Impacts of the 2014 severe drought on the Microcystis bloom in San Francisco Estuary

The increased frequency and intensity of drought with climate change may cause an increase in the magnitude and toxicity of freshwater cyanobacteria harmful algal blooms (CHABs), including Microcystis blooms, in San Francisco Estuary, California. As the fourth driest year on record in San Francisco Estuary, the 2014 drought provided an opportunity to directly test the impact of severe drought on cyanobacteria blooms in SFE. A field sampling program was conducted between July and December 2014 to sample a suite of physical, chemical, and biological variables at 10 stations in the freshwater and brackish reaches of the estuary. The 2014 Microcystis bloom had the highest biomass and toxin concentration, earliest initiation, and the longest duration, since the blooms began in 1999. Median chlorophyll a concentration increased by 9 and 12 times over previous dry and wet years, respectively. Total microcystin concentration also exceeded that in previous dry and wet years by a factor of 11 and 65, respectively. Cell abundance determined by quantitative PCR indicated the bloom contained multiple potentially toxic cyanobacteria species, toxic Microcystis and relatively high total cyanobacteria abundance. The bloom was associated with extreme nutrient concentrations, including a 20-year high in soluble reactive phosphorus concentration and low to below detection levels of ammonium. Stable isotope analysis suggested the bloom varied with both inorganic and organic nutrient concentration, and used ammonium as the primary nitrogen source. Water temperature was a primary controlling factor for the bloom and was positively correlated with the increase in both total and toxic Microcystis abundance. In addition, the early initiation and persistence of warm water temperature coincided with the increased intensity and duration of the Microcystis bloom from the usual 3 to 4 months to 8 months. Long residence time was also a primary factor controlling the magnitude and persistence of the bloom, and was created by a 66% to 85% reduction in both the water inflow and diversion of water for agriculture during the summer. We concluded that severe drought conditions can lead to a significant increase in the abundance of Microcystis and other cyanobacteria, as well as their associated toxins.     

Beyond hydraulic flushing: Deep water mixing takes the harm out of a haptophyte algal bloom

Inflows are linked to water quality, food web dynamics, and the incidence of harmful algal blooms (HABs). It may be that inflows can be manipulated to mitigate some blooms and accelerate recovery of living natural resources, such as fisheries. Lack of available water, however, can limit this approach to management. Utilizing source water from deeper depths to displace surface waters, however, might effectively mimic aspects of inflow events, such as disrupting ecological processes and community succession through hydraulic displacement. We tested this notion by conducting in-lake mesocosm experiments with natural plankton communities plagued by Prymnesium parvum where we manipulated hydraulic flushing. We found that P. parvum cell densities were reduced by up to 69% and 53%, and ambient toxicity ameliorated, during pre-bloom and bloom development periods. Furthermore, other phytoplankton taxa and zooplankton benefited from these pulsed flushing events. In other words, hydraulic flushing with deep waters not only suppressed P. parvum bloom initiation and development, but also proved beneficial to other aspects of the lower food web. These observations provide the first demonstration that P. parvum bloom initiation may be interrupted, and bloom development may be arrested, through hydraulic manipulations.     

Dynamics of Amoebophrya parasites during recurrent blooms of the ichthyotoxic dinoflagellate Cochlodinium polykrikoides in Korean coastal waters

During the bloom events of the harmful dinoflagellate Cochlodinium polykrikoides in August and October, 2012, infections by two different Amoebophrya species were observed in Korean coastal waters. To investigate the dynamics of the two parasites and their relative impact on the host populations, a quantitative real-time PCR (qPCR) method was applied to detect and quantify the parasites in the free-living and parasitic stages. Each specific primer set of the target species, Amoebophrya sp. 1 and sp. 2 was designed on the large subunit (LSU) and the first internal transcribed spacer (ITS1) of ribosomal RNA (rRNA) gene, respectively. Dynamics of the two Amoebophrya species via qPCR assay showed distinct patterns during the C. polykrikoides bloom events. Amoebophrya sp. 1 showed peaks during both bloom events in August and October with relatively low copies (10⁶ to 10⁷ copies L⁻¹), while Amoebophrya sp. 2 appeared only during the bloom event in October with very high copies (10⁹ to 10¹⁰ copies L⁻¹). Overall, the qPCR measurements for the dynamics of two Amoebophrya species in the parasitic stage (> 5 μm fractions) were consistent with parasite prevalence through microscopic observations. Amoebophrya sp. 1 infections were observed during both bloom events in August and October with relatively low parasite prevalence (0.1–1.5%), while Amoebophrya sp. 2 infections were detected only during the bloom event in October with high prevalence (up to 45%). Taken together, Amoebophrya sp. 1 may be a generalist and C. polykrikoides may not be its primary host, while Amoebophrya sp. 2 may be a specialist which can substantially impact host population dynamics.     

