Spring Diatom Bloom Research Articles

Interaction and assembly processes of abundant and rare microbial communities during a diatom bloom process.

Diatom blooms can significantly influence the dynamics of microbial communities, yet little is known about the interaction and assembly mechanisms of abundant and rare taxa during bloom process. Here, using 16S rRNA gene amplicon sequencing, we investigated the co-occurrence patterns and assembly processes of abundant and rare microbial communities during an early spring diatom bloom in Xiangshan bay. Our results showed that α-diversity indices in the rare subcommunity (RS) were significantly higher than those in the abundant and common subcommunities. β-Diversity of the RS was the highest among three subcommunities, and the variation of β-diversity in the three subcommunities was mainly induced by species turnover, which was also the highest in the RS. The assembly of microbial communities was mainly driven by the neutral processes, but the roles of neutral processes might differ in each subcommunity. Co-occurrence network analysis revealed that abundant and common operational taxonomic units were more often located in central positions within the network. Most of the modules in the network were specific to a particular bloom stage, owing to the succession of Skeletonema costatum. Overall, these findings expand current understanding of the microbial interaction and assembly mechanisms in marine environment suffering harmful algal bloom disturbance.

Isoprene production in seawater of Funka Bay, Hokkaido, Japan

We carried out shipboard observations in Funka Bay, Hokkaido, Japan, monthly or bimonthly from December 2015 to November 2016. We measured vertical profiles of isoprene, chlorophyll-a (chl-a), and other parameters from surface to bottom layer (about 95 m) near the center of the bay. We found substantial increases in isoprene concentration in the surface mixed layer from February to March during the peak of the spring diatom bloom, in the bottom layer from March to April after the peak of the bloom, and in the subsurface layer (below the surface mixed layer) in summer from July to August, where there were also substantial chl-a concentration maxima. We attribute the increased isoprene in the surface and subsurface layers to photosynthetic production of isoprene by the dominant phytoplankton in the spring bloom and in summer, and that in the bottom layer to dark production of isoprene by diatom aggregates that settled from the surface euphotic zone. We also measured isoprene production in laboratory incubation experiments. The rates of in situ production of isoprene per unit chl-a in the surface mixed layer during the spring bloom, in the dark bottom layer during the bloom, and in the subsurface layer in summer (0.82, 0.03–0.13, and 7.38 pmol (μg chl-a)−1 day−1, respectively) were consistent with our incubation results. We believe that this is the first report focused on dark production of isoprene by diatoms; the rate of isoprene production under dark conditions ranged from 4% to 16% of that by photosynthesis.

Silicic acid limitation drives bloom termination and potential carbon sequestration in an Arctic bloom

The spring diatom bloom in the Arctic Ocean accounts for significant annual primary production leading to the most rapid annual drawdown of water-column pCO2. Late-winter waters in the Atlantic Arctic & Subarctic Provinces (AASP) have lower silicic acid concentrations than nitrate, which suggests diatom blooms may deplete Si before N. Here we test a facet of the hypothesis that silicic acid limitation terminates the spring diatom bloom in the AASP and the sinking of the senescent and dead diatoms helps drive carbon sequestration. During a 6-week study, diatoms bloomed and progressively consumed silicic acid to where it limited their growth. The onset of growth limitation was concurrent with the minimum pCO2 in the surface waters and increases in both the proportion of dead diatoms and the diatom assemblage sedimentation rate. Data reanalysis within the AASP shows a highly significant and positive correlation between silicic acid and pCO2 in the surface waters, but no significant relationship with nitrate and pCO2 was observed unless data were smoothed. Therefore, understanding the future of the AASP spring diatom bloom requires models that explicitly consider changes in silicic acid supply as a driver of this process.

