Hypersalinity enhances the production of extracellular polymeric substance (eps) in the texas brown tide alga, aureoumbra lagunensis (PELAGOPHYCEAE)

Laboratory experiments with batch cultures showed that the Texas brown tide alga, Aureoumbra lagunensis Stockwell, DeYoe, Hargraves et Johnson, produced a large amount of extracellular polymeric substance (EPS) that formed an extracellular polysaccharide mucus layer. Both dissolved and particulate carbohydrate and EPS concentrations increased as A. lagunensis cultures progressed from exponential growth phase, through stationary phase, to declining phase. Particulate carbohydrate and EPS concentrations per cell were more than three times higher during the declining phase than that in exponential growth phase, reflecting a large increase in the EPS mucus layer. The amounts of carbohydrate and EPS produced by A. lagunensis were significantly higher under hypersaline conditions. The thicker EPS mucus layer surrounding A. lagunensis cells under hypersaline conditions might be a protective adaptation that permits it to bloom under hypersaline conditions that most other phytoplankton cannot survive. This could be one of the reasons why the Texas brown tide persisted in the Laguna Madre, an often hypersaline coastal lagoon, for 7 years.     

Effects of temperature and salinity on the growth of the red tide flagellates Heterocapsa circularisquama (Dinophyceae) and Chattonella verruculosa (Raphidophyceae)

Growth responses of the red tide flagellates, Heterocapsa circularisquama(Dinophyceae) and Chattonella verruculosa (Raphidophyceae),were examined with 36 different combinations of temperature(5–30°C) and salinity (10–35 PSU). Heterocapsacircularisquama did not grow at or below a temperature of 10°C.The maximum growth rate of H.circularisquama (1.3 divisionsday^–1) was obtained with a combination of 30°C and30 PSU. In contrast, C. verruculosa did not grow at 10 PSU andat temperatures of 25°C or more. The maximum growth rateof C. verruculosa (1.74 divisions day^–1) was obtainedwith a combination of 15°C and 25 PSU. A significant temperature-salinityinteraction on growth was found by factorial analysis. Basedon the physiological characteristics obtained in the presentstudy, these novel flagellates have a potential for future outbreaksof red tides in pre viously unaffected waters.     

EFFECT OF NITROGEN ADDITION ON A MIXED SPECIES PHYTOPLANKTON BLOOM

The Laguna Madre of Texas is a hypersaline lagoon that in recent years has been dominated by the brown tide alga, Aureoumbra lagunensis although a Synechococcus -like sub-dominant is typically present. Dominance of A. lagunensis is thought to be due at least in part to its nitrogen competitive abilities. A mesocosm study was performed to determine if an increase in ambient nitrogen concentration would lead to a shift in the dominant alga. Twelve fiberglass cylinders enclosing 1.2 cubic meters of Laguna Madre water were deployed for 16 days. Four times during this period, ammonium was added (N+ mesocosms) to half the mesocosms to achieve a post-addition concentration of approximately 40 mM. The average initial particulate N/P ratio was 40/1 (SD = 2.9) which recent evidence indicates is within the range of N/P ratios for N-limited A. lagunensis. In control mesocosms, total cell biovolume (TCB) of A. lagunensis cells dropped after 4 days by a factor of four and then increased to a level slightly above the day 0 value by day 16. In N+ mesocosms, A. lagunensis TCB was unchanged after 4 days then doubled by day 16. Despite the differences in final yield, growth rates of A. lagunensis in the two treatments were similar. The Synechococcus -like organism, showed a four-day lag before TCB in both treatments increased although at a slower rate in the control mesocosms. By the end of the experiment, TCB of A. lagunensis was two to six-times greater than that of subdominant. Despite the nitrogen treatment, A. lagunensis retained dominance.     

