protein-like signal, increased dramatically at the height of the bloom. At WNB particle sizes and algal composition varied despite the invariant bulk optical properties; CDOM variations were minimal. Overall, the optical properties in the two bays demonstrated that at QB temporal variations were dominated by biomass and colloidal protein changes, whereas shifts in the algal community occurred at WNB. This study demonstrates the utility of in situ optical observations to resolve temporal changes in the ecological conditions associated with algal bloom development.     

Expansion of harmful brown tides caused by the pelagophyte,Aureoumbra lagunensis DeYoe et Stockwell,to the US east coast

Brown tides caused by the pelagophyte Aureoumbra lagunensis DeYoe et Stockwell have formed ecosystem disruptive algal blooms in shallow lagoons of Texas (TX), USA, for more than two decades but have never been reported elsewhere. During the summer of 2012, a dense brown tide occurred in the Mosquito Lagoon and northern Indian River Lagoon along the east coast of Florida (FL), USA. While chlorophyll a levels in this system have averaged 5 μg L⁻¹ during the past two decades, concentrations during this brown tide reached ∼200 μg L⁻¹. Concurrently, levels of nitrate were significantly lower than average and levels of dissolved organic nitrogen were significantly higher than average (p < 0.001 for both). Sequences of the 18S rRNA gene of the bloom community and of single cell isolates were identical to those of Aureoumbra lagunensis DeYoe et Stockwell from TX. The A. lagunensis brown tide in FL bloomed to densities exceeding 10⁶ cells mL⁻¹ (quantified with a species-specific immuno-label) from July through September, began to dissipate in October, but maintained densities exceeding 10⁵ cells mL⁻¹ in some regions through December of 2012. The decline of the bloom was associated with near-hypoxic conditions and more than 30 fish kills reported within the Mosquito Lagoon in September 2012, a number far exceeding all prior monthly reports in this system dating to 1996. Wild northern quahog populations (a.k.a. hard clam, Mercenaria mercenaria) suffered mass die offs during the brown tide and eastern oysters (Crassostrea virginica) that settled during 2012 were significantly smaller than prior years. Clearance rates of hard clams and eastern oyster were significantly reduced in the presence of Mosquito Lagoon bloom water and A. lagunensis monocultures isolated from the Mosquito Lagoon at densities of ∼10⁶ cells L⁻¹. The expansion of harmful brown tides caused by A. lagunensis to these estuaries represents a new threat to the US southeast coast.     

Microbial herbivory on the brown tide alga, Aureococcus anophagefferens: results from natural ecosystems, mesocosms and laboratory experiments

Experiments were conducted with natural plankton assemblages from two areas in Great South Bay (GSB) and the Peconic Bays Estuary System, NY, to compare the rates of growth and pelagic grazing mortality of Aureococcus anophagefferens with co-occurring phytoplankton. We hypothesized that A. anophagefferens would experience low mortality rates by microbial herbivores (relative to feeding pressure on other algae) thus providing it with a competitive advantage within the phytoplankton community. In fact, substantial rates of mortality were observed in nearly every experiment in our study. However, mortality rates of A. anophagefferens were less than intrinsic growth rates of the alga during late spring and early summer in Great South Bay, resulting in positive net growth rates for the alga during that period. This timing coincided with the development of a brown tide in this estuary. Similarly, growth rates of the alga also exceeded mortality rates during bloom development in natural plankton assemblages from the Peconic Bays Estuary System held in mesocosms. In contrast to the situation for A. anophagefferens, growth rates of the total phytoplankton assemblage, and another common picoplanktonic phytoplankter (Synechococcus spp.), were frequently less than their respective mortality rates. Mortality rates of A. anophagefferens in both systems were similar to growth rates of the alga during later stages of the bloom. Laboratory studies confirmed that species of phagotrophic protists that consume A. anophagefferens (at least in culture) are present during brown tides but preference for or against the alga appears to be species-specific among phagotrophic protists. We conclude that two scenarios may explain our results: (1) protistan species capable of consuming the brown tide alga were present at low abundances during bloom initiation and thus not able to respond rapidly to increases in the intrinsic growth rate of the alga, or (2) the brown tide alga produced substance(s) that inhibited or retarded protistan grazing activities during the period of bloom initiation. The latter scenario seems less likely given that significant mortality of A. anophagefferens was measured during our field study and mesocosm experiment. However, even a minor reduction in mortality rate due to feeding selectivity among herbivores might result in a mismatch between growth and grazing of A. anophagefferens that could give rise to significant net population growth of this HAB species. Either scenario infers an important role for trophic interactions within the plankton as a factor explaining the development of brown tides in natural ecosystems.     

