Distinct Phytoplankton Blooms Research Articles

In situ cell division and mortality rates of SAR11, SAR86, Bacteroidetes, and Aurantivirga during phytoplankton blooms reveal differences in population controls.

Net growth of microbial populations, that is, changes in abundances over time, can be studied using 16S rRNA fluorescence in situ hybridization (FISH). However, this approach does not differentiate between mortality and cell division rates. We used FISH-based image cytometry in combination with dilution culture experiments to study net growth, cell division, and mortality rates of four bacterial taxa over two distinct phytoplankton blooms: the oligotrophs SAR11 and SAR86, and the copiotrophic phylum Bacteroidetes, and its genus Aurantivirga. Cell volumes, ribosome content, and frequency of dividing cells (FDC) co-varied over time. Among the three, FDC was the most suitable predictor to calculate cell division rates for the selected taxa. The FDC-derived cell division rates for SAR86 of up to 0.8/day and Aurantivirga of up to 1.9/day differed, as expected for oligotrophs and copiotrophs. Surprisingly, SAR11 also reached high cell division rates of up to 1.9/day, even before the onset of phytoplankton blooms. For all four taxonomic groups, the abundance-derived net growth (-0.6 to 0.5/day) was about an order of magnitude lower than the cell division rates. Consequently, mortality rates were comparably high to cell division rates, indicating that about 90% of bacterial production is recycled without apparent time lag within 1 day. Our study shows that determining taxon-specific cell division rates complements omics-based tools and provides unprecedented clues on individual bacterial growth strategies including bottom-up and top-down controls. IMPORTANCE The growth of a microbial population is often calculated from their numerical abundance over time. However, this does not take cell division and mortality rates into account, which are important for deriving ecological processes like bottom-up and top-down control. In this study, we determined growth by numerical abundance and calibrated microscopy-based methods to determine the frequency of dividing cells and subsequently calculate taxon-specific cell division rates in situ. The cell division and mortality rates of two oligotrophic (SAR11 and SAR86) and two copiotrophic (Bacteroidetes and Aurantivirga) taxa during two spring phytoplankton blooms showed a tight coupling for all four taxa throughout the blooms without any temporal offset. Unexpectedly, SAR11 showed high cell division rates days before the bloom while cell abundances remained constant, which is indicative of strong top-down control. Microscopy remains the method of choice to understand ecological processes like top-down and bottom-up control on a cellular level.

Rare earth element behaviour in seawater under the influence of organic matter cycling during a phytoplankton spring bloom – A mesocosm study

Rare earth elements (REEs) are used as powerful proxies for a variety of oceanic processes. The understanding of their biogeochemical behaviour in the marine environment is therefore essential. While the influence of OM-cycling on REE patterns in seawater is considered as insignificant, it has been shown that algae and bacteria provide good sorption surfaces for REEs and that components of the dissolved OM pool are able to complex REEs, thus potentially altering their behaviour. To investigate the impact of bio-associated processes on REEs in the bio-productive marine environment, we conducted an indoor mesocosm experiment that mimicked a phytoplankton spring bloom in the neritic coastal North Sea. The incubation period of 38 days covered two distinct phytoplankton bloom phases (diatoms followed by Phaeocystis sp.) and an interjacent bacterioplankton maximum. All dissolved REEs (dREEs) except samarium showed similar temporal concentration patterns, which were closely connected to the bloom succession. The concentration patterns were shaped by the ‘phytoplankton-shuttle’, which summarizes adsorption processes on phytoplankton-derived particulate OM (POM) and resulted in decreasing dREE concentrations alongside chlorophyll-a and POM maxima. The ‘heterotrophic-shuttle’ resulted in increasing dREE concentrations likely linked to heterotrophically mediated regeneration of POM and associated desorption processes. The effect of these processes on dREEs resulted in enhanced fractionation of light REEs (LREEs) relative to heavy REEs (HREEs) during adsorption processes and decreased fractionation as a result of desorption. At times of high dissolved organic carbon (DOC) concentrations, we observed a stabilization of especially dHREEs likely in organic complexes. To test the potential influence of DOC on dREEs, we used a PHREEQC model approach that revealed dREE complexation with components of the DOC pool and an increase in complexation with atomic mass of the REEs. That is, at high DOC concentrations OM-dREE complexation leads to an effective and preferential buffering of dHREE against adsorption. Our findings reveal that OM-cycling influences concentration patterns of dREEs via ad- and desorption processes as well as organic complexation with parts of the OM pool, suggesting these processes can have a significant impact on dREE concentrations in the natural marine environment under high OM conditions.