Influence of Coastal Oyashio water on massive spring diatom blooms in the Oyashio area of the North Pacific Ocean

We hypothesized that the characteristics of Coastal Oyashio water in the North Pacific Ocean during winter–spring have a major influence on the generation of massive spring diatom blooms throughout the Oyashio area. To test this hypothesis, we analyzed in situ data from May during 1990–2016 along a transect known as the “A-line” to monitor conditions in the Oyashio off the southeast coast of Hokkaido, Japan. Chlorophyll-a concentrations were high (≥5 mg m−3) on the shelf and in the offshore Oyashio areas around the First and Second Oyashio Intrusions. About 80% of the high chlorophyll-a concentrations in the Oyashio area were linked with low-salinity (≤33.0) water that was associated with the Coastal Oyashio water and its modified waters. For the remaining Oyashio area, the so-called “Oyashio water” with a high salinity of 33.0–33.7 was near the sea surface and associated with low chlorophyll-a concentrations with a frequency distribution mode at 1–2 mg m−3. Diatom communities with high abundance and diversity were also connected (disconnected) between the shelf and the offshore Oyashio areas with low (high) salinity and high (low) chlorophyll-a concentrations. Analysis of historical temperature–salinity data near the sea surface revealed that the Coastal Oyashio water and its modified waters appear most frequently around the shelf along Hokkaido and the Kuril Islands, and that the frequency of appearance decreases with increasing distance offshore. We suggest that the transport and pathway around the islands of the Coastal Oyashio water and its modified waters, which contain high nutrient concentrations and the seeds for massive spring blooms, are the primary factors regulating the generation and spatial extent of the massive spring blooms over the Oyashio area.

Interannual variation in seasonal diatom sedimentation reveals the importance of late winter processes and their timing for sediment signal formation

AbstractDisentangling the process information contained in a diatom sediment signature is crucial for reliable future predictions based on paleolimnological records. In this study, we combine limnological and paleolimnological monitoring to address the fundamental question: Which environmental information is contained in a diatom sediment signal? We compared annual diatom sequential sediment trap records with the diatom record of the annually varved lake sediment of Nylandssjön (northern Sweden) from three meteorologically different years (2012–2014). The seasonal patterns in diatom sedimentation were strikingly different in varve years 2012 and 2014 compared to varve year 2013. In 2012 and 2014, up to 70% of the annual flux occurred in a single spring month and was dominated by Cyclotella glomerata. In contrast, in 2013, peak fluxes were much lower and more annually integrated. Next, we compared the full‐year diatom trap results with year round in‐lake physical, chemical, and biological monitored parameters, as well as meteorological variables. Annual averages of environmental conditions did not explain the interannual variability in diatom sedimentation. Instead, the seasonality of diatom sedimentation was determined by the timing of the spring diatom bloom relative to lake over‐turn in winters with warm vs. cold air temperature. With our combined limnological and paleolimnological monitoring approach, we find that an annual diatom signal can either contain primarily seasonal climate information from a short time period or be annually integrated. We synthesize our results in a novel conceptual model, which describes the response of sediment diatom signals to two distinct sequences of late‐winter conditions.

Reprint of: High prey-predator size ratios and unselective feeding in copepods: A seasonal comparison of five species with contrasting feeding modes

There has been an upsurge of interest in trait-based approaches to zooplankton, modelling the seasonal changes in the feeding modes of zooplankton in relation to phytoplankton traits such as size or motility. We examined this link at two English Channel plankton monitoring sites south of Plymouth (L4 and E1). At L4 there was a general transition from diatoms in spring to motile microplankton in summer and autumn, but this was not mirrored in the succession of copepod feeding traits; for example the ambushing Oithona similis dominated during the spring diatom bloom. At nearby E1 we measured seasonality of food and grazers, finding strong variation between 2014 and 2015 but overall low mesozooplankton biomass (median 4.5 mg C m−3). We also made a seasonal grazing study of five copepods with contrasting feeding modes (Calanus helgolandicus, Centropages typicus, Acartia clausi, Pseudocalanus elongatus and Oithona similis), counting the larger prey items from the natural seston. All species of copepod fed on all food types and differences between their diets were only subtle; the overriding driver of diet was the composition of the prey field. Even the smaller copepods fed on copepod nauplii at significant rates, supporting previous suggestions of the importance of intra-guild predation. All copepods, including O. similis, were capable of tackling extremely long (>500 µm) diatom chains at clearance rates comparable to those on ciliates. Maximum observed prey:predator length ratios ranged from 0.12 (C. helgolandicus) up to 0.52 (O. similis). Unselective feeding behaviour and the ability to remove highly elongated cells have implications for how copepod feeding is represented in ecological and biogeochemical models.