The interactive roles of nutrient loading and zooplankton grazing in facilitating the expansion of harmful algal blooms caused by the pelagophyte,Aureoumbra lagunensis,to the Indian River Lagoon,FL, USA

Confined to Texas, USA, for more than 20 years, brown tides caused by Aureoumbra lagunensis emerged in the Indian River Lagoon and Mosquito Lagoon, Florida, USA, during 2012 and 2013, affording the opportunity to assess whether hypotheses developed regarding the occurrence of these blooms are ecosystem-specific. To examine the extent to which top-down (e.g. grazing) and bottom-up (e.g. nutrients) processes controlled the development of Aureoumbra blooms in Florida, nitrogen (N) uptake, nutrient amendment, and seawater-dilution, zooplankton grazing experiments were performed and the responses of Aureoumbra and competing phytoplankton were evaluated. During the study, Aureoumbra comprised up to 98% of total phytoplankton biomass, achieved cell densities exceeding 2 × 10⁶ cells mL⁻¹, and contained isotopically lighter N compared to non-bloom plankton populations, potentially reflecting the use of recycled N. Consistent with this hypothesis, N-isotope experiments revealed that urea and ammonium accounted for >90% of N uptake within bloom populations whereas nitrate was a primary N source for non-bloom populations. Low levels (10 μM) of experimental ammonium enrichment during blooms frequently enhanced the growth of Aureoumbra and resulted in the growth rates of Aureoumbra exceeding those of phycoerythrin-containing, but not phycocyanin-containing, cyanobacteria. A near absence of grazing pressure on Aureoumbra further enabled this species to out-grow other phytoplankton populations. Given this alga is generally known to resist zooplankton grazing under hypersaline conditions, these findings collectively suggest that moderate loading rates of reduced forms of nitrogenous nutrients (e.g ammonium, urea) into other subtropical, hypersaline lagoons could make them susceptible to future brown tides caused by Aureoumbra.     

Stimulation of the brown tide organism, Aureococcus anophagefferens, by selective nutrient additions to in situ mesocosms

The influence of nutrient additions and sediment exchange on Aureococcus anophagefferens growth was studied using 200 l mesocosms deployed in situ at the Southampton College Marine Science Center in Long Island, New York. A. anophagefferens cell density increased in mesocosms with separate additions of the following materials: urea + glucose and desiccation-stressed Enteromorpha tissue. A decrease in A. anophagefferens cell density was observed in mesocosms with either no additions (control) or with added nitrate. A treatment containing a sediment layer exhibited an increase in average cell densities, but the increase was not statistically significant (P = 0.15). In the 9 day experiment, net growth of A. anophagefferens was only observed during the last 3 days, which corresponded to a period of high solar irradiation. Total chlorophyll concentration declined in all treatments during the first 6 days, which corresponded to relatively low daily irradiance, suggesting low-light stress on the phytoplankton assemblage during the initial phase of the experiment. During the ensuing sunny period, a 4–5-fold increase in chlorophyll concentration was observed in the nitrate and urea treatments with lesser increases in the other treatments. A. anophagefferens density increased relative to total phytoplankton biomass (Chl basis) in the urea + glucose and Enteromorpha treatments. Results are consistent with a prevailing hypothesis that organic nitrogen nutrients favor the growth of A. anophagefferens. Specifically, our evidence indicates that A. anophagefferens exhibited net population growth under organic N, but not inorganic N nutrient (specifically NO₃⁻) loading.     

A comparison of N and C uptake during brown tide (Aureococcus anophagefferens) blooms from two coastal bays on the east coast of the USA