Metaproteomics reveals unique metabolic niches of dominant bacterial groups in response to rapid regime shifts during a mixed dinoflagellate bloom

The dynamics of bacterial composition and metabolic activity during a distinct phytoplankton bloom have been reported. However, there is limited information on the bacterial community response to drastic environmental changes caused by species succession during a mixed-species bloom. This study investigated active bacterial groups and metabolic activity during a mixed bloom formed by dinoflagellates Prorocentrum obtusidens and Karenia mikimotoi using a metaproteomic approach. Bacterial community structure and dominant bacterial groups varied rapidly with the bloom regime shifts caused by species succession. Pseudoalteromonas and Vibrio dominated the bacterial community in the P. obtusidens-dominated regime, while Alteromonas, Cytophaga-Flavobacteria-Bacteroides (CFB) group, and marine Roseobacter clade (MRC) were the major contributors in other regimes, with the most abundant taxa being Alteromonas in the K. mikimotoi-dominated regime and the CFB group in the dissipation regime. Specific metabolic niches and unique substrate specificity of different bacterial groups enabled them to dominate and thrive in different bloom regimes. High metabolic plasticity in signal response, substrate utilization, motility, and adhesion are essential for bacteria to respond to drastic bloom regime shift, and the predominance of specific bacteria under unique bloom regimes may be the result of long-term coevolution between bacteria and bloom-forming phytoplankton species.

Biogeochemical thallium cycling during a mesocosm phytoplankton spring bloom: Biotic versus abiotic drivers

Thallium (Tl) is classified as a non-(bio)-essential and highly toxic element in the marine environment. Despite its active and passive involvement in bio-cycling processes, it is considered a conservative type element in open ocean settings. Previous studies on the Tl-behavior in the coastal waters of the southern North Sea, however, documented non-conservative Tl-behavior in seasonal and tidal patterns. As drivers for the non-conservative depletion, Tl-fixation in redox stratified adjacent sediments as well as its complexation with algae-bloom derived organic matter were suggested. Due to superimposition by resuspended lithogenic particles, it was not possible to distinguish whether the Tl concentration pattern was induced by biotic or abiotic processes. The main motivation of the present study was to investigate the non-conservative Tl-behavior during a phytoplankton bloom in coastal ocean water masses and to identify potential key drivers. We conducted an indoor mesocosm experiment where artificial seawater was inoculated with a natural phytoplankton and bacteria community from the southern North Sea and incubated under natural light and temperature conditions, mimicking a neritic North Sea water column. The incubation of six weeks covered the different stages of two distinct phytoplankton bloom events as well as a subsequent bacteria bloom. Our results reveal a non-conservative Tl-depletion, which seemed to be primarily caused by the coupling to algae bloom derived OM-cycling. The extent of Tl-depletion was dependent on the amount and the composition of organic matter. While the first phytoplankton bloom, dominated by Diatom-species, did not induce significant deviations of Tl from theoretical conservative behavior, especially the colonial stage (hydrogel formation) of the secondary occurring Phaeocystis sp. bloom induced significant depletions of dissolved Tl with rates up to ~27% d-1. Global extrapolations of potential algae-induced deficits in dissolved Tl and its potential export Tl from the open water column have shown that the processes identified for Tl removal in this study could be responsible for a flux in the range of 4–20% of the total removal previously assumed in Tl mass balances. Our study emphasizes that although Tl is classified as a conservative-type element, biological processes have an impact on the global Tl budget and thus must be considered in the respective oceanographic models.