Biogenic silica production and diatom dynamics in the Svalbard region during spring

Abstract. Diatoms are generally the dominant contributors to the Arctic Ocean spring bloom, which is a key event in regional food webs in terms of capacity for secondary production and organic matter export. Dissolved silicic acid is an obligate nutrient for diatoms and has been declining in the European Arctic since the early 1990s. The lack of regional silicon cycling information precludes understanding the consequences of such changes for diatom productivity during the Arctic spring bloom. This study communicates the results from a cruise in the European Arctic around Svalbard, which reports the first concurrent data on biogenic silica production and export, export of diatom cells, the degree of kinetic limitation by ambient silicic acid, and diatom contribution to primary production. Regional biogenic silica production rates were significantly lower than those achievable in the Southern Ocean and silicic acid concentration limited the biogenic silica production rate in 95 % of samples. Compared to diatoms in the Atlantic subtropical gyre, regional diatoms are less adapted for silicic acid uptake at low concentration, and at some stations during the present study, silicon kinetic limitation may have been intense enough to limit diatom growth. Thus, silicic acid can play a critical role in diatom spring bloom dynamics. The diatom contribution to primary production was variable, ranging from <10 % to ∼100 % depending on the bloom stage and phytoplankton composition. While there was agreement with previous studies regarding the export rate of diatom cells, we observed significantly elevated biogenic silica export. Such a discrepancy can be resolved if a higher fraction of the diatom material exported during our study was modified by zooplankton grazers. This study provides the most direct evidence to date suggesting the important coupling of the silicon and carbon cycles during the spring bloom in the European Arctic.

The influence of cyanobacteria blooms on the attenuation of nitrogen throughputs in a Baltic coastal lagoon

We combined a mass balance approach with measurements of air–water and sediment–water nitrogen (N) exchange to better understand the mechanisms attenuating N throughputs in a eutrophic coastal lagoon. We were particularly interested in how seasonal shifts in external versus internal N fluxes and the transition from diatom- to cyanobacteria- dominated phytoplankton communities influence N storage and loss to the atmosphere. We found that on an annual basis almost all of the N removed by the lagoon was due to sediment storage following the spring diatom bloom. This period was characterized by high riverine inputs of dissolved inorganic nitrogen, high rates of assimilatory conversion to particulate nitrogen (PN), and net accrual of N in sediments. By contrast, the larger summer bloom was associated with low sediment N storage, which we attribute in part to the presence of positively-buoyant cyanobacteria. Low settling rates during cyanobacteria blooms favored export of PN to the Baltic Sea over sediment accrual in the lagoon. In addition, summer dinitrogen (N2) fixation by cyanobacteria largely offset annual N2 losses via denitrification. These findings show that cyanobacteria blooms diminish N attenuation within the lagoon by altering the balance of N exchange with the atmosphere and by promoting export of particulate N over sediment burial.