Chytrid epidemics may increase genetic diversity of a diatom spring-bloom

Contrary to expectation, populations of clonal organisms are often genetically highly diverse. In phytoplankton, this diversity is maintained throughout periods of high population growth (that is, blooms), even though competitive exclusion among genotypes should hypothetically lead to the dominance of a few superior genotypes. Genotype-specific parasitism may be one mechanism that helps maintain such high-genotypic diversity of clonal organisms. Here, we present a comparison of population genetic similarity by estimating the beta-dispersion among genotypes of early and peak bloom populations of the diatom Asterionella formosa for three spring-blooms under high or low parasite pressure. The Asterionella population showed greater beta-dispersion at peak bloom than early bloom in the 2 years with high parasite pressure, whereas the within group dispersion did not change under low parasite pressure. Our findings support that high prevalence parasitism can promote genetic diversification of natural populations of clonal hosts.     

Combined physical,chemical and biological factors shape Alexandrium ostenfeldii blooms in The Netherlands

Harmful algal blooms (HABs) are globally expanding, compromising water quality worldwide. HAB dynamics are determined by a complex interplay of abiotic and biotic factors, and their emergence has often been linked to eutrophication, and more recently to climate change. The dinoflagellate Alexandrium is one of the most widespread HAB genera and its success is based on key functional traits like allelopathy, mixotrophy, cyst formation and nutrient retrieval migrations. Since 2012, dense Alexandrium ostenfeldii blooms (up to 4500 cells mL⁻¹) have recurred annually in a creek located in the southwest of the Netherlands, an area characterized by intense agriculture and aquaculture. We investigated how physical, chemical and biological factors influenced A. ostenfeldii bloom dynamics over three consecutive years (2013–2015). Overall, we found a decrease in the magnitude of the bloom over the years that could largely be linked to changing weather conditions during summer. More specifically, low salinities due to excessive rainfall and increased wind speed corresponded to a delayed A. ostenfeldii bloom with reduced population densities in 2015. Within each year, highest population densities generally corresponded to high temperatures, low DIN:DIP ratios and low grazer densities. Together, our results demonstrate an important role of nutrient availability, absence of grazing, and particularly of the physical environment on the magnitude and duration of A. ostenfeldii blooms. Our results suggest that predicted changes in the physical environment may enhance bloom development in future coastal waters and embayments.     

Alteration of plankton communities and biogeochemical cycles by harmful Cochlodinium polykrikoides (Dinophyceae) blooms

Cochlodinium polykrikoides is a globally distributed, ichthyotoxic, bloom-forming dinoflagellate. Blooms of C. polykrikoides manifest themselves as large (many km²) and distinct patches with cell densities exceeding 10³ ml⁻¹ while water adjacent to these patches can have low cell densities (<100 cells ml⁻¹). While the effect of these blooms on fish and shellfish is well-known, their impacts on microbial communities and biogeochemical cycles are poorly understood. Here, we investigated plankton communities and the cycling of carbon, nitrogen, and B-vitamins within blooms of C. polykrikoides and compared them to areas in close proximity (<100 m) with low C. polykrikoides densities. Within blooms, C. polykrikoides represented more than 90% of microplankton (>20 μm) cells, and there were significantly more heterotrophic bacteria and picoeukaryotic phytoplankton but fewer Synechococcus. Terminal restriction fragment length polymorphism analysis of 16S and 18S rRNA genes revealed significant differences in community composition between bloom and non-bloom samples. Inside the bloom patches, concentrations of vitamin B₁₂ were significantly lower while concentrations of dissolved oxygen were significantly higher. Carbon fixation and nitrogen uptake rates were up to ten times higher within C. polykrikoides bloom patches. Ammonium was a more important source of nitrogen, relative to nitrate and urea, for microplankton within bloom patches compared to non-bloom communities. While uptake rates of vitamin B₁ were similar in bloom and non-bloom samples, vitamin B₁₂ was taken up at rates five-fold higher (>100 pmol⁻¹ L⁻¹ d⁻¹) in bloom samples, resulting in turn-over times of hours during blooms. This high vitamin demand likely led to the vitamin B₁₂ limitation of C. polykrikoides observed during nutrient amendment experiments conducted with bloom water. Collectively, this study revealed that C. polykrikoides blooms fundamentally change microbial communities and accelerate the cycling of carbon, some nutrients, and vitamin B₁₂.     

