Spring Phytoplankton Bloom Research Articles

Mesoplankton across the North Atlantic: Composition and environmental drivers at 59.5th latitude

North Atlantic zooplankton communities vary in terms of abundance, biomass and species composition depending on various environmental factors. In this study we analyzed the influence of hydrological factors (water temperature, salinity and density) on mesoplankton and retrieve boundaries between mesoplankton assemblages across the Subpolar North Atlantic. The material was collected in July 2013 with the use of Juday plankton net vertical hauls (0–300 m layer) at 55 stations arranged along a longitudinal transect along 59.5° N during the 41-th expedition of R/V “Akademik Ioffe”. Both total abundance and biomass were higher in the eastern part of the transect and greatly decreased in the western part except the Greenland shelf and slope where the values were high. Four mesoplankton assemblages were retrieved by cluster analysis: two of them (above the Rockall Plateau and the Island Basin) were associated with warm Atlantic waters, the other two assemblages (in the Irminger Basin and above the Greenland slope and shelf) inhabited cold Arctic waters. The major biogeographic boundary was linked to the Subarctic Front above the Reykjanes Ridge. The best correlations of zooplankton characteristics with the surface chlorophyll a amount were obtained when averaging the surface chlorophyll data over May and 3 latitudinal and longitudinal degrees around each station. This can be connected to the main spring phytoplankton bloom, occurring during May in this area. The exception was the western-most part of the transect above the Greenland shelf and slope that harbored separate mesoplankton assemblages with high abundance and biomass, dominated by early mesoplankton stages (e.g. young copepodites and nauplii). These assemblages were likely supported by summer subsurface phytoplankton bloom caused by a water discharge from the melting Greenland Ice Shield.

Feeding Characteristics of Deep-Sea Acorn Worm (Hemichordata, Enteropneusta, Torquaratoridae) from the Bering Sea.

Deep-sea hemichordates Torquaratoridae gen. sp. reach high abundance up to 12 spec. m-2 at the depths of 1830-2130 m on the slope of the Volcanologists Massif in the south-western part of the Bering Sea, dominating in the benthic community at these depths. Their abundance exceeds by two orders the values recorded earlier. In order to clarify this phenomenon, we examined the gut contents of Torquaratoridae gen. sp. The detritus particles and frustules of planktonic diatoms Thalassiosira, Coscinodiscus, Actinocyclus, Chaetoceros, Neodenticula, and Grammatophora were the most common in the gut, as well as the remains of skeletons of benthic invertebrates with little admixture of mineral particles. According to obtained data, Torquaratoridae gen. sp. are mobile deposit feeders with high selectivity to fresh phytodetritus, able to compete with holothurians occupying similar trophic niche. Unusually high abundance of acorn worms is apparently related to high organic matter flux to the seafloor as a result of spring phytoplankton bloom in the surface water layer of the Bering Sea.

Spring Accumulation Rates in North Atlantic Phytoplankton Communities Linked to Alterations in the Balance Between Division and Loss.

For nearly a century, phytoplankton spring blooms have largely been explained in the context of abiotic factors regulating cellular division rates (e.g., mixed-layer light levels). However, the accumulation of new phytoplankton biomass represents a mismatch between phytoplankton division and mortality rates. The balance between division and loss, therefore, has important implications for marine food webs and biogeochemical cycles. A large fraction of phytoplankton mortality is due to the combination of microzooplankton grazing and viral lysis, however, broad scale simultaneous measurements of these mortality processes are scarce. We applied the modified dilution assay along a West-to-East diagonal transect in the North Atlantic during spring. Our results demonstrate positive accumulation rates with losses dominated by microzooplankton grazing. Considering the dynamic light environment phytoplankton experience in the mixed surface layer, particularly in the spring, we tested the potential for incubation light conditions to affect observed rates. Incubations acted as short-term ‘light’ perturbations experiments, in which deeply mixed communities are exposed to elevated light levels. These “light perturbations” increased phytoplankton division rates and resulted in proportional changes in phytoplankton biomass while having no significant effect on mortality rates. These results provide experimental evidence for the Disturbance-Recovery Hypothesis, supporting the tenet that biomass accumulation rates co-vary with the specific rate of change in division.