Submesoscale physicochemical dynamics directly shape bacterioplankton community structure in space and time

AbstractSubmesoscale eddies and fronts are important components of oceanic mixing and energy fluxes. These phenomena occur in the surface ocean for a period of several days, on scales between a few hundred meters and few tens of kilometers. Remote sensing and modeling suggest that eddies and fronts may influence marine ecosystem dynamics, but their limited temporal and spatial scales make them challenging for observation and in situ sampling. Here, the study of a submesoscale filament in summerly Arctic waters (depth 0–400 m) revealed enhanced mixing of Polar and Atlantic water masses, resulting in a ca. 4 km wide and ca. 50 km long filament with distinct physical and biogeochemical characteristics. Compared to the surrounding waters, the filament was characterized by a distinct phytoplankton bloom, associated with depleted inorganic nutrients, elevated chlorophyll a concentrations, as well as twofold higher phyto‐ and bacterioplankton cell abundances. High‐throughput 16S rRNA gene sequencing of bacterioplankton communities revealed enrichment of typical phytoplankton bloom‐associated taxonomic groups (e.g., Flavobacteriales) inside the filament. Furthermore, linked to the strong water subduction, the vertical export of organic matter to 400 m depth inside the filament was twofold higher compared to the surrounding waters. Altogether, our results show that physical submesoscale mixing can shape distinct biogeochemical conditions and microbial communities within a few kilometers of the ocean. Hence, the role of submesoscale features in polar waters for surface ocean biodiversity and biogeochemical processes need further investigation, especially with regard to the fate of sea ice in the warming Arctic Ocean.

Phytoplankton Blooms Below the Antarctic Landfast Ice During the Melt Season Between Late Spring and Early Summer.

Antarctic regions are known to be mainly dominated by diatoms in the water column under sea ice. In this study, we report for the first time two distinct phytoplankton blooms dominated by nanoflagellates (<15µm) under the landfast ice in Terra Nova Bay during the late spring-early summer 2015/2016. The taxa included the pelagic Bolidophyceae Pentalamina corona, the Chrysophyceae Ochromonas spp. and the Chlorophyceae Chlamydomonas spp., typically found in fresh waters, and the Prymnesiophyceae Phaeocystis antarctica usually observed dominating in polynya areas. These species represented from 40% to 91% of the total phytoplankton community, a percentage contrasting with the prevalence of diatoms found previously. The dominance of nanoflagellates, rather than diatoms, during late spring and early summer may have important implications for trophic relationships in Antarctic waters and the presence of typical freshwater species could indicate a great input of continental waters related to environmental changes.

Biogeochemical Response of Apalachicola Bay and the Shelf Waters to Hurricane Michael Using Ocean Color Semi-Analytic/Inversion and Hydrodynamic Models

Hurricanes are increasingly being recognized as important episodic drivers in ocean biogeochemical cycling; however, spatiotemporal response of their impacts on coastal and estuarine ecosystems are limited. Hurricane Michael, which made landfall just west of Apalachicola Bay (ApB) on October 10, 2018 as a Category 5 hurricane with sustained winds of 250 km h-1, caused widespread damage to the northwest Florida coast, and adverse effects on oyster reefs and water quality in ApB due to winds and coastal flooding associated with a strong storm surge. The impact of wind forcing and retreating storm surges on coastal and shelf biogeochemical properties remains however, largely unknown. In this study, we use a combination of pre-hurricane field observations, ocean-color satellite imagery and the outputs (salinity, currents, sea surface height and temperature) of a nested high-resolution 3-dimensional hydrodynamic model (NCOM) to examine the biogeochemical response of ApB and the surrounding shelf waters to Hurricane Michael. MODIS-derived optical proxies (e.g., absorption of colored dissolved organic matter or CDOM and particle backscattering coefficients) of dissolved and particulate organic carbon (DOC and POC) were derived for a series of clear-sky imagery (prior to and following the hurricane) using a combination of estuarine-tuned semi-analytic and empirical algorithms. Following the hurricane, spatiotemporal distribution of both DOC and POC in ApB and the nearshore coastal waters showed a strong response to storm surge, increasing river discharge, currents, and wind field. Average flux estimates of organic carbon exported from ApB between October 5-21, 2018 to the coastal ocean were much greater for DOC (0.86106 kg C d-1) than POC (0.21106 kg C d-1) and increased with increasing river discharge and the wind field. A bio-optical inversion algorithm applied to Sentinel-3A OLCI imagery of 13 October, 2018 immediately following the hurricane’s passage, showed a strong, week-long biological response with spatially distinct phytoplankton blooms of Karenia brevis and Emiliania Huxleyi, as detected by satellite imagery of pigments, an approach that could revolutionize our understanding of environmental impacts on phytoplankton. This study revealed spatiotemporal changes in estuarine and coastal ocean biogeochemistry reflective of a systematic regional ecosystem response to Hurricane Michael.