Community composition and photophysiology of phytoplankton assemblages in coastal Oyashio waters of the western North Pacific during early spring

Globally, the western subarctic Pacific is known as the region with the largest seasonal drawdown in the partial pressure of CO2 due to biological activity, i.e., high spring primary production and particulate organic carbon flux. These distinctive features are mainly caused by intense spring diatom blooms in coastal Oyashio (COY) and Oyashio (OY) waters. Although phytoplankton assemblages in OY waters are rather well studied, little is known about COY waters. In this study, photophysiological properties and phytoplankton community composition in COY waters were investigated during the pre-bloom and bloom periods from March to April 2015. Next-generation sequencing targeting the 18S rRNA gene revealed that the diatom Thalassiosira generally dominated the phytoplankton community and showed distinct differences in the diatom communities in shelf and offshore waters of the COY. Additionally, the relative contribution of Thalassiosira to the total diatom assemblages showed a positive correlation with maximum photosynthetic rates (PBmax) occurring throughout this study. Chlorophyll a concentration and primary productivity were also positively correlated with sea surface temperature, suggesting that temperature was a critical factor for bloom development. Short-term on-deck incubation experiments were carried out to examine the role of temperature in determining planktonic photosynthetic processes. Our results showed an increase in PBmax with rising temperature in assemblages from the shelf COY waters. Similarly, transcription levels of the diatom-specific rbcL gene, which encodes the large subunit of RuBisCO, also increased with rising temperature in the shelf assemblages. In contrast, temperature had little effect on the maximum photochemical quantum efficiency (Fv/Fm) of photosystem II. The results suggested that the transcription activity of the diatom-specific rbcL gene was upregulated by the increase in temperature, and that led to the higher PBmax values and the spring diatom bloom in the shelf COY region.

Within-lake spatio-temporal dynamics of cladoceran and diatom communities in a deep subtropical mountain lake (Lugu Lake) in southwest China

Within-lake spatio-temporal variability of remains of cladocerans and diatoms were examined, using trap and surficial sediment sampling approaches in Lugu Lake, one of the deep mountain lakes in the subtropical region of southwest China, to understand the response to recent environmental change. Seasonality played a strong role in the distribution patterns of both cladocerans and diatoms, but their responses to seasonal change varied. The rich resources of food supported a cladoceran population peak during summer, while increased mixing and higher nutrient triggered diatom blooms in spring. The summer also witnessed increased grazing effects of primary consumers on diatoms when the water column was nutrient-enriched. In particular, Ceriodaphnia intensified grazing on small diatoms (Cyclotella ocellata), consequently affecting community patterns during summer, while increased wind activity during spring induced turbulence, remixing, transportation and depositional processes of remains of littoral Alona guttata and benthic diatoms. The distribution pattern of cladocerans in surface sediments was similar to that of diatoms. Seasonal community patterns and trophic interactions between cladocerans and diatoms in trap and surface sediments of differential depth gradients provide evidence that high-resolution sampling of multi-proxy biological remains in deep mountain lakes of southwest China can help reduce biases in paleoenvironmental reconstructions.

Structural and functional response of phytoplankton to reduced river inputs and anomalous physical-chemical conditions in the Gulf of Trieste (northern Adriatic Sea)

We studied the influence of anomalous meteorological and hydrological conditions that occurred in the Gulf of Trieste from March 2006 to February 2007 on phytoplankton structure and function. We computed monthly mean (or median) air temperature, total precipitation, wind speed, river discharge, seawater temperature, salinity, photosynthetic available radiation (PAR), cyanobacteria, nano- and microphytoplankton abundances during the study year and compared them to climatological (1999–2014 for PAR; 1999–2007 for nanophytoplankton; 1998–2015 for the other variables) mean/median data. We then related the cyanobacteria (0.2–2 μm), nano- (2–20 μm) and microphytoplankton (20–200 μm) of the study year to inorganic nutrient concentrations. Median river inputs in October and November were 9- and 15-fold lower, respectively, than the time series medians, with consequent high salinity from May to November (up to +1.26 compared to the climatological data). Monthly mean seawater temperatures were lower than the climatological values (−2.95 °C at the surface) from March to August 2006 and higher (+2.15 °C at the surface) from September to February 2007. Reductions in freshwater input and nutrient depletion were likely responsible for a decrease in microphytoplankton (median annual abundance over 60% lower than the climatologic median) and cyanobacteria (up to 47% lower than the climatology). Significant seasonal differences in cyanobacteria and microphytoplankton abundances (RANOSIM = 0.52; p < 0.05), as well as in seawater temperature and salinity (RANOSIM = 0.73; p < 0.05) between the study period and the climatology were highlighted. The late spring diatom bloom was not reflected in high photosynthetic rates whereas an unusually high primary production was estimated in November (7.11 ± 1.01 μgC L−1 h−1), when a mucilage event occurred due to very stable atmospheric and oceanographic conditions. The typical seasonal succession of pelagic phototrophs (micro-, nanophytoplankton and cyanobacteria) was altered since an exceptional cyanobacteria bloom first developed in April, followed by a delayed diatom bloom in May.