Spatial-temporal distribution of Aureoumbra lagunensis (“brown tide”) in Baffin Bay,Texas

Once limited to the Laguna Madre of Texas, blooms of the brown tide organism, Aureoumbra lagunensis, have recently been reported elsewhere. Previous studies have focused on the role of extreme hypersalinity and lack of grazing pressure as facilitators of brown tide blooms. However, development of blooms in systems that are not experiencing extreme hypersalinity, and also that are undergoing eutrophication, suggests that our understanding of A. lagunensis bloom dynamics requires additional refinement. The goal of this study was to quantify the spatial-temporal distribution of, and potential controls upon, A. lagunensis in Baffin Bay, Texas. Five sites were sampled monthly over a three-year period, encompassing nearly two years of drought and hypersaline conditions, followed by a high rainfall, lower salinity period. A. lagunensis abundances were higher during drought in May 2013 – March 2015 compared to the higher rainfall period of April 2015 – April 2016. Abundances typically peaked in summer months, though the seasonal pattern was disrupted in 2015 during the shift from high to low salinity conditions. Persistently high abundances of A. lagunensis were observed in the Laguna Salada tributary of Baffin Bay, which typically has higher dissolved organic nitrogen concentrations and may be less well flushed than other parts of Baffin Bay. Thus this location may serve as a reservoir for A. lagunensis in the system. Overall, A. lagunensis abundance was positively correlated with DOC and salinity, and negatively correlated with ammonium, orthophosphate, and ciliate biovolume. These results suggest a variety of physical, chemical and biological factors affect A. lagunensis population dynamics and stress the need for more research on nutrient-A. lagunensis relationships.     

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 Effects of Salinity upon Cellular Volume of the Marine Red Alga Porphyra purpurea (Roth) C.Ag.

Changes in cell volume of the marine red alga Porphyra purpureahave been investigated using photomicroscopic and radioisotopictechniques. There is an inverse relationship between cell volumeand external salt content. The alga responds to changes in thewater potential of its bathing medium by rapid swelling in hyposalinemedia and shrinkage in hypersaline conditions. Cells P. purpureabehave as osmometers in concentrated sea-waters, obeying theBoyle-Van't Hoff law. A non-osmotic volume, 20–25% ofthe total cell volume in sea-water, can be predicted from thelinear plot of volume versus reciprocal pressure in concentratedsea-water media. In dilute sea-waters the presence of non-rigidcell walls serves to limit any increases in cell volume. Theprimary response to dilution stress is thus an increase in turgor.Cell volume is not returned to its original value followingprolonged immersion in either hyposaline or hypersaline media,showing that the alga does not ‘osmoregulate’ sensustricto.     

Effects of mesozooplankton removal and ammonium addition on planktonic trophic structure during a bloom of the Texas 'brown tide': a mesocosm study

A bloom of the alga Aureoumbra lagunensis, known as the Texas‘brown tide’, persisted in the Laguna Madre of Texasfor most of the 1990s. The dominant mesozooplankter in LagunaMadre, Acartia tonsa, does not feed on A. lagunensis, and duringblooms there are few other suitably sized phytoplankton cellsavailable to feed on. We hypothesized that these copepods increasedtheir feeding on microzooplankton, thereby reducing grazingpressure by microzooplankton on A. lagunensis and contributingto the persistence of this bloom. A mesocosm experiment wascarried out to test this hypothesis during the summer of 1999.Twelve fiberglass corral-type mesocosms were deployed in thefield for 16 days, each enclosing 1.2 m³ of Laguna Madre waterand 1.1 m² of natural benthos. Mesozooplankton were removedfrom six mesocosms with a 153 µm mesh dip net every 4days; the other six mesocosms were treated in the same way,except that the contents of the net were returned to the mesocosm.For each zooplankton treatment, half of the mesocosms were dosedwith 40 µm NH₄ at 4 day intervals, and half received noadditions. Phytoplankton populations in these mesocosms at thestart of the experiment were dominated by A. lagunensis andthe cyanobacterium Synechococcus spp. Growth rates of A. lagunensiswere higher in mesocosms without ammonium additions, providingno evidence for nitrogen limitation. Acartia tonsa populationswere reduced by 50% in the zooplankton removal mesocosms, andciliate populations were significantly higher. The increasein ciliate population had no significant impact on A. lagunensispopulation dynamics, however, providing no evidence to supportthe hypothesis that a trophic cascade reducing microzooplanktonpopulations contributed to the persistence of the brown-tidebloom. In contrast, populations of Synechococcus sp. showedevidence of both ‘top–down’ and ‘bottom–up’control; they grew faster in nutrient addition mesocosms andhad lower populations in mesocosms with increased densitiesof ciliate grazers.     