Long-term trends and causal factors associated with Microcystis abundance and toxicity in San Francisco Estuary and implications for climate change impacts

The impacts of climate change on Microcystis blooms in San Francisco Estuary are uncertain because factors associated with the abundance and distribution of Microcystis blooms since their inception in 1999 are poorly understood. Discrete and continuous data collected between 2004 and 2008 were used to assess what factors controlled bloom initiation and persistence, if there was an impact of the bloom on mesozooplankton abundance and toxicity or dissolved organic carbon concentration, and how these might vary with climate change. Microcystis abundance was greater in dry years than wet years and both total microcystins concentration and the microcystins content of mesozooplankton tissue increased with abundance. The bloom began in the upstream portions of the estuary and spread farther west during dry years. Bloom initiation required water temperature above 19°C and surface irradiance in the visible range above 100 W m^?2. The bloom persisted during a wide range of water quality conditions but was closely correlated with low turbidity. The intensity of Microcystis blooms will likely increase with climate change due to increased water temperature and low streamflow during droughts. Elevated water temperature earlier in the spring could also extend the duration of Microcystis blooms by up to 3 months.     

Temporal evolution and vertical stratification of Microcystis toxic potential during a first bloom event

In order to study the setup of a Microcystis bloom and the evolution of its toxic potential, we studied the temporal and vertical variations in Microcystis aeruginosa abundance, microcystins (MC) concentrations (intracellular and extracellular), and the relative proportion of potentially microcystin-producing cells (MC-producing cells) in relation to physicochemical parameters in the recently setup Moroccan reservoir “Yaacoub Al Mansour.” The Microcystis bloom appeared relatively late in the season and was associated with a low proportion of MC-producing cells in the water surface layer, probably related to non-limiting nutrient concentrations. Interestingly, the setup of the bloom leads to a vertical gradient, showing a decrease in Microcystis cell abundance inversely coupled with an increase in the proportion of MC-producing cells. Thus, this can be the result of the growth where non-MC-producing cells remain in the lighted water layer easier than MC-producing ones. Nevertheless, parameters other than light intensity may influence the toxic potential of bloom as no vertical pattern was observed concerning microcystins cellular quotas. The high microcystins concentrations measured in the deep water layer have also proved the importance of considering the deep part of aquatic ecosystem in the management of health risks associated with cyanobacterial proliferations.     

Copepod grazing during an iron-induced diatom bloom in the Antarctic Circumpolar Current (EisenEx): I. Feeding patterns and grazing impact on prey populations

Feeding activity, selective grazing and the potential grazing impact of two dominant grazers of the Polar Frontal Zone, Calanus simillimus and Rhincalanus gigas, and of copepods < 2 mm were investigated with incubation experiments in the course of an iron fertilized diatom bloom in November 2000. All grazers were already actively feeding in the low chlorophyll waters prior to the onset of the bloom. C. simillimus maintained constant clearance rates and fed predominantly on diatoms. R. gigas and the small copepods strongly increased clearance and ingestion of diatoms in response to their enhanced availability. All grazers preyed on microzooplankton, most steadily on ciliates, confirming the view that pure herbivory appears to be the exception rather than the rule in copepod feeding. The grazers exhibited differences in feeding behavior based on selectivity indices. C. simillimus and R. gigas showed prey switching from dinoflagellates to diatoms in response to the phytoplankton bloom. All grazers most efficiently grazed on large diatoms leading to differences in daily losses for large and small species, e.g. Corethron sp. or Thalassionema nitzschioides. Species-specific diatom mortality rates due to grazing suggest that the high feeding activity of C. simillimus prior to and during the bloom played a role in shaping diatom population dynamics.     