Gradient flux measurements of sea–air DMS transfer during the Surface Ocean Aerosol Production (SOAP) experiment

Abstract. Direct measurements of marine dimethylsulfide (DMS) fluxes are sparse, particularly in the Southern Ocean. The Surface Ocean Aerosol Production (SOAP) voyage in February–March 2012 examined the distribution and flux of DMS in a biologically active frontal system in the southwest Pacific Ocean. Three distinct phytoplankton blooms were studied with oceanic DMS concentrations as high as 25 nmol L−1. Measurements of DMS fluxes were made using two independent methods: the eddy covariance (EC) technique using atmospheric pressure chemical ionization–mass spectrometry (API-CIMS) and the gradient flux (GF) technique from an autonomous catamaran platform. Catamaran flux measurements are relatively unaffected by airflow distortion and are made close to the water surface, where gas gradients are largest. Flux measurements were complemented by near-surface hydrographic measurements to elucidate physical factors influencing DMS emission. Individual DMS fluxes derived by EC showed significant scatter and, at times, consistent departures from the Coupled Ocean–Atmosphere Response Experiment gas transfer algorithm (COAREG). A direct comparison between the two flux methods was carried out to separate instrumental effects from environmental effects and showed good agreement with a regression slope of 0.96 (r2= 0.89). A period of abnormal downward atmospheric heat flux enhanced near-surface ocean stratification and reduced turbulent exchange, during which GF and EC transfer velocities showed good agreement but modelled COAREG values were significantly higher. The transfer velocity derived from near-surface ocean turbulence measurements on a spar buoy compared well with the COAREG model in general but showed less variation. This first direct comparison between EC and GF fluxes of DMS provides confidence in compilation of flux estimates from both techniques, as well as in the stable periods when the observations are not well predicted by the COAREG model.

Phytoplankton bloom triggered by an anticyclonic eddy: The combined effect of eddy‐Ekman pumping and winter mixing

AbstractRecent composite based researches reveal that anticyclonic eddies facilitate the growth of phytoplankton in the subtropical gyres. Two dynamical mechanisms, eddy‐Ekman pumping and winter mixing, have been examined individually, but their relative and combined effects remain unclear. Using satellite observations and model simulations, this study investigated the process of a distinct phytoplankton bloom generated in an anticyclonic eddy in the nutrient‐depleted southeastern Indian Ocean. The bloom propagated westward along with the eddy for more than 600 km from late April to August in 2010. The peak of surface chlorophyll concentration in the eddy is 2.2 times larger than the mean value of the ambient. The development of the bloom is dominated by the winter deepening of mixed layer, whose velocity in vertical nutrient flux is on average 3 times larger than that of eddy‐Ekman pumping. The results of a 1‐D physical‐biogeochemical model demonstrate that the role of eddy‐Ekman pumping is also indispensable, because it not only transports extra nutrients into the mixed layer, but also results in significant chlorophyll enrichment in subsurface water. The superposition of eddy‐Ekman pumping on winter mixing triples the chlorophyll both at the surface and in the upper layer, and the entrainment of subsurface phytoplankton into the mixed layer contributes significantly to the surface bloom, especially in its initial stage. Both the satellite observations and model simulation show that eddy‐Ekman pumping can lead to an early occurrence of the bloom for more than 2 weeks.