Effect of temperature and light intensity on the growth rate of Synura sphagnicola

The chrysophyte Synura sphagnicola Korsch. was isolated froma hypolimnion bloom in a Canadian Shield lake and its abilityto grow at low light and temperature was studied. Growth saturatinglight was much higher than in situ intensities and independentof temperature while compensation intensity decreased with decreasingtemperature. Optimum temperature decreased with decreasing lightintensity. While optimum temperature was lower than generallyseen among temperate water algae, compensating and saturatinglight were similar to those seen in other algae. At low lightand temperature, the growth rate of S. sphagnicola was lowerthan the growth rates under similar conditions of other algae,and appeared insufficient to account for the net rate of chlorophyllaccumulation observed in the bloom from which the alga was isolated.     

Sublethal effects of Texas brown tide on Streblospio benedicti (Polychaeta) larvae

The Texas brown tide bloom is noted for a concordant decline in benthic biomass and species diversity. However, the link between harmful effects induced by Texas brown tide and benthos has not been demonstrated. It has been proposed there may be a larval bottleneck, where larvae, but not adults, suffer adverse effects. This study was performed to test the effect of brown tide alga, Aureoumbra lagunensis, on mortality, growth and behavior of Streblospio benedicti larvae. Growth rates and swimming speeds, but not mortality rates, of polychaete larvae were reduced in cultures with brown tide relative to Isochrysis galbana, which is about the same size as brown tide. Results from this research indicate that brown tide does have harmful sublethal effects for one dominant species of meroplanktonic larvae, which could help explain reduced adult population size.     

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.     

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.     

Growth and decline of a diatom spring bloom phytoplankton species composition, formation of marine snow and the role of heterotrophic dinoflagellates

During the spring of 1994, we determined the factors responsiblefor the decline of the seasonal diatom bloom in the Gullmarfjord, on the west coast of Sweden. Four species constituted>75% of the biomass—Detonula confervacea, Chaetocerosdiadema, Skeletonema costatum and Thalassiosira nordenskioeldii—reachingconcentrations of 4900, 350, 8200 and 270 cells ml^–1,respectively. Growth of phytoplankton was exponential (growthrate = 0.12 day^–1) from 3 to 21 March, after which a galewith winds >15 m s^–1 caused massive aggregation. Amaximum of 130 p.p.m. (v/v) of marine snow aggregates was observedby in situ video at the peak of the bloom. Critical concentrations(Jackson, Deep-Sea Res., 37, 1197–1211, 1990) were similarto observed showing that coagulation theory could explain thesudden decline of the bloom. The heterotrophic dinoflagellateGyrodinium cf. spirale increased exponentially after the peakof the bloom with maximum (temperature-adjusted) growth rates.After the rapid aggregation and sedimentation of the bloom,they were able to control any further growth of diatoms. Nitrateand silicate were never depleted, but phosphate may have beenlimiting by the end of the study period. We conclude that massaggregation during a gale marked the end of the bloom, and thatintense grazing by heterotrophic dinoflagellates prevented anysubsequent increase of diatoms.     

NUTRIENT-LIMITED GROWTH OF AUREOUMBRA LAGUNENSIS (PELAGOPHYCEAE), WITH IMPLICATIONS FOR ITS CAPABILITY TO OUTGROW OTHER PHYTOPLANKTON SPECIES IN PHOSPHATE-LIMITED ENVIRONMENTS

Growth and chemical compositional characteristics of the brown tide-forming alga, Aureoumbra lagunensis Stockwell, DeYoe, Hargraves et Johnson, were studied through a series of nitrogen-limited and phosphate-limited continuous cultures over a range of growth rates. The specific growth rate of A. lagunensis was hyperbolically related to the cell quota of the limiting nutrient in ammonium-limited cultures. In phosphate-limited cultures, the relationship was best described by a straight line. The N cell quota of A. lagunensis ranges from about 20 fmol at zero growth rate under N-limited conditions to a high of roughly 85 fmol under N-replete conditions. Similarly, the P cell quota of A. lagunensis ranges from about 0.15 fmol at zero growth rate under P-limited conditions to a high of 2 fmol under P-replete conditions. Aureoumbra lagunensis has a very high N:P critical ratio (>100). The high N:P critical ratio, as well as the organism's apparent ability to use forms of phosphorus other than phosphate under severe phosphate deficiency, may partially explain its success in P-limited environments, such as the Laguna Madre. In addition, a uniqe quadratic relationship between the productivity index (PI) and growth rate was discovered. Such a relationship supports an earlier argument that PI may not be a good indicator of nutritional status.     