The occurrence of a Euglena pascheri and Lepocinclis ovum bloom in an oxbow lake in southern Poland under extreme environmental conditions

Euglena pascheri is not an abundant species in water habitats, with no previous reports of bloom formation for this species. In 2005, we observed a spring bloom of E. pascheri with Lepocinclis ovum as a subdominant species in a shallow oxbow lake located within the Niepo?omice Forest of southern Poland. Rapid increases in the populations of both species were accompanied with low pH values, oxygen deficits, and high concentrations of nitrogen compounds, chlorides, Fe, Mn, Zn, and Cd, but with low concentrations of easily assimilated orthophosphates. Water properties within the habitat were extremely different during and after the bloom. E. pascheri and L. ovum have not previously been considered as index species of any specific water condition or environmental requirements. However, according to our observations, they are organisms tolerant to water acidification, increased salinity, and the presence of heavy metals.     

Bacterial community dynamics during a bloom caused by Akashiwo sanguinea in the Xiamen sea area,China

Phytoplankton blooms are a worldwide ecological problem and one of the major algae that cause phytoplankton blooms is Akashiwo sanguinea. Though much research has addressed the abiotic causes (e.g. growth condition) of A. sanguinea blooms, few studies have examined the dynamics of microbial communities associated with these blooms. In this study, polymerase chain reaction (PCR)-based denaturing gradient gel electrophoresis (DGGE) analysis of 16S rDNA genes was used to document changes in the phylogenetic diversity of microbial communities associated with an A. sanguinea bloom that occurred in the Xiamen sea in May 2010. Surface sea water was sampled once a day within five consecutive days at four sites, and the microbial community composition was determined using DGGE. Sea water concentrations of chlorophyll a, nitrate and phosphate were also measured. The results indicated that the A. sanguinea bloom was probably stimulated by low salinity (26–30‰) and ended probably because inorganic nutrients were consumed and resulted in a N/P ratio unfavorable for this alga. Gammaproteobacteria populations increased significantly during bloom declines and then decreased post-bloom. Divergences in the microbial community composition during different bloom periods were the result of changes in Candidatus, Pelagibacter, Alteromonas, Rhodobacteraceae, Vibrio and Pseudoalteromonas populations. Sediminimonas qiaohouensis was the first bacterium shown to be significantly negatively correlated with A. sanguinea concentration. This study indicated that bacteria may play an important role in A. sanguinea–bloom regulation and provides a deeper insight into bacterial community succession during and after an A. sanguinea–bloom.     

Devastating farmed abalone mortalities attributed to yessotoxin-producing dinoflagellates

A large dinoflagellate bloom in Walker Bay (South Africa) in January 2017 impacted 3 land-based abalone farms resulting in the death of several million animals. Satellite-derived images of Chl-a from the Ocean and Land Colour Imager (OLCI) on board the European Space Agency Sentinel-3 A showed bloom initiation in late December 2016 and dispersal in mid-February 2017. The bloom was dominated by two dinoflagellate species identified by light microscopy as Gonyaulax spinifera (Claparède & Lachmann) Diesing, 1866 and Lingulodinium polyedrum (Stein) Dodge, 1989. These morphologically based identifications were confirmed by phylogenetic analysis using partial sequences of the large subunit rDNA of both dinoflagellates. The appearance of yessotoxins (YTX) in abalone clearly coincided with increases in dinoflagellate concentrations. Yessotoxins in both the plankton and abalone were dominated by the two analogues homo-YTX and 45-hydroxy-YTX. The absence of toxins in a clonal culture of L. polyedrum implicated G. spinifera as the likely source of YTX. Toxin concentrations were found to be highest in the gills which showed the most significant pathology, including severe, generalized disruption of the gill epithelium characterized by degeneration and necrosis of epithelial cells accompanied by a modest inflammatory response. Some farms undertook pre-emptive or emergency harvesting to reduce financial losses.     