High prey-predator size ratios and unselective feeding in copepods: A seasonal comparison of five species with contrasting feeding modes

There has been an upsurge of interest in trait-based approaches to zooplankton, modelling the seasonal changes in the feeding modes of zooplankton in relation to phytoplankton traits such as size or motility. We examined this link at two English Channel plankton monitoring sites south of Plymouth (L4 and E1). At L4 there was a general transition from diatoms in spring to motile microplankton in summer and autumn, but this was not mirrored in the succession of copepod feeding traits; for example the ambushing Oithona similis dominated during the spring diatom bloom. At nearby E1 we measured seasonality of food and grazers, finding strong variation between 2014 and 2015 but overall low mesozooplankton biomass (median 4.5 mg C m−3). We also made a seasonal grazing study of five copepods with contrasting feeding modes (Calanus helgolandicus, Centropages typicus, Acartia clausi, Pseudocalanus elongatus and Oithona similis), counting the larger prey items from the natural seston. All species of copepod fed on all food types and differences between their diets were only subtle; the overriding driver of diet was the composition of the prey field. Even the smaller copepods fed on copepod nauplii at significant rates, supporting previous suggestions of the importance of intra-guild predation. All copepods, including O. similis, were capable of tackling extremely long (>500 µm) diatom chains at clearance rates comparable to those on ciliates. Maximum observed prey:predator length ratios ranged from 0.12 (C. helgolandicus) up to 0.52 (O. similis). Unselective feeding behaviour and the ability to remove highly elongated cells have implications for how copepod feeding is represented in ecological and biogeochemical models.

Development of a conceptual warning system for toxic levels of Alexandrium fundyense in the Bay of Fundy based on remote sensing data

Harmful algal blooms (HABs) present a potential danger for human health and commercial activities, especially in coastal regions. Observing systems increasingly rely on remote sensors to monitor and possibly predict the locations and intensity of such blooms. Here we present a novel approach for detecting HABs of Alexandrium fundyense in the Bay of Fundy, Canada. A. fundyense is considered toxic for individuals who consume shellfish when cell abundances adjacent to shellfish harvesting areas are as low as 200 cells·L−1, making it difficult to use direct remote sensing techniques to assess the threat in the early stages of the development of the bloom. Using in situ A. fundyense cell abundance measurements, together with satellite observations of sea-surface temperature and the occurrence of diatom-dominated phytoplankton populations, a warning system was developed based on three levels of alerts: green (low abundance of A. fundyense), orange (possible threat of A. fundyense) and red (high probability of A. fundyense concentrations that would result in shellfish toxicity above safe levels for human consumption). Combined information on diatom phenology and variations in sea-surface temperature are key to the timing of A. fundyense blooms: our data reveal that the termination of the diatom spring bloom, associated with the warming of the water, can trigger an increase in A. fundyense cell abundance. The objective criteria for a HAB warning system was developed and tested in the Bay of Fundy using two different datasets: one to develop the algorithm (data collected between 1998 and 2007) and one to assess its performance (data collected in 2011). The warning system is based on the cautionary principle that a false negative (warning not issued when it should have been) is far more serious than a false positive (warning issued when it should not have been). The overall success of the algorithm when tested on the validation dataset is about 70% using a threshold of 150 A. fundyense cells·L−1, with a low occurrence of false negative red alerts (<8%). The satellite-data-based warning can be used to optimize an in situ monitoring system, which can be designed to be more intensive when the warning status is orange or red. This study demonstrates that combined satellite information on phytoplankton phenology and sea-surface temperature can help predict low abundances of toxic A. fundyense cells. It also highlights the importance of an integrated approach combining satellite and in situ observations to monitor HABs.