Growth and development of Neocalanus flemingeri/plumchrus in the northern Gulf of Alaska: validation of the artificial-cohort method in cold waters

In situ growth and development of Neocalanus flemingeri/plumchrusstage C1–C4 copepodites were estimated by both the artificial-cohortand the single-stage incubation methods in March, April andMay of 2001–2005 at 5–6°C. Results from thesetwo methods were comparable and consistent. In the field, C1–C4stage durations ranged from 7 to >100 days, dependent ontemperature and chlorophyll a (Chl a) concentration. Averagestage durations were 12.4–14.1 days, yielding an averageof 56 days to reach C5, but under optimal conditions stage durationswere closer to 10 days, shortening the time to reach C5 (fromC1) to 46 days. Generally, growth rates decreased with increasingstage, ranging from 0.28 day^–1 to close to zero but weretypically between 0.20 and 0.05 day^–1, averaging 0.110± 0.006 day^–1 (mean ± SE) for single-stageand 0.107 ± 0.005 day^–1 (mean ± SE) forartificial-cohort methods. Growth was well described by equationsof Michaelis–Menten form, with maximum growth rates (G_max)of 0.17–0.18 day^–1 and half saturation Chl a concentrations(K_chl) of 0.45–0.46 mg m^–3 for combined C1–3,while G_max dropped to 0.08–0.09 day^–1 but K_chl remainedat 0.38–0.93 mg m^–3 for C4. In this study, in situgrowth of N. flemingeri/plumchrus was frequently food limitedto some degree, particularly during March. A comparison withglobal models of copepod growth rates suggests that these modelsstill require considerable refinement. We suggest that the artificial-cohortmethod is the most practical approach to generating the multispeciesdata required to address these deficiencies.     

Changes in the Levels of Ribulose-l,5-bisphosphate Carboxylase/Oxygenase (Rubisco) in Tetraedron minimum (Chlorophyta) during Light and Shade Adaptation

Exponentially growing cultures of the chlorophyta Tetraedronminimum were allowed to photoadapt to low (50µmole quantam^–2s^–1) and high (500µmole quanta m^–2–1)irradiance levels. In these cultures, various aspects of theorganization of the photosynthetic apparatus and related differencesin its performance were studied. In this organism, the observed five-fold increase in pigmentationof low-light adapted cells was due to increases in the numbersof PSU's, while their sizes remained constant. Using radioimmunoassay technique, we found that high-light adaptedalgae had over five times more Rubisco per PSU than their low-lightadapted counterparts. The high-light adapted algae also exhibited far higher (x2.3)light saturated photosynthetic rates per chl a. This increasewas the result of a reduction of tau, , the turnover time ofPS II reaction centers. We propose that the increase in Rubisco per PSU in high-lightadapted algae explains the reduction in , which results in thehigher P_max rates per chl a in these algae. The relationship is non linear, since the increase in Rubiscoper PSU was x5.3 whereas that in P_mM per chl a was only x2.3. (Received July 30, 1988; Accepted December 2, 1988)     

A recurrent and localized dinoflagellate bloom in a Mediterranean beach

A recurrent, prolonged and singular bloom of Alexandrium tayloriBalech in an open beach (La Fosca, Spain, NW Mediterranean)is described. Alexandrium taylori appears at several placesalong a wide area of the NW Mediterranean (Costa Brava) duringthe summer, reaching concentrations up to 10⁵ cells l^–1,but it only proliferates persistently, massively (densities>10⁶ cells l^–1) and recurrently during August in LaFosca beach. The A.taylori bloom can be considered a manifestationof large-scale proliferation in a restricted area, where couplingbetween resting cysts in the sediment and bloom outbreak isnot a major factor compared to the interaction of local environmentalconditions with the planktonic organism's life history. Fromobservations of environmental conditions (the environmentalwindow) and the multiscale spatio-temporal distributions andlife history of A.taylori, we describe the bloom dynamics andanswer some critical questions about the different phases ofthe bloom. Some of these answers are: (i) the source of theA.taylori population is widespread offshore and is not locateddirectly at the beach; (ii) high cell densities are reachedand maintained with a moderate in situ growth and low loss rates;(iii) temporary cysts act as a reserve of the population.     