Biodiversity and dynamics of cyanobacterial communities during blooms in temperate lake (Harsha Lake,Ohio, USA)

Cyanobacterial blooms are intensifying global ecological hazards. The fine structure and dynamics of bloom community are critical to understanding bloom development but little understood. Here, the questions whether dominant bloomers have high diversity and whether dominant OTUs (operational taxonomical units) compete with one another were addressed. 16S rRNA gene amplicons from an annual bloom at five locations in Harsha Lake (Ohio, USA) showed cyanobacteria were the dominant phylum, and co-existing major bacterial phyla included Proteobacteria, Bacteroidetes, Actinoacteria, and Verrucomicrobia. On the genus level, the initial dominance by Dolichospermum in June yielded to Planktothrix in July, which were replaced by Microcystis and Cylindrospermopsis in August throughout the bloom. Based on the number of verified unique OTUs (a within-genus biodiversity metric), dominant genera tended to have high within-genus diversity. For example, Dolichospermum had 57 unique OTUs, Planktothrix had 36, Microcystis had 12, and Cylindrospermopsis had 4 unique OTUs. Interestingly, these different OTUs showed different dynamics and association with other OTUs. First, no between-OTU competitions were observed during the bloom cycle, and dominant OTUs were abundant throughout the bloom. Such biodiversity of OTUs and their dynamics were verified in Microcystis aeruginosa with two microcystin synthetase genes (mcyA and mcyG): the relative abundance of both genes varied during the bloom based on quantitative PCR. Two Dolichospermum circinale OTUs and one P. rubescens OTU were most abundant and persistently present throughout the entire bloom. Second, these OTUs differed in the OTUs they were associated with. Third, these OTUs tended to have different levels of association with the environmental factors, even they belonged to the same genera. These findings suggest the structure and dynamics of a cyanobacterial bloom community is complex, with only few OTUs dominating the bloom. Thus, high-resolution molecular characterization will be necessary to understand bloom development.     

Three-dimensional structure of a Karenia brevis bloom: Observations from gliders,satellites, and field measurements

Autonomous underwater gliders with customized sensors were deployed in October 2011 on the central West Florida Shelf to measure a Karenia brevis bloom, which was captured in satellite imagery since late September 2011. Combined with in situ taxonomy data, satellite measurements, and numerical circulation models, the glider measurements provided information on the three-dimensional structure of the bloom. Temperature, salinity, fluorescence of colored dissolved organic matter (CDOM) and chlorophyll-a, particulate backscattering coefficient, and K. brevis-specific chlorophyll-a concentrations were measured by the gliders over >250 km from the surface to about 30-m water depth on the shallow shelf. At the time of sampling the bloom was characterized by uniform vertical structures, with relatively high chlorophyll-a and CDOM fluorescence, low temperature, and high salinity. Satellite data extracted along the glider tracks demonstrated coherent spatial variations as observed by the gliders. Further, the synoptic satellite observations revealed the bloom evolution during the 7 months between late September 2011 and mid April 2012, and showed the maximum bloom size of ∼3000 km² around 23 November. The combined satellite and in situ data also confirmed that the ratio of satellite-derived fluorescence line height (FLH) to particulate backscattering coefficient at 547 nm (b_bp(547)) could be used as a better index than FLH alone to detect K. brevis blooms. Numerical circulation models further suggested that the bloom could have been initiated offshore and advected onshore via the bottom Ekman layer. The case study here demonstrates the unique value of an integrated coastal ocean observing system in studying harmful algal blooms (HABs).     

Controls on the initiation and development of blooms of the dinoflagellate Cochlodinium polykrikoides Margalef in lower Chesapeake Bay and its tributaries

Massive blooms of the dinoflagellate Cochlodinium polykrikoides occur annually in the Chesapeake Bay and its tributaries. The initiation of blooms and their physical transport has been documented and the location of bloom initiation was identified during the 2007 and 2008 blooms. In the present study we combined daily sampling of nutrient concentrations and phytoplankton abundance at a fixed station to determine physical and chemical controls on bloom formation and enhanced underway water quality monitoring (DATAFLOW) during periods when blooms are known to occur. While C. polykrikoides did not reach bloom concentrations until late June during 2009, vegetative cells were present at low concentrations in the Elizabeth River (4 cells ml⁻¹) as early as May 27. Subsequent samples collected from the Lafayette River documented the increase in C. polykrikoides abundance in the upper branches of the Lafayette River from mid-June to early July, when discolored waters were first observed. The 2009 C. polykrikoides bloom began in the Lafayette River when water temperatures were consistently above 25 °C and during a period of calm winds, neap tides, high positive tidal residuals, low nutrient concentrations, and a low dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorous (DIP) ratio. The pulsing of nutrients associated with intense but highly localized storm activity during the summer months when water temperatures are above 25 °C may play a role in the initiation of C. polykrikoides blooms. The upper Lafayette River appears to be an important area for initiation of algal blooms that then spread to other connected waterways.     