Blooms of the brown tide pelagophyte, Aureococcus anophagefferens, have been reported in coastal bays along the east coast of the USA for nearly two decades. Blooms appear to be constrained to shallow bays that have low flushing rates, little riverine input and high salinities (e.g., >28). Nutrient enrichment and coastal eutrophication has been most frequently implicated as the cause of A. anophagefferens and other blooms in coastal bays. We compare N and C dynamics during two brown tide blooms, one in Quantuck Bay, on Long Island, NY in 2000, and the other in Chincoteague Bay, at Public Landing, MD in 2002, with a physically similar site in Chincoteague Bay that did not experience a bloom. We found that the primary forms of nitrogen (N) taken up during the bloom in Quantuck Bay were ammonium and dissolved free amino acids (DFAA) while the primary form of N fueling production at both sites in Chincoteague Bay was urea. At both Chincoteague sites, amino acid carbon (C) was taken up while urea C was not. Even though A. anophagefferens has the ability to take up organic C, during the bloom at Chincoteague Bay, photosynthetic uptake of bicarbonate was the dominant pathway of C acquisition by the >1.2 μm size fraction during the day. C uptake by cells <5.0 μm was insufficient to meet cellular C demand based on the measured N uptake rates and the C:N ratio of particulate material. While cells >1.2 μm did not take up much organic C during the day, smaller cells (>0.2 μm) did. Peptide hydrolysis appeared to play an important role in mobilizing organic matter in Quantuck Bay, where amino acids contributed substantially to N and C uptake, but not in Chincoteague Bay. Dissolved organic N (DON), dissolved organic C (DOC) concentrations and the DOC/DON ratio were higher and total dissolved inorganic N (DIN) concentrations were lower at the bloom site in Chincoteague Bay than at the nonbloom site in the same bay. We conclude that A. anophagefferens is capable of using a wide variety of N and C compounds, and that nutrient inputs, biotic interactions and the dominant recycling pathways determine which compounds are available and which metabolic pathways are active at a particular site.     

Mediation of benthic–pelagic coupling by microphytobenthos: an energy- and material-based model for initiation of blooms of Aureococcus anophagefferens

We present a conceptual model for initiation of blooms of the estuarine brown-tide pelagophyte Aureococcus anophagefferens. The model is based on the observation that in addition to its well-documented stimulation by organic nutrients, Aureococcus is pre-adapted to low light levels. Its relatively low maximum (light-saturated) growth rate makes it a poor competitor with other estuarine species at high light under acclimated conditions. Its large photosynthetic antenna and relatively low quota of photoprotective pigments make it more susceptible to photoinhibition than other species to which it is compared. These same characteristics give it a competitive advantage at low light levels. In its shallow habitat, both the light level and the rate of nutrient supply from groundwater and benthic porewater are determined by the degree of benthic coupling. Experimental manipulations in a microcosm and a survey of the literature demonstrate the ability of the sediment-associated microphytobenthos (MPB) to regulate both the light- and nutrient-environment in the overlying water column. The model predicts that the growth dynamics of the MPB are such that the benthic/water column interactions tend towards one of two stable states. In one, a well-developed population of MPB restricts resuspension of particulate material and efflux of dissolved nutrients, resulting in clear and nutrient-poor overlying waters. This condition does not favor growth of Aureococcus. In the alternative state, erosion of the MPB results in turbid, nutrient-rich waters that do favor bloom initiation. Alternation between the states is caused by external physical forcing, through wind-driven mixing of the water column. Field data from Quantuck Bay, New York (USA), failed to document the transition from non-bloom to bloom conditions. Even so, they are consistent with the model’s predictions.     

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.     

Intraspecific variability in Karlodinium veneficum: Growth rates, mixotrophy, and lipid composition

We isolated eleven strains of the harmful algal bloom (HAB)-forming dinoflagellate Karlodinium veneficum during a bloom event in the NW Mediterranean coastal waters and we studied the inter-strain variability in several of their physiological and biochemical traits. These included autotrophic growth parameters, feeding capabilities (mixotrophy), lipid composition, and, in some cases, their responses to biotic and abiotic factors. The strains were found to differ in their growth rates (0.27–0.53 d⁻¹) and in the maximum cell concentrations achieved during stationary phase (6.1 × 10⁴–8.6 × 10⁴ cells mL⁻¹). Their ingestion performance, when offered Rhodomonas salina as prey, was also diverse (0.22–1.3 cells per K. veneficum per day; 8–52% of their daily ration). At least two strains survived for several months under strict heterotrophic conditions (no light, low inorganic nutrients availability, and R. salina as food source). These strains also showed very distinct fatty acid compositions, with very low contents of monounsaturated and polyunsaturated fatty acids. According to a Bray Curtis similarity analysis, three or four strain groups able to perform different roles in bloom development were identified. We further analyzed one strain from each of the two most distinct groups with respect to prey concentration, light intensity, nutrient availability, and we determined the functional responses (growth and feeding rates) to food concentration. Taken together, the results served to highlight the role of mixotrophy and clone variability in the formation of HABs.     