The influence of environmental variability on the biogeography of coccolithophores and diatoms in the Great Calcite Belt

Abstract. The Great Calcite Belt (GCB) of the Southern Ocean is a region of elevated summertime upper ocean calcite concentration derived from coccolithophores, despite the region being known for its diatom predominance. The overlap of two major phytoplankton groups, coccolithophores and diatoms, in the dynamic frontal systems characteristic of this region provides an ideal setting to study environmental influences on the distribution of different species within these taxonomic groups. Samples for phytoplankton enumeration were collected from the upper mixed layer (30 m) during two cruises, the first to the South Atlantic sector (January–February 2011; 60° W–15° E and 36–60° S) and the second in the South Indian sector (February–March 2012; 40–120° E and 36–60° S). The species composition of coccolithophores and diatoms was examined using scanning electron microscopy at 27 stations across the Subtropical, Polar, and Subantarctic fronts. The influence of environmental parameters, such as sea surface temperature (SST), salinity, carbonate chemistry (pH, partial pressure of CO2 (pCO2), alkalinity, dissolved inorganic carbon), macronutrients (nitrate + nitrite, phosphate, silicic acid, ammonia), and mixed layer average irradiance, on species composition across the GCB was assessed statistically. Nanophytoplankton (cells 2–20 µm) were the numerically abundant size group of biomineralizing phytoplankton across the GCB, with the coccolithophore Emiliania huxleyi and diatoms Fragilariopsis nana, F. pseudonana, and Pseudo-nitzschia spp. as the most numerically dominant and widely distributed. A combination of SST, macronutrient concentrations, and pCO2 provided the best statistical descriptors of the biogeographic variability in biomineralizing species composition between stations. Emiliania huxleyi occurred in silicic acid-depleted waters between the Subantarctic Front and the Polar Front, a favorable environment for this species after spring diatom blooms remove silicic acid. Multivariate statistics identified a combination of carbonate chemistry and macronutrients, covarying with temperature, as the dominant drivers of biomineralizing nanoplankton in the GCB sector of the Southern Ocean.

A winter dinoflagellate bloom drives high rates of primary production in a Patagonian fjord ecosystem

A dense winter bloom of the dinoflagellate Heterocapsa triquetra was observed at a fixed station (44°35.3′S; 72°43.6′W) in the Puyuhuapi Fjord in Chilean Patagonia during July 2015. H. triquetra dominated the phytoplankton community in the surface waters between 2 and 15 m (13–58 × 109 cell m−2), with abundances some 3 to 15 times higher than the total abundance of the diatom assemblage, which was dominated by Skeletonema spp. The high abundance of dinoflagellates was reflected in high rates of gross primary production (GPP; 0.6–1.6 g C m−2 d−1) and chlorophyll-a concentration (Chl-a; 70–199.2 mg m−2) that are comparable to levels reported in spring diatom blooms in similar Patagonian fjords. We identify the main forcing factors behind a pulse of organic matter production during the non-productive winter season, and test the hypothesis that low irradiance levels are a key factor limiting phytoplankton blooms and subsequent productivity during winter.Principal Component Analysis (PCA) indicated that GPP rates were significantly correlated (r = −0.8, p < 0.05) with a decrease in salinity/temperature and the presence of the Heterocapsa bloom. The bloom occurred under low surface irradiance levels characteristic of austral winter and was accompanied by strong northern winds, associated with the passage of a low-pressure system, and a water column dominated by double diffusive layering. To our knowledge, this is the first report of a dense dinoflagellate bloom during deep austral winter in a Patagonian fjord, and our data challenge the paradigm of light limitation as a factor controlling phytoplankton blooms in this region in winter.