North Sea spring bloom-associated Gammaproteobacteria fill diverse heterotrophic niches

BackgroundThe planktonic bacterial community associated with spring phytoplankton blooms in the North Sea is responsible for a large amount of carbon turnover in an environment characterised by high primary productivity. Individual clades belonging to the Gammaproteobacteria have shown similar population dynamics to Bacteroidetes species, and are thus assumed to fill competing ecological niches. Previous studies have generated large numbers of metagenome assembled genomes and metaproteomes from these environments, which can be readily mined to identify populations performing potentially important ecosystem functions. In this study we attempt to catalogue these spring bloom-associated Gammaproteobacteria, which have thus far attracted less attention than sympatric Alphaproteobacteria and Bacteroidetes.MethodsWe annotated 120 non-redundant species-representative gammaproteobacterial metagenome assembled genomes from spring bloom sampling campaigns covering the four years 2010–2012 and 2016 using a combination of Prokka and PfamScan, with further confirmation via BLAST against NCBI-NR. We also matched these gene annotations to 20 previously published metaproteomes covering those sampling periods plus the spring of 2009.ResultsMetagenome assembled genomes with clear capacity for polysaccharide degradation via dedicated clusters of carbohydrate active enzymes were among the most abundant during blooms. Many genomes lacked gene clusters with clearly identifiable predicted polysaccharide substrates, although abundantly expressed loci for the uptake of large molecules were identified in metaproteomes. While the larger biopolymers, which are the most abundant sources of reduced carbon following algal blooms, are likely the main energy source, some gammaproteobacterial clades were clearly specialised for smaller organic compounds. Their substrates range from amino acids, monosaccharides, and DMSP, to the less expected, such as terpenoids, and aromatics and biphenyls, as well as many ‘unknowns’. In particular we uncover a much greater breadth of apparent methylotrophic capability than heretofore identified, present in several order level clades without cultivated representatives.ConclusionsLarge numbers of metagenome assembled genomes are today publicly available, containing a wealth of readily accessible information. Here we identified a variety of predicted metabolisms of interest, which include diverse potential heterotrophic niches of spring bloom-associated Gammaproteobacteria. Features such as those identified here could well be fertile ground for future experimental studies.

Climate Change Induced Trends and Uncertainties in Phytoplankton Spring Bloom Dynamics

Spring phytoplankton blooms in the southern North Sea substantially contribute to annual primary production and largely influence food web dynamics. Studying long-term changes in spring bloom dynamics is therefore crucial for understanding future climate responses and predicting implications on the marine ecosystem. This paper aims to study long term changes in spring bloom dynamics in the Dutch coastal waters, using historical coastalin-situdata and satellite observations as well as projected future solar radiation and air temperature trajectories from regional climate models as driving forces covering the twenty-first century. The main objective is to derive long-term trends and quantify climate induced uncertainties in future coastal phytoplankton phenology. The three main methodological steps to achieve this goal include (1) developing a data fusion model to interlace coastalin-situmeasurements and satellitechlorophyll-aobservations into a single multi-decadal signal; (2) applying a Bayesian structural time series model to produce long-term projections of chlorophyll-a concentrations over the twenty-first century; and (3) developing a feature extraction method to derive the cardinal dates (beginning, peak, end) of the spring bloom to track the historical and the projected changes in its dynamics. The data fusion model produced an enhanced chlorophyll-a time series with improved accuracy by correcting the satellite observed signal within-situobservations. The applied structural time series model proved to have sufficient goodness-of-fit to produce long term chlorophyll-a projections, and the feature extraction method was found to be robust in detecting cardinal dates when spring blooms were present. The main research findings indicate that at the study site location the spring bloom characteristics are impacted by the changing climatic conditions. Our results suggest that toward the end of the twenty-first century spring blooms will steadily shift earlier, resulting in longer spring bloom duration. Spring bloom magnitudes are also projected to increase with a 0.4%year−1trend. Based on the ensemble simulation the largest uncertainty lies in the timing of the spring bloom beginning and -end timing, while the peak timing has less variation. Further studies would be required to link the findings of this paper and ecosystem behavior to better understand possible consequences to the ecosystem.

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.