A large and prolonged bloom of Karenia mikimotoi in Scottish waters in 2006

A prolonged bloom of Karenia mikimotoi was observed during 2006 in Scottish waters. This bloom is thought to be unique in the region in terms of its large spatial extent. From its first detection in the west of the country, the bloom moved clockwise around the coast eventually reaching the east coast and the Shetland Isles to the north. The bloom resulted in extensive mortalities of benthic organisms including annelids and molluscs and some species of fish. Farmed fish mortalities were absent but gill damage was reported. The availability of satellite remote sensing, phytoplankton counts from multiple sites, meteorological data and some water chemistry, as well as information on the physical characteristics of the sampling sites, provided an extensive spatial and temporal data set. Analysis of remotely sensed chlorophyll-a data from Aqua-MODIS indicated that this parameter is a useful early warning indicator of K. mikimotoi in shelf waters off the Scottish west coast, and suggested that the bloom developed in this region prior to its advection to coastal waters. An earth observation (EO) based harmful bloom classifier for K. mikimotoi recognised areas of elevated K. mikimotoi cell density but generated false positives in areas of high reflectance. Data were also used to evaluate, in Scottish waters, various hypotheses that exist to explain the formation of K. mikimotoi blooms including phototaxis, nutrient availability, cell transport and elevated water temperature. Specifically, we sought to evaluate if routinely collected environmental data (water temperature, insolation, wind strength and direction, and sea-loch aspect) could be used as a predictor of bloom magnitude near aquaculture facility locations, which typically lie within fjordic sea lochs. Path analysis was used to derive intuitive models linking environmental drivers to bloom magnitude. Once the effects of latitude such as northward water cooling were taken into account, only rainfall was a significant predictor of bloom magnitude at the sampling sites. Therefore, while the offshore development and progression of a bloom may be predicted from satellite information, it is likely that local hydrodynamic influences are crucial in determining its magnitude at coastal aquaculture sites.     

Life-history stages of natural bloom populations and the bloom dynamics of a tropical Asian ribotype of Alexandrium minutum

In 2015, a remarkably high density bloom of Alexandrium minutum occurred in Sungai Geting, a semi-enclosed lagoon situated in the northeast of Peninsular Malaysia, causing severe discoloration and contaminated the benthic clams (Polymesoda). Plankton and water samples were collected to investigate the mechanisms of bloom development of this toxic species. Analysis of bloom samples using flow cytometry indicated that the bloom was initiated by the process of active excystment, as planomycetes (>4C cells) were observed in the early stage of the bloom. Increase in planozygotes (2C cells) was evident during the middle stage of the bloom, coinciding with an abrupt decrease in salinity and increase of temperature. The bloom was sustained through the combination of binary division of vegetative cells, division of planozygotes, and cyst germination through continuous excystment. Nutrient depletion followed by precipitation subsequently caused the bloom to terminate. This study provides the first continuous record of in situ life-cycle stages of a natural bloom population of A. minutum through a complete bloom cycle. The event has provided a fundamental understanding of the pelagic life-cycle stages of this tropical dinoflagellate, and demonstrated a unique bloom development characteristic shared among toxic Alexandrium species in coastal embayments.     

Environmental drivers of paralytic shellfish toxin producing Alexandrium catenella blooms in a fjord system of northern Southeast Alaska