Dimethylsulfide gas transfer coefficients from algal blooms in the Southern Ocean

Abstract. Air–sea dimethylsulfide (DMS) fluxes and bulk air–sea gradients were measured over the Southern Ocean in February–March 2012 during the Surface Ocean Aerosol Production (SOAP) study. The cruise encountered three distinct phytoplankton bloom regions, consisting of two blooms with moderate DMS levels, and a high biomass, dinoflagellate-dominated bloom with high seawater DMS levels (&gt; 15 nM). Gas transfer coefficients were considerably scattered at wind speeds above 5 m s−1. Bin averaging the data resulted in a linear relationship between wind speed and mean gas transfer velocity consistent with that previously observed. However, the wind-speed-binned gas transfer data distribution at all wind speeds is positively skewed. The flux and seawater DMS distributions were also positively skewed, which suggests that eddy covariance-derived gas transfer velocities are consistently influenced by additional, log-normal noise. A flux footprint analysis was conducted during a transect into the prevailing wind and through elevated DMS levels in the dinoflagellate bloom. Accounting for the temporal/spatial separation between flux and seawater concentration significantly reduces the scatter in computed transfer velocity. The SOAP gas transfer velocity data show no obvious modification of the gas transfer–wind speed relationship by biological activity or waves. This study highlights the challenges associated with eddy covariance gas transfer measurements in biologically active and heterogeneous bloom environments.

Episodic vertical nutrient fluxes and nearshore phytoplankton blooms in Southern California

Three distinct phytoplankton blooms lasting 4–9 d were observed in approximately 15‐m water depth near Huntington Beach, California, between June and October of 2006. Each bloom was preceded by a vertical NO3 flux event 6–10 d earlier. NO3 concentrations were estimated using a temperature proxy that was verified by comparison with the limited NO3 observations. The lower–water‐column vertical NO3 flux from vertical advection was inferred from observed vertical isotherm displacement. Turbulent vertical eddy diffusivity was parameterized based on the observed background (&lt; 0.3 cycles h−1) stratification and vertical shear in the horizontal currents. The first vertical nitrate flux event in June contained both advective and turbulent fluxes, whereas the later two events were primarily turbulent, driven by shear in the lower part of the water column. The correlation between the NO3 flux and the observed chlorophyll a (Chl a) was maximum (r2 = 0.40) with an 8‐d lag. A simple nitrate–phytoplankton model using a linear uptake function and driven with the NO3 flux captured the timing, magnitude, and duration of the three Chl a blooms (skill = 0.61) using optimal net growth rate parameters that were within the expected range. Vertical and horizontal advection of Chl a past the measurement site were too small to explain the observed Chl a increases during the blooms. The vertical NO3 flux was a primary control on the growth events, and estimation of both the advective (upwelled) and turbulent fluxes is necessary to best predict these episodic blooms.

Mesocosm study of particle dynamics and control of particle-associated bacteria by flagellate grazing

The goal of this study was to attain an integrated understanding of the dynamics of attached microbial communities in nature as governed by the component processes of particle forma- tion, colonization, detachment, growth and interspecific interactions among the microbes. The study was conducted in mesocosms in a Danish fjord over a 2 wk period in April to May 2005. Despite nutrient additions, chlorophyll and particle concentrations were low and no distinct phytoplankton blooms were observed. Particle volume concentration was dominated by particles >100 µm. The abundance of attached bacteria was weakly but positively correlated with total particle volume. Phylogenetic composition of attached bacteria became increasingly different from that of free bacte- ria through time. Growth rates of free and attached bacteria were 0.31 and 0.023 d -1 , respectively. Colonization experiments with model agar aggregates showed that the bacterial community had lim- ited apparent diffusivity but strong chemotaxis, which resulted in an up to 8-fold increase in bacter- ial colonization rates with organic-enriched aggregates. Estimated residence time of attached bacte- ria on aggregates was 21 min or less. Grazing mortality derived from temporal changes in attached bacterial abundance in the presence or absence of grazing activities ranged from 0 to 9.8 d -1 . In com- parison, grazing mortality of free bacteria was 0.06 to 0.91 d -1 , suggesting that the bacterivorous fla- gellates were more adapted to grazing on surfaces than in free suspension. Very few attached flagel- lates were observed in the experiments, and microscopic observations revealed that many flagellates were attached only temporarily to surfaces. Our study showed that grazing mortality is a critical but often underappreciated factor in regulating marine snow bacterial population abundance. The loose association of bacterivorous flagellates with aggregates requires that their role be evaluated by direct grazing measurements instead of simple quantification of attached flagellate abundance.