Managed nutrient reduction impacts on nutrient concentrations, water clarity, primary production, and hypoxia in a north temperate estuary

Except for the Providence River and side embayments like Greenwich Bay, Narragansett Bay can no longer be considered eutrophic. In summer 2012 managed nitrogen treatment in Narragansett Bay achieved a goal of reducing effluent dissolved inorganic nitrogen inputs by over 50%. Narragansett Bay represents a small northeast US estuary that had been heavily loaded with sewage effluent nutrients since the late 1800s. The input reduction was reflected in standing stock nutrients resulting in a statistically significant 60% reduction in concentration. In the Providence River estuary, total nitrogen decreased from 100 μm to about 40 μm, for example. We tested four environmental changes that might be associated with the nitrogen reduction. System apparent production was significantly decreased by 31% and 45% in the upper and mid Bay. Nutrient reductions resulted in statistically improved water clarity in the mid and upper Bay and in a 34% reduction in summer hypoxia. Nitrogen reduction also reduced the winter spring diatom bloom; winter chlorophyll levels after nutrient reduction have been significantly lower than before the reduction. The impact on the Bay will continue to evolve over the next few years and be a natural experiment for other temperate estuaries that will be experiencing nitrogen reduction. To provide perspective we review factors effecting hypoxia in other estuaries with managed nutrient reduction and conclude that, as in Narragansett Bay, physical factors can be as important as nutrients. On a positive note managed nutrient reduction has mitigated further deterioration in most estuaries.

Dynamics and Origin of Transparent Exopolymer Particles in the Oyashio Region of the Western Subarctic Pacific during the Spring Diatom Bloom

The seasonal biological drawdown of the partial pressure of CO2 (pCO2) in the surface waters of the Oyashio region of the western subarctic Pacific is one of the greatest among the world’s oceans. This is attributable to spring diatom blooms. Transparent exopolymer particles (TEPs) are known to affect efficiency of the biological carbon pump, and higher TEP levels are frequently associated with massive diatom blooms.However, TEP dynamics in the Oyashio region remain unclear. This study investigates the TEP distribution from three cruises during the spring diatom bloom periods in 2010 and 2011. TEP concentrations varied from -1 above 300 m and generally declined with depth. Vertical TEP concentrations were significantly related not only to chlorophyll a concentrations but also to bacterial abundance. Average TEP concentrations within the mixed layer were significantly higher during the bloom (155 ± 12 µg Xanthan gum equiv. L-1) than in the post-bloom phase (90 ± 32 µg Xanthan gum equiv. L-1). In contrast, bacteria abundance within the mixed layer changed little during the bloom to post-bloom phases. These results suggest that the abundance of phytoplankton greatly contributed to dynamics of the TEP distribution. To evaluate the ability of the phytoplankton to produce TEP, an axenic strain of the diatom Thalassiosira nordenskioeldii, which is a representative species of Oyashio blooms, was examined within a batch culture system. Cell abundance-normalized TEP and dissolved organic carbon (DOC) production rates changed simultaneously with growth of the strain. Although these production rates were significantly higher in the stationary phase than in the exponential growth period, values of the TEP/DOC ratio changed little throughout incubation. These findings suggest that TEP production in the Oyashio region may be enhanced by an increase in DOC production from spring diatoms.