Paralytic shellfish poisoning (PSP) is a persistent problem that threatens human health and the availability of shellfish resources in Alaska. Regular outbreaks of marine dinoflagellates in the genus Alexandrium produce paralytic shellfish toxins (PSTs) that make shellfish consumption unsafe, and impose economic hardships on Alaska’s shellfish industry. Phytoplankton and environmental monitoring spanning 2008–2016, and a pilot benthic cyst survey in 2016, were focused in the Juneau region of Southeast Alaska to investigate Alexandrium catenella distributions and conditions favorable to bloom development. Overwintering Alexandrium cysts were found in near-shore sediments throughout the study region. Alexandrium catenella cells were present in the water column across a range of sea surface temperatures (7–15 °C) and surface salinities (S = 4–30); however, an optimal temperature/salinity window (10–13 °C, 18–23) supported highest cell concentrations. Measurable levels of PSTs were associated with lower concentrations (100 cells L⁻¹) of A. catenella, indicating high cell densities may not be required for shellfish toxicity to occur. Several interacting local factors were identified to support A. catenella blooms: 1) sea surface temperatures ≥7 °C; 2) increasing air temperature; 3) low to moderate freshwater discharge; and 4) several consecutive days of dry and calm weather. In combination, these bloom favorable conditions coincide with toxic bloom events during May and June in northern Southeast Alaska. These findings highlight how integrated environmental and phytoplankton monitoring can be used to enhance early warning capacity of toxic bloom events, providing more informed guidance to shellfish harvesters and resource managers in Alaska.     

Physical transport processes affect the origins of harmful algal blooms in estuaries

The effects of physical transport processes on the initiation of harmful algal blooms (HABs) in estuaries were investigated through both mathematical model analysis and numerical model experiments. This study highlights the influence of the flushing effect due to physical transport processes on the location of bloom initiation, which is comparable to or even more important than local processes. The theoretical analysis suggests that the differential flushing effect at different waterbodies due to complex geometry is one of the dominant factors causing inhomogeneous distribution of algal density during HAB initiation. The ratio of residence time to volume is one of the key variables that determine the differential timing of HAB occurrence in estuary-subestuary systems with multiple interconnected waterbodies. As a result, a HAB tends to be observed first in those locations with relatively long residence time and small waterbodies, such as tributaries or areas with large eddies. Multiple unconnected originating locations can co-exist within an estuary.Two three-dimensional model experiments with realistic forcings were conducted to demonstrate the flushing effect on annual Cochlodinium polykrikoides bloom in the lower James River. The results show that while the environmental conditions that affect local processes, such as salinity and temperature, are important in determining the originating locations of HABs, the differential flushing effect is the dominant factor driving the spatial difference in the density of C. polykrikoides in this region during the bloom initiation. This explains why the occurrence of the first bloom in this region is frequently observed in the Lafayette River, a relatively small waterbody with long residence time. Because of the relatively low growth rate of C. polykrikoides and because of the high water-exchange between the mainstem and tributaries of the James River, initial cyst distribution is suggested to have a relatively small impact on originating locations of the bloom compared to flushing effect and salinity, and the HAB originating locations do not have to be in the waterbody with abundant cysts.     

Seasonal variation of nutrients,organic carbon,ATP, and microbial standing crops in a vertical profile of Pyramid Lake,Nevada

Previous studies of Pyramid Lake, Nevada, led to the hypothesis that detritus could be an important food source for zooplankton because abundance of palatable algal species did not seem to be enough to support the zooplankton community throughout the year. Furthermore, a large portion of the annual primary productivity was attributed to a nonpalatable blue-green alga, Nodularia spumigena. We felt this alga became important to the Pyramid Lake aquatic community upon death, as edible detritus and a source of new nitrogen. Changes in pelagic detritus concentrations and microbial standing crops were monitored to determine the availability of these potential foods. Epilimnetic particulate organic carbon (POC) was primarily living phytoplankton. During holomixis and following spring primary production, hypolimnetic POC was 60–97% detrital, but these profundal POC concentrations were low (ca 650 µg l^-1). Detritus-bacteria aggregates were observed only following the September cyanophyte bloom. Although pelagic detritus availability for zooplankton was low, bacterial populations were sufficient to be at least a supplemental food source. Bacteria numbers ranged from 0.50 10⁶ to 24.7 10⁶ ml^-1 and increased in response to photosynthetic peaks. Microbial diversity, contribution to POC, and particle association were notable after July. The percentage of living carbon (assessed with ATP measurements) attributable to bacteria was highest in late summer and fall hypolimnetic samples. Patterns of change in organic phosphorus and nitrogen, the presence of a nitrogen-fixing cyanophyte, the N:P ratio, and results of other research demonstrated that non-nitrogen-fixing algae of Pyramid Lake are limited by inorganic nitrogen. The importance of N. spumigena to the aquatic community appeared to be as a source of new nitrogen, rather than as a forage; its mineralization is critical for the growth of palatable diatoms and green algae following winter mixing.