Physiological responses of Aureoumbra lagunensis and Synechococcus sp. to nitrogen addition in a mesocosm experiment

Aureoumbra lagunensis is the causative organism of the Texas brown tide and is notable because it dominated the Laguna Madre ecosystem from 1990 to 1997. This species is unusual because it has the highest known critical nitrogen to phosphorus ratio (N:P) for any microalgae ranging from 115 to 260, far higher than the 16N:1P Redfield ratio. Because of its high N:P ratio, Aureoumbra should be expected to respond to N additions that would not stimulate the growth of competitors having the Redfield ratio. To evaluate this prediction, a mesocosm experiment was performed in the Laguna Madre, a South Texas coastal lagoon, in which a mixed Aureoumbra–Synechococcus (a cyanobacterium) community was enclosed in 12 mesocosms and subjected to nitrogen addition (6 controls, 6 added ammonium) for 16 days. After day 4, added nitrogen did not significantly increase Aureoumbra specific growth rate but the alga retained dominance throughout the experiment (64–75% of total cell biovolume). In control mesocosms, Aureoumbra became less abundant during the first 4 days of the experiment but rebounded by the end of the experiment and was dominant over Synechococcus. Despite the lack of a strong positive growth response, Aureoumbra did respond physiologically to N addition. By the end of the experiment, the average N:P ratio of the Aureoumbra-dominated community was 86 in the N+ treatment and 41 in the control, indicating that the alga became less N-limited in the N+ treatment. The average C:N ratio was 6.6 in the N+ treatment (8.6 in the control) and suggests that the alga was not N-limited, however, C:N ratio may not be a good indicator of nitrogen limitation since this alga can produce significant quantities of carbon-containing extracellular polysaccharides, depending on growth conditions. Both Aureoumbra cellular chlorophyll fluorescence and cell size increased in response to added N, indicating a reduction in N limitation. It appeared that the N additions were not large and/or frequent enough to stimulate Aureoumbra growth. The main competitor, the unicellular cyanobacterium Synechococcus, responded positively to the nitrogen addition by increased specific growth rate. Unlike Aureoumbra, no significant effect on Synechococcus cellular pigment fluorescence or cell size was noted. Literature data suggest that Synechococcus, like Aureoumbra, may have a critical N:P ratio much higher than 16:1, which could explain its response.     

Long-term increase in Karenia brevis abundance along the Southwest Florida Coast

Data collected along the southwest coast of Florida between Tampa Bay and Sanibel Island on the abundance of the toxic dinoflagellate Karenia brevis from 1954 to 2002 were examined for spatial and temporal patterns. K. brevis was found to be approximately 20-fold more abundant within 5 km of the shoreline than 20–30 km offshore. Overall, K. brevis was approximately 13–18-fold more abundant in 1994–2002 than in 1954–1963. In 1954–1963, K. brevis occurred primarily in the fall months. In 1994–2002, it was more abundant not only in the fall, but also in the winter and spring months. It is hypothesized that greater nutrient availability in the ecosystem is the most likely cause of this increase in K. brevis biomass, and the large increase in the human population and its activities in South Florida over the past half century is a major factor.     

Attempts to model the bloom dynamics of Pyrodinium, a tropical toxic dinoflagellate

For the first time, several models have been used to aid in the understanding of the bloom dynamics of Pyrodinium bahamense var. compressum, the major causal organism of toxic algal blooms in Manila Bay and several areas in the tropical world. The complex life cycle of Pyrodinium includes the formation of cysts that settle at the sediments, which can serve as the inoculum for the next bloom.The seasonal variation of temperature and salinity reflects the combined effects of convection and water column stability, which can control vertical movement of plankton and other parameters essential to its growth. The significance of wind forcing appears to be related to the potential to resuspend cysts. In the absence of wind, tidal currents in the inner part of the bay may be too weak to induce resuspension. The addition of wind results in a significant increase in bottom current velocity. Off Cavite at the southeast, bottom velocity is enhanced by orbital motion due to waves, one of the reasons why sediments off this area are dominated by sandy material. The strong vertical mixing of the water column at depths of less than 10 m may influence nutrient and consequently, plankton populations.The wave field during the southwest monsoon indicates that its contribution to the bottom velocity dominates in this area of the bay.Bloom simulations using combined bio-physical parameters show that direction of advection is almost always along wind direction. The dispersal distances increases if the Pyrodinium cells are found higher in the water column. For cells originating from southeastern (Cavite) sources, the direction of transport is slightly towards the north. In either case, the formation of cysts after a bloom is adjacent to the northern area (Pampanga) for blooms originating from the western side (Bataan) and along the eastern side (Parañaque–Manila) for blooms originating from the southeastern side (Cavite). Comparison with a few records of bloom occurrences in Manila Bay shows some consistent features. Reports of these blooms also showed that they occurred almost always during spring tides. There appears to be two main systems for bloom formation: one fed by cyst beds in the west (Bataan) which is advected along the west–northwest coast (Bataan–Bulacan) while the other one is fed by the southeast (Cavite) cyst beds that dominates in the east-southeast (Parañaque–Cavite) area.