Annual cycles of sea ice and phytoplankton in Cape Bathurst polynya, southeastern Beaufort Sea, Canadian Arctic

The relationship between the dynamics of sea ice and phytoplankton abundance were investigated for the Cape Bathurst polynya region of the Canadian Arctic using five years (1998–2002) of satellite data from SSM/I and SeaWiFS. The Cape Bathurst polynya exhibited marked interannual variability in sea ice dynamics, both in the timing of initial polynya formation and in the extent and persistence of open water. Related to this, although all years exhibited two distinct phytoplankton blooms, these also varied in their intensity and timing. Generally, the late bloom of each year was the most intense, after surface waters had stratified in summer. Blooms were most intense in spring 1998, following anomalous warming and early stratification, and late summer 2002, following a summer ice melt event. Changes in the timing of phytoplankton bloom development in polar waters can impact food web structure and the relative importance of top‐down versus bottom‐up control of marine ecosystems.

Dynamics in bacterioplankton production in a shallow, temperate lake (Lake Neusiedl, Austria): evidence for dependence on macrophyte production rather than on phytoplankton

The seasonal dynamics in bacterioplankton abundance and production were studied at 3 characteristic stations (open water, large pond within the reed belt and within the reed Phragmites australis) in the shallow Lake Neusiedl, Austria, and related to phytoplankton primary production and dissolved organic carbon (DOC). DOC concentrations ranged from about 1 to 2.5 mm01 1-' with humic DOC contributing between 4 0 % during the winter and 55% during the summer. Phytoplankton production was highest in the pond withln the reed belt, where the attenuation was lowest, reaching 110 mg C m-' d-' during a distinct phytoplankton bloom in August. Bacterial abundance ranged from 2 X 106 cells ml-' during winter to about 10 X 10' cells ml-l during summer. Bacterial production calculated by thymidine (TdR) and leucine (leu) incorporation, respectively, were in good agreement at the stations in the reed belt, but bacterial production based on leu incorporation was significantly lower than bacterial production based on TdR incorporation at the open water station. Based on a bacterial growth yield of 16% determined in an earlier study, bacterioplankton carbon demand was always at least 1 order of magnitude higher than carbon production of phytoplankton, indcating that bacterioplankton metabohsm in Lake Neusiedl is heavily dependent on non-phytoplankton sources of DOC. The bacterial carbon demand (ranging from 225 to 870 mg C m-2 d-' depending on the sampling site and substrate used) could be matched by the production of the reed P. australis amounting to 750 to 4510 mg C m-* d-'. Since there is no major allochthonous organic matter input from other sources, this macrophyte production is obviously channeled to the pelagic food web via the bacterioplankton.