Cultivation of particle-associated heterotrophic bacteria during a spring phytoplankton bloom in the North Sea

Seawater contains free-living and particle-attached bacteria. Only a small fraction is cultivable on plates. As free-living and particle-associated bacteria differ in their physiological traits, their cultivability on plates may coincide with particle association. Using filtration and Imhoff sedimentation cones, particles were collected during a spring phytoplankton bloom off Helgoland (North Sea) in order to obtain particle-associated bacteria as inocula. Direct dilution plating resulted in 526 strains from 3 µm filtration retentates and 597 strains from settled particles. Motile Gammaproteobacteria from the genera Pseudoalteromonas, Shewanella, Psychrobacter, Vibrio and Colwellia, as well as particle-attached Flavobacteriia affiliating with the genera Tenacibaculum and Gramella, were frequently isolated. As a result, a diverse collection comprised of 266 strains was deposited. Two strains were most likely to represent novel genera and 78 strains were probably novel species. Recently, a high-throughput cultivation study from the same site using seawater as an inoculum had retrieved 271 operational phylogenetic units (OPUs) that represented 88% of the 4136 characterized strains at the species level. A comparison of 16S rRNA gene sequences revealed that the collection obtained matched 104 of the 271 seawater OPUs at the species level and an additional 113 at the genus level. This large overlap indicated a significant contribution of particle-associated bacteria to the cultivable microbiome from seawater. The presence of 49 genera not identified in the larger seawater study suggested that sample fractionation was an efficient strategy to cultivate rare members of the planktonic microbiome. The diverse collection of heterotrophic bacteria retrieved in this study will be a rich source for future studies on the biology of particle-associated bacteria.

Drivers of Atmosphere-Ocean CO2 Flux in Northern Norwegian Fjords

High-latitude fjords and continental shelves are shown to be sinks for atmospheric CO2, yet large spatial-temporal variability and poor regional coverage of sea-air CO2 flux data, especially from fjord systems, makes it difficult to scale our knowledge on how they contribute to atmospheric carbon regulation. The magnitude and seasonal variability of atmosphere-sea CO2 flux was investigated in high-latitude northern Norwegian coastal areas over 2018 and 2019, including four fjords and one coastal bay. The aim was to assess the physical and biogeochemical factors controlling CO2 flux and partial pressure of CO2 in surface water via correlation to physical oceanographic and biological measurements. The results show that the study region acts as an overall atmospheric CO2 sink throughout the year, largely due to the strong undersaturation of CO2 relative to atmospheric concentrations. Wind speed exerted the strongest influence on the instantaneous rate of sea-air CO2 exchange, while exhibiting high variability. We concluded that the northernmost fjords (Altafjord and Porsangerfjord) showed stronger potential for instantaneous CO2 uptake due to higher wind speeds. We also found that fixation of CO2 was likely a significant factor controlling ΔpCO2 from April to June, which followed phenology of spring phytoplankton blooms at each location. Decreased ΔpCO2 and the resulting sea-air CO2 flux was observed in autumn due to a combined reduction of the mixed layer with entrain of high CO2 subsurface water, damped biological activity and higher surface water temperatures. This study provides the first measurements of atmospheric CO2 flux in these fjord systems and therefore an important new baseline for gaining a better understanding on how the northern Norwegian coast and characteristic fjord systems participate in atmosphere carbon regulation.

Spring phytoplankton blooms in the Northern Satsunan region, Japan, stimulated by the intrusion of Kuroshio Branch water

The Kuroshio current greatly affects biological processes in coastal areas. The Kagoshima Bay has good spawning ground for major migratory fish, serves as an important feeding area for fish larvae, and is affected by the Kuroshio current; however, there is little information concerning the biological response to the Kuroshio intrusion in this location. During the mixing period, the Kuroshio current approaches the Kagoshima Bay and a density flow, consisting of surface inflow and bottom outflow, is generated. We hypothesize that the bottom water in the bay, having a high nutrient concentration, mixes with the surface layer when the water flows out of the bay and that this contributes to the high biological production at the bay mouth. In this study, we conducted a field survey of the water quality in March 2016 and, using a three-end member mixing model, examined whether the bottom water of the bay could be a nutrient source supporting the phytoplankton bloom in the bay mouth. We discuss the fate of the organic matter derived from the primary production promoted by a nutrient supply event. The results indicate that the N-based phytoplankton biomass represented less than 80% of the N uptake by the phytoplankton, as estimated by subtracting the measured nitrate concentration from the supplied nitrate concentration based on the mixing model, indicating that the phytoplankton bloom at the bay mouth was supported by the supply of bottom water from the bay. Based on the C/N molar ratio (6.4) calculated by C/chlorophyll a (Chl-a) and N/Chl-a in this study, the highest value of nitrogen uptake was 12 mg C m−3 d−1 in terms of the carbon assimilation. The feeding rate of the mesozooplankton was estimated to be 13.6 mg C m−3 d−1 using a physiological model. An analysis of the food web structure is required to assess the effects of feeding by mesozooplankton; however, most of the phytoplankton bloom caused by mixing could be efficiently transferred to mesozooplankton in this study area.