Influence of hydrography on the spatiotemporal variability of phytoplankton assemblages and primary productivity in Funka Bay and the Tsugaru Strait

Phytoplankton community structures and primary productivity were assessed in relation to the oceanographic conditions in the coastal waters of Funka bay and the eastern end of the Tsugaru Strait, adjacent to southwestern Hokkaido, Japan, from April 2010 to January 2012. Phytoplankton community compositions, as estimated from chemotaxonomic analysis based on high-performance liquid chromatography of pigments, showed diatom blooms during spring in both 2010 and 2011. However, spatial heterogeneity of chlorophyll a (Chl a) concentration and primary productivity were found between regions investigated within and outside of Funka Bay during the spring diatom blooms in April 2010. The low Chl a concentrations within Funka Bay in April 2010 were related to the depletion of dissolved inorganic macronutrients, which implies that this difference was related to both the small inflow of the cold Coastal Oyashio Current (COW) into the bay and the development of clockwise circulation caused by discharge of fresh water into the bay. After the spring diatom blooms, the major phytoplankton groups in the study area were Chl b-containing phytoplanktons (chlorophytes and prasinophytes) because of changes in salinity associated with river discharge during the melting season. The results indicate that these phytoplanktons play an important role in the carbon cycle after the spring bloom in Funka Bay and the eastern end of the Tsugaru Strait. The thermohaline fronts created by the COW and the Tsugaru Warm Water in late February produced north–south differences in phytoplankton community structures in the eastern end of the Tsugaru Strait. Diatoms with high Chl a concentrations dominated in the northern section of the front. In the southern section, the proportions of chlorophytes and cryptophytes were high. Increases in cyanobacterial abundance and temperature were detected in both regions. Additionally, the contribution of pico- plus nano-sized phytoplankton productivity to the total primary productivity at the surface was concomitant with increases in temperature. Our results suggest that small-sized phytoplankton become more important part of the food web during summer, even in the coastal waters.

Are benthic fluxes important for the availability of Si in the Gulf of Finland?

Abstract We estimated the efflux of dissolved silicon (DSi) from sediments in the Gulf of Finland and compared it to sedimentation fluxes, burial of Si and existing data on Si loading and stocks, reassessing the reliability of existing Si budgets. Benthic fluxes of DSi measured in situ and in vitro were several times higher than estimates from diffusion calculations. The spatial variability in the open Gulf of Finland was relatively small, while both very high and low fluxes were measured from coastal areas. Fluxes were highest in late summer and lowest in early spring. In our re-assessed budget we present a new lower estimate for Si burial in the sediments, ca. 6 Gmol a− 1 and show that more than half of the sedimentation flux of Si is released back into the water column. Changes in the efficiency of internal DSi recycling may thus affect the prevalence of siliceous phytoplankton within the ecosystem, and the diatom spring bloom may be regulated by the functioning of this internal recycling pump. We also show that the seasonal variation in benthic DSi fluxes and dissolved phosphate fluxes is similar, and that a tentative connection between hypoxia and high DSi efflux exists.

Particle flux in the oceans: Challenging the steady state assumption

AbstractAtmospheric carbon dioxide levels are strongly controlled by the depth at which the organic matter that sinks out of the surface ocean is remineralized. This depth is generally estimated from particle flux profiles measured using sediment traps. Inherent in this analysis is a steady state assumption that export from the surface does not significantly change in the time it takes material to reach the deepest trap. However, recent observations suggest that a significant fraction of material in the mesopelagic zone sinks slowly enough to bring this into doubt. We use data from a study in the North Atlantic during July/August 2009 to challenge the steady state assumption. An increase in biogenic silica flux with depth was observed which we interpret, based on vertical profiles of diatom taxonomy, as representing the remnants of the spring diatom bloom sinking slowly (&lt;40 m d−1). We were able to reproduce this behavior using a simple model using satellite‐derived export rates and literature‐derived remineralization rates. We further provide a simple equation to estimate “additional” (or “excess”) particulate organic carbon supply to the dark ocean during nonsteady state conditions, which is not captured by traditional sediment trap deployments. In seasonal systems, mesopelagic net organic carbon supply could be wrong by as much as 25% when assuming steady state. We conclude that the steady state assumption leads to misinterpretation of particle flux profiles when input fluxes from the upper ocean vary on the order of weeks, such as in temperate and polar regions with strong seasonal cycles in export.