Bio‐optical properties of the southwestern Ross Sea

The bio‐optical properties of the southwestern Ross Sea were measured as part of the Antarctic research program Research on Atmospheric Variability and Atmospheric Response in the Ross Sea (ROAVERRS). The study area contained three distinct phytoplankton blooms, distinguishable by species composition. The largest in area was located to the north of the Ross Ice Shelf and was dominated by the prymnesiophyte Phaeocystis antarctica; chlorophyll a (Chl a) ranged from 0.45 to 8.2 mg m−3. Beam attenuation and particle absorption at 435 nm were as high as 3.4 m−1 and 0.35 m−1, respectively. A bloom of diatoms was more spatially restricted, located to the north and west of the P. antarctica bloom, with Chl a generally below 4 mg m−3. Neither diatoms nor P. antarctica exhibited evidence of the level of pigment packaging measured in waters near the Antarctic Peninsula during the Research on Antarctic Coastal Ecosystem Rates (RACER) program, possibly because of their smaller sizes. A much smaller cryptophyte bloom, located south of the Drygalski Ice Tongue, displayed a lower pigment‐specific absorption spectra than did P. antarctica or diatoms, a sign of greater pigment packaging. Pigment‐specific diffuse attenuation coefficients were consistent with the pigment‐specific particle absorption coefficients (aph*), both being ∼3 times greater than similar measurements made during RACER. Spectral absorption by solutes determined through regression analysis of Kd against Chl a for the ROAVERRS data set was nearly identical to that measured during RACER. Total diffuse attenuation spectra at a given station could be reconstructed by summing the inherent optical properties of the major optical components (pure water, soluble material, detritus, phytoplankton) measured there. Differences in the absorption ratio of aph*(λ) at 490 nm to aph*(λ) at 555 nm among the three dominant phytoplankton taxa in the southwestern Ross Sea were responsible for most of the variability in the ratio of remote sensing reflectance (Rrs) at these same wavelengths. At a given concentration of Chl a, the ratio log [Rrs(490):Rrs(555)] was greatest in cryptophyte‐dominated waters, which also possessed the lowest aph*(490):aph*(555) ratio, and lowest in P. antarctica–dominated waters. These bio‐optical differences suggest that no simple empirical relationship between Chl a and log [Rrs(490):Rrs(555)] will apply to all three taxonomically distinct phytoplankton blooms in the southwestern Ross Sea.

Ecology and biogeochemistry of the antarctic circumpolar current during austral spring: a summary of southern ocean JGOFS cruise ANT X/6 of R.V. Polarstern

The R.V. Polarstern cruise ANT X/6, part of the international Southern Ocean JGOFS programme, investigated phytoplankton spring bloom development and its biogeochemical effects in different water masses of the Atlantic sector of the Southern Ocean: the Polar Frontal region (PFr), the southern Antarctic Circumpolar Current zone (sACC), its boundary with the Weddell Gyre (AWB) and the marginal ice zone (MIZ). The relative roles of physical stability, iron limitation and grazing pressure in enhancing or constraining phytoplankton biomass accumulation were examined. Three sections were carried out between the PFr and the ice edge along the 6°W meridian from early October to late November 1992. This paper summarises the major findings of the cruise and discusses their implications for our understanding of Southern Ocean ecology and biogeochemistry. A major finding was the negligible build-up of plankton biomass and concomitant absence of CO 2 drawdown associated with seasonal retreat of the ice cover. In striking contrast to this unexpected poverty of both the MIZ and the frontal region of the AWB, distinct phytoplankton blooms, dominated by different diatom species, accumulated in the PFr. Chlorophyll stocks in the sACC remained monotonously low throughout the study. Our findings confirm those of other studies that frontal regions are the major productive sites in the Southern Ocean and that input of meltwater and associated ice algae to the surface layer from a retreating ice edge is by itself an insufficient condition for induction of phytoplankton blooms. The blooms in the PFr developed under conditions of shallow mixing layers, high iron concentrations and relatively low grazing pressure. However, in all three blooms, high biomass extended to deeper than 70 m, which cannot be explained by either in situ growth or sinking out of a part of the population from the upper euphotic zone. Subduction of adjoining, shallower layers to explain depth distribution is invoked. Despite a clear CO 2 drawdown in the Polar Frontal region, the highly variable conditions encountered render reliable estimation of annual CO 2 fluxes in the Southern Ocean difficult.

Ecological Study of Spring-Early Summer Phytoplankton Blooms in A Semi-Enclosed Estuary

Abstract A sampling station in the Eastern Harbour of Alexandria was operated for 51 days between 2 March and 12 June, 1991. the harbour had different wide physical and chemical variations. Six distinct phytoplankton blooms occurred during this period. It is concluded that the physicochemical conditions affected the phytoplankton community structure and species composition. A dense diatom bloom could be maintained in a very weak and/or stabilized water column. A diatom bloom does not necessarily accompany an enhanced nutrient period. the depletion of nutrients and establishment of thermal stratification probably created favourable conditions for dinoflagellate and flagellate species to achieve blooms. the species composition could be shifted over the short term and a dense phytoplankton bloom could possibly dissipate in a few days.