An estimation of the quantitative impacts of copepod grazing on an under sea-ice spring phytoplankton bloom in western Baffin Bay, Canadian Arctic

This study aimed to quantify the impact of copepod grazing on the productivity of phytoplankton during an under sea-ice spring phytoplankton bloom (USPB) in western Baffin Bay. To quantify positive and/or negative impacts of copepod grazing on primary production and the interaction between copepod grazing and phytoplankton species, we sampled seawater and zooplankton under the landfast sea ice every 2–3 days between May 24 and July 10, 2016. Samples were analyzed for estimation of primary production, chlorophyll-a (chl-a) concentration, diatom abundance, and copepod fecal pellet (FP) production/grazing rate. Analyses of chl-a concentration, primary production, and FP production/grazing rate revealed clear temporal changes and a mismatch between primary production and copepod consumption. The FP production/grazing rate reached a maximum (9.4/31.2 mg C m–2 d–1) on June 16 before the USPB phase and suddenly decreased to 0.7/2.4 mg C m–2 d–1 on June 21, despite an increase in primary production to 74.0 mg C m–2 d–1. The copepod grazing rate (3.7 mg C m–2 d–1) was low relative to primary production (344.6 mg C m–2 d–1) during the USPB phase (after June 20). While our estimates illustrate that copepod grazing did not limit the maximum daily primary production during the USPB, the low grazing pressure (2% of primary production) may have been an additional contributor to the reduction in total primary productivity at the end of the USPB period due primarily to the low supply of regenerated nitrogen-containing nutrients to drive regenerated production.

Meroplankton Diversity, Seasonality and Life-History Traits Across the Barents Sea Polar Front Revealed by High-Throughput DNA Barcoding

In many species of marine benthic invertebrates, a planktonic larval phase plays a critical role in dispersal. Very little is known about the larval biology of most species, however, in part because species identification has historically been hindered by the microscopic size and morphological similarity among related taxa. This study aimed to determine the taxonomic composition and seasonal distribution of meroplankton in the Barents Sea, across the Polar Front. We collected meroplankton during five time points seasonally and used high-throughput DNA barcoding of individual larvae to obtain species-level information on larval seasonality. We found that meroplankton was highly diverse (72 taxa from eight phyla) and present in the Barents Sea year-round with a peak in abundance in August and November, defying the conventional wisdom that peak abundance would coincide with the spring phytoplankton bloom. Ophiuroids, bivalves, and polychaetes dominated larval abundance while gastropods and polychaetes accounted for the bulk of the taxon diversity. Community structure varied seasonally and total abundance was generally higher south of the Polar Front while taxon richness was overall greater to the north. Of the species identified, most were known inhabitants of the Barents Sea. However, the nemertean Cephalothrix iwatai and the brittle star Ophiocten gracilis were abundant in the meroplankton despite never having been previously recorded in the northern Barents Sea. The new knowledge on seasonal patterns of individual meroplanktonic species has implications for understanding environment-biotic interactions in a changing Arctic and provides a framework for early detection of potential newcomers to the system.

Contemporary variability in dimethylsulfide flux in the Barents Sea and simulated change under 4×CO2 climate conditions