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.     

The seasonal abundance of the phototrophic ciliate Mesodinium rubrum in Southampton Water, England

The abundance of the marine phototrophic planktonic ciliateMesodinium rubrum was monitored throughout an annual cycle attwo stations in the Southampton Water estuary. Seasonal changesin the concentration of nitrate, ammonia and phosphate weremonitored both at the inner estuary station (NW Netley) andouter estuary station (Calshot). Nutrient levels in the winterwere similar at both stations, and were diminished during sequentialdiatom blooms dominated initially by Sketetonema costatum andthen by Rhizosolenia dclicatula. Nitrate was reduced to a seasonalminimum in the outer estuary following these spring diatom blooms,but in the inner estuary was sustained >500 µg I^–1until the onset of the M.rubrum bloom. During the developmentof a visible red tide of M.rubrum in June/July at NW Netley,nutrient concentrations were considerably reduced. Cell numbersof M.rubrum varied between 2 and 3 cells ml^– during winterto >400 cells ml^–1 during the bloom at NW Netley, whereasat Calshot cell abundance did not increase above 25 cells ml^–1at any time of the year. At NW Netley, dense accumulations ofthe ciliate occurred over restricted depth intervals duringthe bloom and possible factors influencing the observed verticaldistribution of cells are considered. ¹Present address: Biology Department, Faculty of Science, AddisAbaba University, PO Box 1176, Addis Ababa, Ethiopia     

An immunofluorescent survey of the brown tide chrysophyte Aureococcus anophagefferens along the northeast coast of the United States

Surveys were conducted along the northeast coast of the USA.between Portsmouth, NH, and the Chesapeake Bay in 1988 and 1990,to determine the population distribution of Aureococcus anophagefferens,the chrysophyte responsible for massive and destructive ‘browntides’ in Long Island and Narragansett Bay beginning in1985. A species-specific immunofluorescent technique was usedto screen water samples, with positive identification possibleat cell concentrations as low as 10–20 cells ml^–1.Both years.A.anophagefferens was detected at numerous stationsin and around Long Island and Barnegat Bay, NJ, typically athigh cell concentrations. To the north and south of thus ‘center’,nearly half of the remaining stations were positive for A.anophagefferens,but the cells were always at very low cell concentrations. Manyof the positive identifications in areas distant from Long Islandwere in waters with no known history of harmful brown tides.The species was present in both open coastal and estuanne locations,in salinities between 18 and 32 practical salinity units (PSU).The observed population distributions apparently still reflectthe massive 1985 outbreak when this species first bloomed, giventhe number of positive locations and high abundance of A.anophagefferensin the immediate vicinity of Long Island. However, the frequentoccurrence of this species in waters far from this population‘center’ is disturbing. Aureococcus anophagefferensis more widely distributed than was previously thought. Numerousareas thus have the potential for destructive brown tides suchas those associated with the sudden appearance of the speciesin 1985.     

How does eutrophication affect the role of grazers in harmful algal bloom dynamics?

Population dynamics of harmful algal bloom species are regulated both from the “bottom-up” by factors that affect their growth rate and from the “top-down” by factors that affect their loss rates. While it might seem apparent that eutrophication would have the greatest impact on factors affecting growth rates of phytoplankton (nutrient supply, light availability) the roles of top-down controls, including grazers and pathogens, cannot be ignored in studies of harmful bloom dynamics. Lags between the growth of phytoplankton and zooplankton populations, or disruption of zooplankton populations by adverse environmental conditions may be important factors in the initiation of plankton blooms under eutrophic conditions. Grazers that avoid feeding on harmful species and actively graze on competing species may also play important roles in bloom initiation. Grazers that are not affected by phytoplankton toxins and have growth rates comparable to phytoplankton (e.g. protozoan grazers) may have the potential to control the initiation of blooms. If the inhibition of grazers varies with cell density for blooms of toxic phytoplankton, eutrophication may increase the chances of blooms reaching threshold densities for grazer inhibition. In addition, secondary effects of eutrophication, including hypoxia and change in pH may adversely affect grazer populations, and further release HAB species from top-down control. The Texas brown tide (Aureoumbra lagunensis) blooms provide evidence for the role of grazer disruption in bloom initiation and the importance of high densities of brown tide cells in continued suppression of grazers.