The impacts of climate change will likely be most pronounced in the Arctic, due to various climate feedbacks such as the sea-ice albedo effect and changes in the biogenic emission of marine aerosols. The Barents Sea is one of the most productive ecosystems in the Arctic Ocean, seems to be a hot spot of warming, and as such merits special attention. A biogeochemical model is used to simulate phytoplankton variability and dimethylsulfide (DMS) flux in year 2005. A Genetic Algorithm (GA) technique is used to calibrate the biogeochemical model parameters. The CMIP5 (Coupled Model Inter-comparison Project Phase 5) model output is used to provide changes in relevant model forcings under 1×CO 2 (contemporary) and 4×CO 2 (which could be reached by 2100 under worst case greenhouse gas emissions scenarios). Satellite data and meteorological forcings are used to characterize contemporary biophysical conditions in a region of the Barents Sea (70°N-80°N, 30°E-40°E). Under contemporary climate, meteorological forcings are studied for year 2003–2013. Mean concentrations of chlorophyll-a (CHL) reach a peak in late May and increase from north to south. Sea ice cover (ICE) decreases from March to September and is almost zero after year 2005 in the southern region (70°N-75°N). In the northern region (75°N-80°N), seasonal ice melt is closely related to the timing of the spring phytoplankton bloom. In general, the vernal increase of CHL coincides with an increase of sea surface temperature (SST) and decrease in surface wind speed (WIND). Under 4×CO 2 climate conditions, spring ICE decreases around 60% and the mixed layer depth (MLD) increases around 112%. The simulated deepening of the mixed layer leads to less favorable light conditions for phytoplankton growth. DMS sea-to-air flux hence decreases annual around 12%–46% due to a delay in the vernal bloom of phytoplankton. However, sea ice melt with concomitant increase in open waters promotes a DMS flux increase of around 9%–20% from May to mid of July. The change in DMS flux is influenced more by deepening of the MLD, rather than the increase in open water due to reduced sea ice cover. Importantly, these results contrast with previous studies that simulated increases in DMS sea-air flux in the Arctic Ocean under warming. • Meteorological forcings are studied and A biogeochemical model is used to simulate DMS flux. • A Genetic Algorithm (GA) technique is used to calibrate the biogeochemical model parameters. • Under 4×CO 2 condition, ice cover decreases around 60% and mixed layer depth increases around 112%. • DMS sea-to-air flux decrease around 12–46% in the Barents Sea by 4×CO 2. • This DMS emission reduction may lead to serious ramifications for the regional marine ecosystems.

Estimation of ocean export production ratios based on mixed layer depth and satellite chlorophyll observations

We introduced a simple methodology to determine basin-wide marine export production ratios during spring phytoplankton bloom periods by combining monthly varying mixed layer depths (MLD) derived from observed temperature depth profiles, satellite time series of chlorophyll-a (Chl-a) concentrations, and hypothetical critical depth. The MLD-Chl-a based approach yielded monthly export ratios reflecting seasonal variability of net community production in correlation with the MLD variations. Regional averages of export ratio in the spring for the southern part of the East Sea (36–39°N, 130–134°E), North Pacific Ocean (42–45°N, 151–155°E), and North Atlantic Ocean (57–60°N, 17–21°W) are 0.44 ± 0.22, 0.42 ± 0.16, and 0.73 ± 0.29, respectively, which compare favorably with prior field data. This suggests that the approach enables estimation of export production ratios with high spatial and temporal resolutions, complementing the methods that rely on extrapolation of limited field measurements to larger space and time scales.

Particle Collection in Imhoff Sedimentation Cones Enriches Both Motile Chemotactic and Particle-Attached Bacteria.

Marine heterotrophic microorganisms remineralize about half of the annual primary production, with the microbiomes on and around algae and particles having a major contribution. These microbiomes specifically include free-living chemotactic and particle-attached bacteria, which are often difficult to analyze individually, as the standard method of size-selective filtration only gives access to particle-attached bacteria. In this study, we demonstrated that particle collection in Imhoff sedimentation cones enriches microbiomes that included free-living chemotactic bacteria and were distinct from particle microbiomes obtained by filtration or centrifugation. Coastal seawater was collected during North Sea phytoplankton spring blooms, and the microbiomes were investigated using 16S rRNA amplicon sequencing and fluorescence microscopy. Enrichment factors of individual operational taxonomic units (OTUs) were calculated for comparison of fractionated communities after separation with unfractionated seawater communities. Filtration resulted in a loss of cells and yielded particle fractions including bacterial aggregates, filaments, and large cells. Centrifugation had the lowest separation capacity. Particles with a sinking rate of >2.4 m day–1 were collected in sedimentation cones as a bottom fraction and enriched in free-living chemotactic bacteria, i.e., Sulfitobacter, Pseudoalteromonas, and Vibrio. Subfractions of these bottom fractions, obtained by centrifugation, showed enrichment of either free-living or particle-attached bacteria. We identified five distinct enrichment patterns across all separation techniques: mechano-sensitive and mechano-stable free-living bacteria and three groups of particle-attached bacteria. Simultaneous enrichment of particle-attached and chemotactic free-living bacteria in Imhoff sedimentation cones is a novel experimental access to these groups providing more insights into the diversity, structure, and function of particle-associated microbiomes, including members of the phycosphere.

Organic biomarkers and Meiofauna diversity reflect distinct carbon sources to sediments transecting the Mackenzie continental shelf

During the Arctic spring freshet, the Mackenzie River delivers large amounts of dissolved and particulate terrestrial organic carbon to the Canadian Beaufort Sea shelf. This pulse of material and nutrients fuels phytoplankton blooms in late spring and summer and together with under ice primary production contributes to the organic carbon reservoir of the coastal shelf sediments. As a component of the Marine Arctic Ecosystem Study (MARES), samples transecting the Canadian Mackenzie Trough were collected in fall before ice cover to examine the balance of marine and terrestrial carbon in sediments and its relationship with sediment meiobenthic diversity and abundance. Organic biomarker analysis of lipid proxies together with total hydrolyzable amino acids (THAA) defined organic matter sources and potential lability. Across the shelf transect, consistent values observed in sedimentary total organic carbon (1.09%–1.62%) belied a shift in the balance of terrestrial and marine carbon sources. Predominately terrestrial inputs at the shallow site near the delta outflow appeared to constrain meiofaunal populations while the deepest location off shelf reflected lower labile marine organic carbon reaching the sediments that limited benthic production. Surface sediments at the mid-shelf, however, showed a near 3-fold increase (to 1.16 mg C OC-1) in contributions by diatoms and dinoflagellates markers plus a peak in total amino acids (9.53 mg g OC-1) which corresponded with higher meiofauna abundance. The combination of detailed organic analysis together with meiofaunal community measures suggest that mid-shelf sediments receive higher contributions of labile marine carbon which drive increased meiofauna diversity and abundance despite the large background of terrestrial organic material exiting the Mackenzie River outflow.

The Prominent Spring Bloom and Its Relation to Sea‐Ice Melt in the Sea of Okhotsk, Revealed by Profiling Floats

AbstractSeven profiling floats equipped with oxygen sensors deployed in the Sea of Okhotsk provide time series data for 33 cases of spring phytoplankton bloom period, including 9 cases in which sea ice existed just before the bloom (prior‐ice case). As an index of biological productivity, we calculated net community production (NCP) based on the increasing oxygen rate using the Redfield ratio. The total NCP in the euphotic layer averaged for prior‐ice cases is 31.3 mmolC m−2 day−1), ∼3 times higher than that of non‐ice cases. In addition to intensification of surface stratification, other factors of sea‐ice melt likely enhance the bloom. The influence of sea‐ice melt is particularly large in the southwestern region, where the iron availability likely limits phytoplankton growth. A suggested scenario is that when the sea ice containing sediment/iron is transported from the northern shelves, a prominent bloom is induced via the iron supply by sea‐ice melt.

Towards a Multi‐Platform Assimilative System for North Sea Biogeochemistry

AbstractOceanography has entered an era of new observing platforms, such as biogeochemical‐Argo floats and gliders, some of which will provide three‐dimensional maps of essential ecosystem variables on the North‐West European (NWE) Shelf. In a foreseeable future operational centers will use multi‐platform assimilation to integrate those valuable data into ecosystem reanalysis and forecast systems. Here we address some important questions related to glider biogeochemical data assimilation (DA) and introduce multi‐platform DA in a preoperational model of the NWE Shelf sea ecosystem. We test the impact of the different multi‐platform system components (glider vs. satellite, physical vs. biogeochemical) on the simulated biogeochemical variables. To characterize the model performance, we focus on the period around the phytoplankton spring bloom, since the bloom is a major ecosystem driver on the NWE Shelf. We found that the timing and magnitude of the phytoplankton bloom is insensitive to the physical DA, which is explained in the study. To correct the simulated phytoplankton bloom one needs to assimilate chlorophyll observations from glider or satellite Ocean Color (OC) into the model. Although outperformed by the glider chlorophyll assimilation, we show that OC assimilation has mostly desirable impact on the sub‐surface chlorophyll. Since the OC assimilation updates chlorophyll only in the mixed layer, the impact on the sub‐surface chlorophyll is the result of the model dynamical response to the assimilation. We demonstrate that the multi‐platform assimilation combines the advantages of its components and always performs comparably to its best performing component.

Effects of sea ice and wind speed on phytoplankton spring bloom in central and southern Baltic Sea.

In this study, the effects of sea ice and wind speed on the timing and composition of phytoplankton spring bloom in the central and southern Baltic Sea are investigated by a hydrodynamic-biogeochemical model and observational data. The modelling experiment compared the results of a reference run in the presence of sea ice with those of a run in the absence of sea ice, which confirmed that ecological conditions differed significantly for both the scenarios. It has been found that diatoms dominate the phytoplankton biomass in the absence of sea ice, whereas dinoflagellates dominate the biomass in the presence of thin sea ice. The study concludes that under moderate ice conditions (representing the last few decades), dinoflagellates dominate the spring bloom phytoplankton biomass in the Baltic Sea, whereas diatoms will be dominant in the future as a result of climate change i.e. in the absence of sea ice.

Changing expression patterns of TonB-dependent transporters suggest shifts in polysaccharide consumption over the course of a spring phytoplankton bloom

Algal blooms produce large quantities of organic matter that is subsequently remineralised by bacterial heterotrophs. Polysaccharide is a primary component of algal biomass. It has been hypothesised that individual bacterial heterotrophic niches during algal blooms are in part determined by the available polysaccharide substrates present. Measurement of the expression of TonB-dependent transporters, often specific for polysaccharide uptake, might serve as a proxy for assessing bacterial polysaccharide consumption over time. To investigate this, we present here high-resolution metaproteomic and metagenomic datasets from bacterioplankton of the 2016 spring phytoplankton bloom at Helgoland island in the southern North Sea, and expression profiles of TonB-dependent transporters during the bloom, which demonstrate the importance of both the Gammaproteobacteria and the Bacteroidetes as degraders of algal polysaccharide. TonB-dependent transporters were the most highly expressed protein class, split approximately evenly between the Gammaproteobacteria and Bacteroidetes, and totalling on average 16.7% of all detected proteins during the bloom. About 93% of these were predicted to take up organic matter, and for about 12% of the TonB-dependent transporters, we predicted a specific target polysaccharide class. Most significantly, we observed a change in substrate specificities of the expressed transporters over time, which was not reflected in the corresponding metagenomic data. From this, we conclude that algal cell wall-related compounds containing fucose, mannose, and xylose were mostly utilised in later bloom stages, whereas glucose-based algal and bacterial storage molecules including laminarin, glycogen, and starch were used throughout. Quantification of transporters could therefore be key for understanding marine carbon cycling.

Spatial patterns of phytoplankton community and biomass along the Kuroshio Extension and adjacent waters in late spring

The Kuroshio Extension (KE) is a natural boundary region characterized by complex hydrodynamic processes in the western North Pacific Ocean that fuel spring phytoplankton blooms in high latitudes. This study looked at the spatial patterns and diversity of phytoplankton community and biomass associated with oceanographic profiles across the KE from the north to south in June 2011. Results showed that upper-ocean gradients were strongly correlated with observed spatial variability in phytoplankton communities (> 10 μm). Phytoplankton abundance, species richness, and Chl a biomass were generally higher in the Kuroshio–Oyashio transition region than in the KE axis and the southern KE. No bloom event was recorded in late spring. A total of 81 phytoplankton taxa belonging to 45 genera of four phyla were identified. Dominant taxa in both abundance and occurrence included the diatoms Chaetoceros radicans, C. curvisetus, C. convolutes, C. deblis, C. peruvianus, Pseudonitzchia delicatissima, Coscinodiscus asteromphalus, Thalassionema nitzschioides, T. frauenfeldii, Thalassiothrix longissima, the dinoflagellates Protoperidinium pyrifoprmus, and the Cyanobacterium Trichodesmium thiebautii. Depth-averaged abundance and Chl a were 2076 ± 3,615 cells L−1 and 0.20 ± 0.10 mg m−3, respectively. Subsurface Chl a maxima (SCM) was mainly found at 50 m, while the abundance maximum differed regionally in the water column. A cluster analysis grouped phytoplankton communities into three assemblages related to geographic locations influenced by Oyashio and Kuroshio currents. Nutrient limitation rather than light led to low Chl a and abundance during late spring. Redundancy correspondence and spearman’s correlation further suggest that dissolved inorganic nitrogen was the major factor regulating phytoplankton distribution north of KE, while phosphate limitation was more prominent in the KE axis and the southern KE. The Oyashio and Kuroshio interaction caused evident variability in the hydrodynamic environment including the MLD and Zeu of the region, and changes in chemical profiles such as nutrients, pH, and DO. The observed distributional shift in the spatial pattern of phytoplankton was attributed to upper-ocean environmental gradients within the KE axis and its northern and southern sections.